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Germicidal UV light (UV-C) has been shown to effectively suppress several plant pathogens as well as some arthropod pests. Recent reports describe the efficacy of nighttime applications of UV-C at doses from 100 to 200 J/m 2 in vineyards to reduce grape powdery mildew ( Erysiphe necator ). Our in vitro studies confirmed the efficacy of UV-C to inhibit germination of E. necator and Botrytis cinerea conidia, demonstrated a range of tolerances to UV-C within a collection of E. necator isolates, and showed growth stage-specific effects of UV-C on B. cinerea . Nighttime use of UV-C was evaluated at 48 to 96 J/m 2 in small plot trials (<1,000 vines) from 2020 to 2023. Once- or twice-weekly UV-C applications significantly reduced the incidence of foliar powdery mildew compared with non-UV-C-treated controls ( P < 0.02). Suppression of powdery mildew on fruit was less consistent, where once or twice weekly UV-C exposure reduced powdery mildew disease severity in 2020 ( P = 0.04), 2021 ( P = 0.02), and 2023 ( P = 0.003) but less so in 2022 ( P = 0.07). Bunch rot severity was not significantly reduced with UV-C treatment in any year of the study. Application of UV-C until the onset of fruit color change (veraison) also had a minimal effect on the fruit-soluble solids, pH, anthocyanins, or phenolics in harvested fruit at any UV-C dose or frequency ( P > 0.10). Suppression of powdery mildew by nighttime application of UV-C at lower doses in small plots suggests that such treatments merit further evaluation in larger-scale studies in Western Oregon., Competing Interests: The author(s) declare no conflict of interest.

Cercospora leaf spot (CLS), caused by the hemibiotrophic fungus Cercospora beticola , is a destructive disease affecting table beet. Multiple applications of fungicides are needed to reduce epidemic progress to maintain foliar health and enable mechanized harvest. The sustainability of CLS control is threatened by the rapid development of fungicide resistance, the need to grow commercially acceptable yet CLS-susceptible cultivars, and the inability to manipulate agronomic conditions to mitigate disease risk. Nighttime applications of germicidal UV light (UV-C) have recently been used to suppress several plant diseases, notably those caused by ectoparasitic biotrophs such as powdery mildews. We evaluated the efficacy of nighttime applications of UV-C for suppression of CLS in table beet. In vitro lethality of UV-C to germinating conidia increased with increasing dose, with complete suppression at 1,000 J/m 2 . Greenhouse-grown table beet tolerated relatively high doses of UV-C without lethal effects despite some bronzing on the leaf blade. A UV-C dose >1,500 J/m 2 resulted in phytotoxicity severities greater than 50%. UV-C exposure to ≤750 J/m 2 resulted in negligible phytotoxicity. Older (6-week-old) greenhouse-grown plants were more susceptible to UV-C damage than younger (2- and 4-week-old) plants. Suppression of CLS by UV-C was greater when applied within 6 days of C. beticola inoculation than if delayed until 13 days after infection in greenhouse-grown plants. In field trials, there were significant linear relationships between UV-C dose and CLS control and phytotoxicity severity, and a significant negative linear relationship between phytotoxicity and CLS severity at the final assessment. Significant differences between UV-C doses on the severity of CLS and phytotoxicity indicated an efficacious dose near 800 J/m 2 . Collectively, these findings illustrate significant and substantial suppression by nighttime applications of UV-C for CLS control on table beet, with potential for incorporation in both conventional and organic table beet broadacre production systems., Competing Interests: The author(s) declare no conflict of interest.

Nighttime applications of germicidal UV light (UV-C) have been used to suppress several fungal diseases of plants, but less is known of UV-C's potential to suppress bacterial plant pathogens. Fire blight of apple and pear, caused by the bacterium Erwinia amylovora , is difficult to suppress using cultural practices, antibiotics, and host resistance. We therefore investigated the potential of UV-C as an additional means to manage the disease. Laboratory assays confirmed that in vitro exposure of cultures E. amylovora to UV-C at doses ranging from 0 to 400 J/m 2 in the absence of visible light was more than 200% as effective as cultures exposed to visible light after the same UV-C treatments. In a 2-year orchard study, we demonstrated that with only two nighttime applications of UV-C at 200 J/m 2 made at bloom resulted in an incidence of blossom blight and shoot blight equivalent to the results viewed when antibiotic and biopesticide commercial standards were applied. In vitro dose-response studies indicated consistency in pathogen response to suppressive UV-C doses, including pathogen isolates that were resistant to streptomycin. Based on these results, UV-C may be useful in managing bacterial populations with antibiotic resistance. Concurrent measurements of host growth after UV-C applications indicated that the dose required to suppress E. amylovora had no significant ( P > 0.05) effects on foliar growth, shoot extension, internode length, or fruit finish but substantially reduced epiphytic populations of E. amylovora on host tissues., Competing Interests: The author(s) declare no conflict of interest.

Ultraviolet light at wavelengths from 254 to 283 nm/has been reported to effectively suppress powdery mildews in several crops, including some cucurbits. Its use to suppress powdery mildew ( Podosphaera xanthii ) specifically in cantaloupe has not been previously reported. We evaluated the foregoing technology in cantaloupe fields for suppression of powdery mildew and possible effects on plant growth and yield. In a controlled laboratory study, greenhouse-grown cantaloupe plants were exposed to a gradient of UV-C (254 nm) doses during darkness, and the effects upon powdery mildew development and the plant were evaluated. We also evaluated the efficacy of nighttime applications of UV-C at 100 and 200 J/m 2 against powdery mildew on adaxial leaf surfaces in greenhouse, high-tunnel, and open-field plantings. UV-C at the foregoing doses reduced sporulation and germination of P. xanthii conidia without damaging plants. On cantaloupe seedlings in the greenhouse, disease severity was equivalently suppressed at all doses and frequencies of applications of the light. In high-tunnel and open-field experiments, the most effective control of powdery mildew was provided by UV-C applied at 200 J/m 2 twice every week, where suppression provided by UV-C was generally equal to and sometimes better than the fungicide treatment. The foregoing UV-C dose and frequency of application also provided the highest yield under field conditions, indicating that UV-C treatment is a promising technology for commercially relevant suppression of powdery mildew on cantaloupe in a variety of growing systems., Competing Interests: The author(s) declare no conflict of interest.

Nighttime applications of germicidal ultraviolet were evaluated as a means to suppress three diseases of grapevine. In laboratory studies, UV-C light (peak 254 nm, FWHM 5 nm) applied during darkness strongly inhibited the germination of conidia of Erysiphe necator , and at a dose of 200 J/m 2 , germination was zero. Reciprocity of irradiance and duration of exposure with respect to conidial germination was confirmed for UV-C doses between 0 and 200 J/m 2 applied at 4 or 400 s. When detached grapevine leaves were exposed during darkness to UV-C at 100 J/m 2 up to 7 days before they were inoculated with zoospores of Plasmopara viticola , infection and subsequent sporulation was reduced by over 70% compared to untreated control leaves, indicating an indirect suppression of the pathogen exerted through the host. A hemicylindrical array of low-pressure discharge UV-C lamps configured for trellised grapevines was designed and fitted to both a tractor-drawn carriage and a fully autonomous robotic carriage for vineyard applications. In 2019, in a Chardonnay research vineyard with a history of high inoculum and severe disease, weekly nighttime applications of UV-C suppressed E. necator on leaves and fruit at doses of 100 and 200 J/m 2 . In the same vineyard in 2020, UV-C was applied once or twice weekly at doses of 70, 100, or 200 J/m 2 , and severity of E. necator on both leaves and fruit was significantly reduced compared to untreated controls; twice-weekly applications at 200 J/m 2 provided suppression equivalent to a standard fungicide program. None of the foregoing UV-C treatments significantly reduced the severity of P. viticola on Chardonnay vines compared to the untreated control in 2020. However, twice-weekly applications of UV-C at 200 J/m 2 to the more downy mildew-resistant Vitis interspecific hybrid cultivar Vignoles in 2021 significantly suppressed foliar disease severity. In commercial Chardonnay vineyards with histories of excellent disease control in Dresden, NY, E. necator remained at trace levels on foliage and was zero on fruit following weekly nighttime applications of UV-C at 200 J/m 2 in 2020 and after weekly or twice-weekly application of UV-C at 100 or 200 J/m 2 in 2021. In 2019, weekly nighttime applications of UV-C at 200 J/m 2 also significantly reduced the severity of sour rot, a decay syndrome of complex etiology, on fruit of 'Vignoles' but not the severity of bunch rot caused by Botrytis cinerea . A similar level of suppression of sour rot was observed on 'Vignoles' vines treated twice-weekly with UV-C at 200 J/m 2 in 2021. Nighttime UV-C applications did not produce detectable indications of metabolic abnormalities, phytotoxicity, growth reduction, or reductions of fruit yield or quality parameters, even at the highest doses and most frequent intervals employed., Competing Interests: The author(s) declare no conflict of interest.

Strawberry powdery mildew, caused by Podosphaera aphanis , can be particularly destructive in glasshouse and plastic tunnel production systems, which generally are constructed of materials that block ultraviolet (UV) solar radiation (about 280 to 400 nm). We compared epidemic progress in replicated plots in open fields and under tunnels constructed of polyethylene, which blocks nearly all solar UV-B, and two formulations of ethylene tetrafluoroethylene (ETFE), one of which contained a UV blocker and another that transmitted nearly 90% of solar UV-B. Disease severity under all plastics was higher than in open-field plots, indicating a generally more favorable environment in containment structures. However, the foliar severity of powdery mildew within the tunnels was inversely related to their UV transmissibility. Among the tunnels tested, incidence of fruit infection was highest under polyethylene and lowest under UV-transmitting ETFE. These effects probably transcend crop, and the blocking of solar UV transmission by glass and certain plastics probably contributes to the widely observed favorability of greenhouse and high-tunnel growing systems for powdery mildew.

Grapevine is one of the most widely-planted fruit crops in the world, and is the most economically important fruit crop in the state of New York, USA. Symptoms of anthracnose on grapevine are similarly widely-reported on grapevine fruit and foliage, and such symptoms are commonly attributed to Elsinöe ampelina (Wilcox et al., 2015). However, similar symptoms, if not identical, to those associated with E. ampelina have been sporadically attributed to various species in the genus Colletotrichum. In September 2021, a survey was conducted in three research vineyards at Cornell AgriTech in Geneva, NY. Symptoms of anthracnose werebserved on four Vitis interspecific hybrid breeding lines in a 1 ha vineyard. Leaves, fruit, and petioles showing symptoms of anthracnose, i.e., sunken necrotic lesions with grayish centers and brownish margins, were collected. Symptomatic and healthy portions of surface-sterilized tissues were placed on PDA medium and incubated at 23oC for 7 days. Several petiole samples yielded colonies of white to greyish mycelium, with some red to orange pigmentation (Fig. 1A and 1B), similar to those described by Chowdappa et al. (2009) for Colletotrichum species isolated from grapevine in India. Cultures were allowed to sporulate. Slides from cultures were prepared and examined at 400X magnification. Conidia from cultures were cylindrical with rounded ends, 13.5-15.2 μm in length and 7.6-9.0 μm in width (Fig. 1C). Koch's postulates were fulfilled by inoculating detached healthy leaves of V. vinifera 'Chardonnay' that had been surface sterilized in 10% sodium hypochlorite and triple-rinsed in sterile distilled water. Drop inoculation was used from a suspension of 105 conidia/ml from the foregoing pure cultures as five 2 µL droplets per leaf. Inoculated detached leaves were maintained on water agar in a Petri dish at 23oC. Four days after inoculation, symptoms were observed and compared with the originally collected samples. Inoculated leaves displayed symptoms typically found on the collected tissues, and the original pathogen, as confirmed by colony morphology and conidial characteristics and dimensions, was reisolated from inoculated leaves, and not from non-inoculated controls. For molecular characterization, fungal DNA was isolated by using Qiagen DNeasy kit and amplified using the following primer pairs: ITS1/ITS4, TEF (Hyun et al., 2009), E. ampelina F/R (Santos et al. 2018), TUB2, ACT, HIS3, GAPDH and CHS1 (Damm et al., 2001). PCR products were purified using ExoSAP-IT, and samples were Sanger sequenced. Sequences were analyzed using Geneious Prime software, and the resulting sequences (NCBI accessions OL720215, OL720216, OL720217, OL720218, OL853836, OM982612, OM982613, OM982614, OM982615 and OM982616) had 94 to 100% identity to Colletotrichum fioriniae NCBI accessions MN944922.1, MK646015.1, MN944922.1, MN856415.1, KU847413.1, MN520490.1, MN544294.1, KY695259.1, MN535117.1 and MN544295.1. Symptoms of grapevine anthracnose caused by Colletotrichum species have been reported from India (Chowdappa et al., 2009) and Korea (Kim et al., 2021). To our knowledge this is the first report of grapevine anthracnose caused by C. fioriniae Anthracnose and ripe rot are diseases of increasing importance, particularly as new grapevine cultivars with resistance to powdery mildew or downy mildew are adopted. Taxonomy of the causal agents (E. ampelina and Colletotrichum spp.) has undergone considerable revision. Consequently, distribution and relative prevalence of the various taxa will require further study.

Hop powdery mildew, caused by the ascomycete fungus Podosphaera macularis , is a consistent threat to sustainable hop production. The pathogen utilizes two reproductive strategies for overwintering and perennation: (i) asexual vegetative hyphae on dormant buds that emerge the following season as infected shoots; and (ii) sexual ascocarps (chasmothecia), which are discharged during spring rain events. We demonstrate that P. macularis chasmothecia, in the absence of any asexual P. macularis growth forms, are a viable overwintering source capable of causing early season infection two to three orders of magnitude greater than that reported for perennation via asexual growth. Two epidemiological models were defined that describe (i) temperature-driven maturation of P. macularis chasmothecia; and (ii) ascosporic discharge in response to duration of leaf wetness and prevailing temperatures. P. macularis ascospores were confirmed to be infectious at temperatures ranging from 5 to 20°C. The organism's chasmothecia were also found to adhere tightly to the host tissue on which they formed, suggesting that these structures likely overwinter wherever hop tissue senesces within a hop yard. These observations suggest that existing early season disease management practices are especially crucial to controlling hop powdery mildew in the presence of P. macularis chasmothecia. Furthermore, these insights provide a baseline for the validation of weather-driven models describing maturation and release of P. macularis ascospores, models that can eventually be incorporated into hop disease management programs.

We designed and deployed an apparatus to apply UV light for suppression of powdery mildew in open field production of strawberry. The unit was evaluated in a commercial production field for one season, and for two additional seasons in open field research plots at the University of Florida Gulf Coast Research and Education Center. The apparatus contained two 180-cm-long hemicylindrical arrays of twenty 55-W low-pressure discharge UV-C lamps (operated at 30 W; peak wavelength = 254 nm) backed by polished aluminum reflectors covering two adjacent beds of the strawberry planting. The lamp arrays were suspended within a steel carriage that was tractor-drawn through the planting at 2.3, 4.6, and 5.6 km h -1 . Nighttime applications of UV-C at doses ranging from 65 to 170 J⋅m -2 either once or twice weekly provided suppression of foliar and fruit disease that was consistently equal to or better than that provided by a commercial calendar-based fungicide spray program.

Obligately biotrophic plant pathogens pose challenges in population genetic studies due to their genomic complexities and elaborate culturing requirements with limited biomass. Hop powdery mildew ( Podosphaera macularis ) is an obligately biotrophic ascomycete that threatens sustainable hop production. P. macularis populations of the Pacific Northwest (PNW) United States differ from those of the Midwest and Northeastern United States, lacking one of two mating types needed for sexual recombination and harboring two strains that are differentially aggressive on the cultivar Cascade and able to overcome the Humulus lupulus R -gene R6 ( V6 ), respectively. To develop a high-throughput marker platform for tracking the flow of genotypes across the United States and internationally, we used an existing transcriptome of diverse P. macularis isolates to design a multiplex of 54 amplicon sequencing markers, validated across a panel of 391 U.S. samples and 123 international samples. The results suggest that P. macularis from U.S. commercial hop yards form one population closely related to P. macularis of the United Kingdom, while P. macularis from U.S. feral hop locations grouped with P. macularis of Eastern Europe. Included in this multiplex was a marker that successfully tracked V6 -virulence in 65 of 66 samples with a confirmed V6 -phenotype. A new qPCR assay for high-throughput genotyping of P. macularis mating type generated the highest resolution distribution map of P. macularis mating type to date. Together, these genotyping strategies enable the high-throughput and inexpensive tracking of pathogen spread among geographical regions from single-colony samples and provide a roadmap to develop markers for other obligate biotrophs.

Powdery mildew ( Podosphaera aphanis ) is a destructive and widespread disease of strawberry ( Fragaria × ananassa ), especially when susceptible cultivars are grown in high plastic tunnels or glasshouses. Many powdery mildews thrive in humid environments but free water films on plant surfaces can inhibit conidial germination of some species. We hypothesized that P. aphanis might be directly suppressed by rain through the action of water films and meteoric water. In repeated experiments, the hydrophobic conidia of P. aphanis collected on the surface of water droplets, resulting in their removal when the droplets rolled over the leaf surfaces and fell to the ground. Meteoric water and water films also damaged conidiophores. Brief midday water mists applied in pulses lasting 1 min each four times per day were as effective as multiple fungicide treatments in suppressing powdery mildew. Rapid drying of the pulsed mists resulted in effective suppression of powdery mildew without consequent increases of fungal pathogens that might benefit from water films. The timing and duration of water sprinkling has been refined to the point where it can provide a commercially relevant degree of suppression of powdery mildew on strawberry in a high-tunnel production system.

In a number of pathosystems involving the powdery mildews (Erysiphales), plant stress is associated with decreased disease susceptibility and is detrimental to pathogen growth and reproduction. However, in strawberry, anecdotal observations associate severe powdery mildew ( Podosphaera aphanis ) with water stress. In a 2017 survey of 42 strawberry growers in Norway and California, 40 growers agreed with a statement that water-stressed strawberry plants were more susceptible to powdery mildew compared with nonstressed plants. In repeated in vitro and in vivo experiments, we found that water stress was consistently and significantly unfavorable to conidial germination, infection, and increases in disease severity. Deleterious effects on the pathogen were observed from both preinoculation and postinoculation water stress in the host. Soil moisture content in the range from 0 to 50% was correlated ( R 2 = 0.897) with germinability of conidia harvested from extant colonies that developed on plants growing at different levels of water stress. These studies confirm that P. aphanis fits the norm for biotrophic powdery mildews and hosts under stress. Mild water stress, compared with a state of optimal hydration, is likely to decrease rather than increase susceptibility of strawberry to P. aphanis . We believe it is possible that foliar symptoms of leaf curling due to diffuse and inconspicuous infection of the lower leaf surfaces by P. aphanis could easily be mistakenly attributed to water stress, which we observed as having a nearly identical leaf curling symptom in strawberry.

Powdery mildew, caused by Podosphaera macularis , is one of the most important diseases of hop. The disease was first reported in the Pacific Northwestern United States, the primary hop-growing region in this country, in the mid-1990s. More recently, the disease has reemerged in newly planted hopyards of the eastern United States, as hop production has expanded to meet demands of local craft brewers. The spread of strains virulent on previously resistant cultivars, the paucity of available fungicides, and the potential introduction of the MAT1-2 mating type to the western United States, all threaten sustainability of hop production. We sequenced the transcriptome of 104 isolates of P . macularis collected throughout the western United States, eastern United States, and Europe to quantify genetic diversity of pathogen populations and elucidate the possible origins of pathogen populations in the western United States. Discriminant analysis of principal components grouped isolates within three to five geographic populations, dependent on stringency of grouping criteria. Isolates from the western United States were phenotyped and categorized into one of three pathogenic races based on disease symptoms generated on differential cultivars. Western U.S. populations were clonal, irrespective of pathogenic race, and grouped with isolates originating from Europe. Isolates originating from wild hop plants in the eastern United States were genetically differentiated from all other populations, whereas isolates from cultivated hop plants in the eastern United States mostly grouped with isolates originating from the west, consistent with origins from nursery sources. Mating types of isolates originating from cultivated western and eastern U.S. hop plants were entirely MAT1-1 . In contrast, a 1:1 ratio of MAT1-1 and MAT1-2 was observed with isolates sampled from wild plants or Europe. Within the western United States a set of highly differentiated loci were identified in P. macularis isolates associated with virulence to the powdery mildew R-gene R6. The weight of genetic and phenotypic evidence suggests a European origin of the P. macularis populations in the western United States, followed by spread of the pathogen from the western United States to re-emergent production regions in the eastern United States. Furthermore, R6 compatibility appears to have been selected from an extant isolate within the western United States. Greater emphasis on sanitation measures during propagation and quarantine policies should be considered to limit further spread of novel genotypes of the pathogen, both between and within production areas.

Temperatures from 2 to 8°C transiently induce quantitative resistance to powdery mildew in several host species (cold stress-induced disease resistance [SIDR]). Although cold SIDR events occur in vineyards worldwide an average of 14 to 21 times after budbreak of grapevine and can significantly delay grapevine powdery mildew ( Erysiphe necator ) epidemics, its molecular basis was poorly understood. We characterized the biology underlying the Vitis vinifera cold SIDR phenotype-which peaks at 24 h post-cold (hpc) treatment and results in a 22 to 28% reduction in spore penetration success-through highly replicated ( n = 8 to 10) RNA sequencing experiments. This phenotype was accompanied by a sweeping transcriptional downregulation of photosynthesis-associated pathways whereas starch and sugar metabolism pathways remained largely unaffected, suggesting a transient imbalance in host metabolism and a suboptimal target for pathogen establishment. Twenty-six cold-responsive genes peaked in their differential expression at the 24-hpc time point. Finally, a subset of genes associated with nutrient and amino acid transport accounted for four of the eight most downregulated transcripts, including two nodulin 1A gene precursors, a nodulin MtN21 precursor, and a Dynein light chain 1 motor protein precursor. Reduced transport could exacerbate localized nutrient sinks that would again be transiently suboptimal for pathogen growth. This study links the transient cold SIDR phenotype to underlying transcriptional changes and provides an experimental framework and library of candidate genes to further explore cold SIDR in several systems, with an ultimate goal of identifying novel breeding or management targets for reduced disease.

Quantitative trait locus (QTL) identification in perennial fruit crops is impeded largely by their lengthy generation time, resulting in costly and labor-intensive maintenance of breeding programs. In a grapevine (genus Vitis ) breeding program, although experimental families are typically unreplicated, the genetic backgrounds may contain similar progenitors previously selected due to their contribution of favorable alleles. In this study, we investigated the utility of joint QTL identification provided by analyzing half-sib families. The genetic control of powdery mildew was studied using two half-sib F 1 families, namely GE0711/1009 (MN1264 × MN1214; N  = 147) and GE1025 (MN1264 × MN1246; N  = 125) with multiple species in their ancestry. Maternal genetic maps consisting of 1077 and 1641 single nucleotide polymorphism (SNP) markers, respectively, were constructed using a pseudo-testcross strategy. Ratings of field resistance to powdery mildew were obtained based on whole-plant evaluation of disease severity. This 2-year analysis uncovered two QTLs that were validated on a consensus map in these half-sib families with improved precision relative to the parental maps. Examination of haplotype combinations based on the two QTL regions identified strong association of haplotypes inherited from 'Seyval blanc', through MN1264, with powdery mildew resistance. This investigation also encompassed the use of microsatellite markers to establish a correlation between 206-bp (UDV-015b) and 357-bp (VViv67) fragment sizes with resistance-carrying haplotypes. Our work is one of the first reports in grapevine demonstrating the use of SNP-based maps and haplotypes for QTL identification and tagging of powdery mildew resistance in half-sib families., Competing Interests: The authors declare no conflict of interest. Ethical standards This article does not contain any studies with human participants or animal performed by any of the authors.

Disease-suppressive effects of nighttime applications of ultraviolet-B (UV-B) were investigated at two irradiance levels (1.6 or 0.8 W/m 2 ) in strawberry and rosemary plants inoculated with Podosphaera aphanis or Golovinomyces biocellatus, respectively. Plants were exposed to each irradiance level every third night for either 9 or 18 min, every night for either 3 or 6 min, or three times every night for either 1 or 2 min. Thus, over time, all plants received the same cumulative dose of UV-B, and severity of powdery mildew was reduced by 90 to 99% compared with untreated controls in both crops. Use of polished aluminum lamp reflectors and UV-B reflective surfaces on greenhouse benches significantly increased treatment efficacy. An automated apparatus consisting of an adjustable boom with directed airflow was used to move UV-B lamps over greenhouse benches at 25 or 50 cm/min. Directed airflow moved leaves on the subtending plants to better expose upper and lower surfaces to UV-B but directed airflow actually decreased the efficacy of UV-B treatments, possibly by dispersing conidia from lesions before they were exposed to a lethal dose of UV-B. Results indicate broad applicability of nighttime applications of UV-B to suppress powdery mildews, and that cumulative UV-B dose is an overriding factor determining efficacy. Finally, enhanced suppression on shaded or obscured tissues is more likely to be affected by reflective bench surfaces than through attempts to physically manipulate the foliage.

The temporal distribution and magnitude of ontogenic resistance in strawberry leaves to Podosphaera aphanis has recently been quantified. However, the degree to which the pathogen is inhibited at sequential stages of conidial germination, hyphal growth, haustoria formation, latent period, colony expansion, and sporulation on the adaxial and abaxial leaf surfaces of various strawberry cultivars remains unclear. Five developmental stages of strawberry leaves ranging from newly emerged and folded leaves to fully expanded and dark-green leaves were inoculated with conidia of P. aphanis. The percentage of germinated conidia significantly declined between leaf stages 3 and 5. Postgermination growth of the pathogen was sequentially reduced in all measured responses, and the latent period was increased. Haustoria were not observed in mature leaves. The failure of the pathogen to penetrate mature leaves was a consistent feature associated with the expression of ontogenic resistance in older, fully expanded leaves.

Recorded severity of grape powdery mildew on berries of untreated, susceptible hybrid cultivars varied from 0.2 to 50.5% across a 30-year period in Geneva, NY; within 7 of those years, cluster disease severity ranged from 3.42 to 99.5% on Vitis vinifera 'Chardonnay'. Although existing temperature-driven risk models could not account for this annual variation, pan evaporation (E pan ), an environmental variable influenced by the collective effects of temperature, vapor pressure deficit, solar radiation, and wind speed, did. Logistic regression analysis (LRA) was used to classify epidemics as either mild or severe. Recursive partition analysis (RPA) provided a simplified decision tree for calculation of powdery mildew risk and incorporated (i) an estimate of the relative primary inoculum levels based on temperatures in the previous late summer and (ii) the current season favorability for pathogen development during the grapevine phenological period critical for berry infection by Erysiphe necator. Although the LRA had fewer instances of misclassification, RPA provided a rapid means for seasonal risk classification. Both the RPA and LRA models are able to describe disease severity risk in real time or can be used to forecast risk, thereby allowing growers to adjust management programs in a responsive manner.

The Toll/interleukin-1 receptor nucleotide-binding site leucine-rich repeat gene, "resistance to Uncinula necator 1" (RUN1), from Vitis rotundifolia was recently identified and confirmed to confer resistance to the grapevine powdery mildew fungus Erysiphe necator (syn. U. necator) in transgenic V. vinifera cultivars. However, sporulating powdery mildew colonies and cleistothecia of the heterothallic pathogen have been found on introgression lines containing the RUN1 locus growing in New York (NY). Two E. necator isolates collected from RUN1 vines were designated NY1-131 and NY1-137 and were used in this study to inform a strategy for durable RUN1 deployment. In order to achieve this, fitness parameters of NY1-131 and NY1-137 were quantified relative to powdery mildew isolates collected from V. rotundifolia and V. vinifera on vines containing alleles of the powdery mildew resistance genes RUN1, RUN2, or REN2. The results clearly demonstrate the race specificity of RUN1, RUN2, and REN2 resistance alleles, all of which exhibit programmed cell death (PCD)-mediated resistance. The NY1 isolates investigated were found to have an intermediate virulence on RUN1 vines, although this may be allele specific, while the Musc4 isolate collected from V. rotundifolia was virulent on all RUN1 vines. Another powdery mildew resistance locus, RUN2, was previously mapped in different V. rotundifolia genotypes, and two alleles (RUN2.1 and RUN2.2) were identified. The RUN2.1 allele was found to provide PCD-mediated resistance to both an NY1 isolate and Musc4. Importantly, REN2 vines were resistant to the NY1 isolates and RUN1REN2 vines combining both genes displayed additional resistance. Based on these results, RUN1-mediated resistance in grapevine may be enhanced by pyramiding with RUN2.1 or REN2; however, naturally occurring isolates in North America display some virulence on vines with these resistance genes. The characterization of additional resistance sources is needed to identify resistance gene combinations that will further enhance durability. For the resistance gene combinations currently available, we recommend using complementary management strategies, including fungicide application, to reduce populations of virulent isolates.

This study demonstrates that the spectral quality of radiation sources applied with ultraviolet-B (UV-B; background radiation) affects the suppression of cucumber powdery mildew (Podosphaera xanthii) by UV-B. Suppression provided by daily UV-B exposure of 1 W/m 2 for 10 min was greatest in the presence of red light or by a complete lack of background light, and powdery mildew suppression was least in the presence of ultraviolet-A (UV-A) or blue radiation compared with plants exposed only to 16 h of daily natural light supplemented with high-pressure sodium lamps that supply broad-spectrum radiation with peaks in the yellow-orange region. Exposure of powdery mildew-inoculated plants to supplemental red light without UV-B, beginning at the end of the daylight period, also reduced disease severity; however, supplemental blue light applied in the same fashion had no effect. Daily application of UV-B at 1 W/m 2 beginning on the day of inoculation significantly reduced the severity of powdery mildew to 15% compared with 100% severity on control plants. Maximum suppression of powdery mildew was observed following 15 min of exposure to UV-B (1.1% severity compared with 100% severity on control plants) but exposure time had to be limited to 5 to 10 min to reduce phytotoxicity. There was no additional disease suppression when plants were exposed to UV-B beginning 2 days prior to inoculation compared with plants exposed to UV-B beginning on the day of inoculation. UV-B inhibited germination, infection, colony expansion, and sporulation of P. xanthii. The results suggest that efficacy of UV-B treatments, alone or in combination with red light, against P. xanthii can be enhanced by exposure of inoculated plants to these wavelengths of radiation during the night, thereby circumventing the counteracting effects of blue light and UV-A radiation. The effect of UV-B on powdery mildew seemed to be directly upon the pathogen, rather than induced resistance of the host. Night exposure of plants to 5 to 10 min of UV-B at 1 W/m 2 and inexpensive, spectral-specific, light-emitting diodes may provide additional tools to suppress powdery mildews of diverse greenhouse crops.

Ontogenic or age-related resistance has been noted in many pathosystems but is less often quantified or expressed in a manner that allows the concept to be applied in disease management programs. Preliminary studies indicated that leaves and fruit of three strawberry cultivars rapidly acquired ontogenic resistance to the powdery mildew pathogen, Podosphaera aphanis. In the present study, we quantify the development of ontogenic resistance in controlled inoculations of 10 strawberry cultivars using diverse isolates of P. aphanis in New York and Florida, USA, and in Norway. We report the differential and organ-specific development of ontogenic resistance in the receptacle and externally borne strawberry achenes. We further report that rapid development of ontogenic resistance prior to unfolding of emergent leaves, rather than differential susceptibility of adaxial versus abaxial leaf surfaces, may explain the commonly observed predominance of powdery mildew on the lower leaf surfaces. Susceptibility of leaves and fruit declined exponentially with age. Receptacle tissue of berries inoculated at four phenological stages from bloom to ripe fruit became nearly immune to infection approximately 10 to 15 days after bloom, as fruit transitioned from the early green to the late green or early white stage of berry development, although the achenes remained susceptible for a longer period. Leaves also acquired ontogenic resistance early in their development, and they were highly resistant shortly after unfolding and before the upper surface was fully exposed. No significant difference was found in the susceptibility of the adaxial versus abaxial surfaces. The rapid acquisition of ontogenic resistance by leaves and fruit revealed a narrow window of susceptibility to which management programs might be advantageously adapted.

The formation of chasmothecia by the strawberry powdery mildew pathogen (Podosphaera aphanis) is widespread but often sporadic throughout the range of strawberry cultivation. In some production regions, notably in warmer climates, chasmothecia are reportedly rare. We confirmed that the pathogen is heterothallic, and that initiation of chasmothecia is not only dependent upon the presence of isolates of both mating types but also largely suppressed at temperatures >13°C. Compared with incubation at a constant temperature of 25°C, progressively more chasmothecia were initiated when temperatures were decreased to 13°C for progressively longer times. At lower temperatures, production of chasmothecia was associated with a decline in but not total cessation of conidial formation, and pairings of compatible isolates sporulated abundantly at 25°C. We developed mating-type markers specific to P. aphanis and used these to confirm the presence of both mating types in populations that had not yet initiated chasmothecia. The geographic discontinuity of chasmothecia production and the sporadic and seemingly unpredictable appearance of chasmothecia in P. aphanis are possibly due to the combined influence of heterothallism and suppression of chasmothecia formation by high temperatures.

Ultraviolet (UV)-B (280 to 315 nm) irradiance from 0.1 to 1.2 W m -2 and exposure times from 2 min to 2 h significantly suppressed powdery mildew (Podosphaera pannosa) in pot rose (Rosa × hybrida 'Toril') via reduced spore germination, infection efficiency, disease severity, and sporulation of surviving colonies. Brief daily exposure to UV-B suppressed disease severity by more than 90% compared with unexposed controls, and severity was held at low levels as long as daily brief exposures continued. Selective removal of wavelengths below 290 nm from the UV lamp sources by cellulose diacetate filters resulted in significant reduction of treatment efficacy. Exposure of plants to 2 h of UV-B during night for 1 week followed by inoculation with P. pannosa did not affect subsequent pathogen development, indicating that the treatment effect was directly upon the exposed pathogen and not operated through the host. Following 20 to 30 days of exposure, chlorophyll and flavonoid content was slightly higher in plants exposed to the highest UV-B levels. Brief daily exposure to UV-B for 5 min at 1.2 W m -2 or 1 h at 0.1 W m -2 substantially reduced mildew severity without significant phytotoxicity, and may represent a useful nonchemical option for suppression of powdery mildew in greenhouse roses and, possibly, other crops.

Unlabelled: Few plant pathogens have had a more profound effect on the evolution of disease management than Erysiphe necator, which causes grapevine powdery mildew. When the pathogen first spread from North America to England in 1845, and onwards to France in 1847, 'germ theory' was neither understood among the general populace nor even generally accepted within the scientific community. Louis Pasteur had only recently reported the microbial nature of fermentation, and it would be another 30 years before Robert Koch would publish his proofs of the microbial nature of certain animal diseases. However, within 6 years after the arrival of the pathogen, nearly 6 million grape growers in France were routinely applying sulphur to suppress powdery mildew on nearly 2.5 million hectares of vineyards (Campbell, 2006). The pathogen has remained a focus for disease management efforts ever since. Because of the worldwide importance of the crop and its susceptibility to the disease, and because conventional management with modern, organic fungicides has been compromised on several occasions since 1980 by the evolution of fungicide resistance, there has also been a renewed effort worldwide to explore the pathogen's biology and ecology, its genetics and molecular interactions with host plants, and to refine current and suggest new management strategies. These latter aspects are the subject of our review., Taxonomy: The most widely accepted classification follows. Family Erysiphaceae, Erysiphe necator Schw. [syn. Uncinula necator (Schw.) Burr., E. tuckeri Berk., U. americana Howe and U. spiralis Berk. & Curt; anamorph Oidium tuckeri Berk.]. Erysiphe necator var. ampelopsidis was found on Parthenocissus spp. in North America according to Braun (1987), although later studies revealed isolates whose host range spanned genera, making the application of this taxon somewhat imprecise (Gadoury and Pearson, 1991). The classification of the genera before 1980 was based on features of the mature ascocarp: (i) numbers of asci; and (ii) morphology of the appendages, in particular the appendage tips. The foregoing has been supplanted by phylogeny inferred from the internal transcribed spacer (ITS) of ribosomal DNA sequences (Saenz and Taylor, 1999), which correlates with conidial ontogeny and morphology (Braun et al., 2002)., Host Range: The pathogen is obligately parasitic on genera within the Vitaceae, including Vitis, Cissus, Parthenocissus and Ampelopsis (Pearson and Gadoury, 1992). The most economically important host is grapevine (Vitis), particularly the European grape, V. vinifera, which is highly susceptible to powdery mildew. Disease symptoms and signs: In the strictest sense, macroscopically visible mildew colonies are signs of the pathogen rather than symptoms resulting from its infection, but, for convenience, we describe the symptoms and signs together as the collective appearance of colonized host tissues. All green tissues of the host may be infected. Ascospore colonies are most commonly found on the lower surface of the first-formed leaves near the bark of the vine, and may be accompanied by a similarly shaped chlorotic spot on the upper surface. Young colonies appear whitish and those that have not yet sporulated show a metallic sheen. They are roughly circular, ranging in size from a few millimetres to a centimetre or more in diameter, and can occur singly or in groups that coalesce to cover much of the leaf. Senescent colonies are greyish, and may bear cleistothecia in various stages of development. Dead epidermal cells often subtend the colonized area, as natural mortality in the mildew colony, the use of fungicides, mycoparasites or resistance responses in the leaf result in the deaths of segments of the mildew colony and infected epidermal cells. Severely affected leaves usually senesce, develop necrotic blotches and fall prematurely. Infection of stems initially produces symptoms similar to those on leaves, but colonies on shoots are eventually killed as periderm forms, producing a dark, web-like scar on the cane (Gadoury et al., 2011). Inflorescences and berries are most susceptible when young, and can become completely coated with whitish mildew. The growth of the berry epidermal tissue stops when severely infected, which may result in splitting as young fruit expand. Berries in a transitional stage between susceptible and resistant (generally between 3 and 4 weeks after anthesis) develop diffuse, nonsporulating mildew colonies only visible under magnification. Diffuse colonies die as berries continue to mature, leaving behind a network of necrotic epidermal cells (Gadoury et al., 2007). Survival over winter as mycelium in buds results in a distinctive foliar symptom. Shoots arising from these buds may be heavily coated with fungal growth, stark white in colour and stand out like white flags in the vine, resulting in the term 'flag shoots'. More commonly, colonization of a flag shoot is less extensive, and infection of a single leaf, or of leaves on one side of the shoot only, is observed (Gadoury et al., 2011)., (© 2011 THE AUTHORS. MOLECULAR PLANT PATHOLOGY © 2011 BSPP AND BLACKWELL PUBLISHING LTD.)

Initiation of asexual sporulation in powdery mildews is preceded by a period of superficial vegetative growth of mildew colonies. We found evidence of a quorum-sensing signal in Erysiphe necator that was promulgated at the colony center and stimulated conidiation throughout the colony. Removal of the colony center after putative signal promulgation had no impact upon timing of sporulation by 48-h-old hyphae at the colony margin. However, removal of the colony center before signaling nearly doubled the latent period. A relationship between inoculum density and latent period was also observed, with latent period decreasing as the number of conidia deposited per square millimeter was increased. The effect was most pronounced at the lowest inoculum densities, with little decrease of the latent period as the density of inoculation increased above 10 spores/mm. Furthermore, light was shown to be necessary to initiate conidiation of sporulation-competent colonies. When plants were inoculated and maintained in a day-and-night cycle for 36 h but subjected to darkness after 36 h, colonies kept in darkness failed to sporulate for several days after plants kept in light had sporulated. Once returned to light, the dark-suppression was immediately reversed, and sporulation commenced within 12 h. Merging of colonies of compatible mating types resulted in near-cessation of sporulation, both in the region of merging and in more distant parts of the colonies. Colonies continued to expand but stopped producing new conidiophores once pairing of compatible mating types had occurred, and extant conidiophores stopped producing new conidia. Therefore, in addition to a quorum-sensing signal to initiate conidiation, there appears to be either signal repression or another signal that causes conidiation to cease once pairing has occurred and the pathogen has initiated the ascigerous stage for overwintering.

Asexual sporulation (conidiation) is coordinately regulated in the grape powdery mildew pathogen Erysiphe necator but nothing is known about its genetic regulation. We hypothesized that genes required for conidiation in other fungi would be upregulated at conidiophore initiation or full conidiation (relative to preconidiation vegetative growth and development of mature ascocarps), and that the obligate biotrophic lifestyle of E. necator would necessitate some novel gene regulation. cDNA amplified fragment length polymorphism analysis with 45 selective primer combinations produced ≈1,600 transcript-derived fragments (TDFs), of which 620 (39%) showed differential expression. TDF sequences were annotated using BLAST analysis of GenBank and of a reference transcriptome for E. necator developed by 454-FLX pyrosequencing of a normalized cDNA library. One-fourth of the differentially expressed, annotated sequences had similarity to fungal genes of unknown function. The remaining genes had annotated function in metabolism, signaling, transcription, transport, and protein fate. As expected, a portion of orthologs known in other fungi to be involved in developmental regulation was upregulated immediately prior to or during conidiation; particularly noteworthy were several genes associated with the light-dependent VeA regulatory system, G-protein signaling (Pth11 and a kelch repeat), and nuclear transport (importin-β and Ran). This work represents the first investigation into differential gene expression during morphogenesis in E. necator and identifies candidate genes and hypotheses for characterization in powdery mildews. Our results indicate that, although control of conidiation in powdery mildews may share some basic elements with established systems, there are significant points of divergence as well, perhaps due, in part, to the obligate biotrophic lifestyle of powdery mildews.

Estimates of ascospore maturity generated by models developed for Venturia pyrina in Victoria, Australia (NV and SV), Oregon, United States (OR), and Italy (IT) or for V. inaequalis in New Hampshire, United States (NH-1) or modified in Norway (NH-2) were compared with observed field ascospore release of V. pyrina from 21 site-year combinations. The models were also compared with ascospore release data from laboratory assays. In the laboratory assays, the forecasts of the NH-1 and NH-2 models provided the best fit to observed spore release. Under field conditions, the lag phases and slope coefficients of all models differed from those of observed release of ascospores. Identifying the precise time of bud break of pear to initiate degree-day accumulation was problematic at both Australian sites. This resulted in a higher deviance between bud break and first released ascospore compared with the sites in Norway and Belgium. Linear regressions of observed release against forecasted maturity generated similarly high concordance correlation coefficients. However, where differences were noted, they most often favored models that included adjustment for dry periods. The NH-2, IT, and NV models using pooled data also provided the most accurate estimates of 95% ascospore depletion, a key event in many disease management programs.

Growth and development of Erysiphe necator (syn. Uncinula necator) has been extensively studied under controlled conditions, primarily with a focus on development of grapevine powdery mildew within the optimal temperature range and the lethal effects of high temperatures. However, little is known of the effect of cold temperatures (above freezing but <8 degrees C) on pathogen development or host resistance. Pretreatment of susceptible Vitis vinifera leaf tissue by exposure to cold temperatures (2 to 8 degrees C for 2 to 8 h) reduced infection efficiency and colony expansion when tissues were subsequently inoculated. Furthermore, nascent colonies exposed to similar cold events exhibited hyphal mortality, reduced expansion, and increased latent periods. Historical weather data and an analysis of the radiational cooling of leaf tissues in the field indicated that early-season cold events capable of inducing the foregoing responses occur commonly and frequently across many if not most viticultural regions worldwide. These phenomena may partially explain (i) the unexpectedly slow development of powdery mildew during the first month after budbreak in some regions and (ii) the sudden increase in epidemic development once seasonal temperatures increase above the threshold for acute cold events.

When rose plants bearing colonies of Podosphaera pannosa were placed in a wind tunnel, the number of conidia trapped was directly proportional to intensity of daylight-balanced (white) light from 5 to 150 μmol m -2 s -1 . Illumination of samples using blue (420 to 520 nm) light-emitting diodes (LEDs) increased the number of conidia trapped by a factor of approximately 2.7 over white light but germination of conidia under blue light was reduced by approximately 16.5% compared with conidia germination under white light. The number of conidia trapped under far-red (>685 nm) LEDs was approximately 4.7 times higher than in white light, and 13.3 times higher than under red (575 to 675 nm) LEDs, and germination was not induced compared with white light. When mildewed plants were exposed to cycles of 18 h of white light followed by 6 h of blue, red, far-red light, or darkness, light from the red LEDs reduced the number of conidia trapped by approximately 88% compared with darkness or far-red light. Interrupting the above dark period with 1 h of light from red LEDs also reduced the number of conidia trapped, while a 1-h period of light from far-red following the 1 h of light from red LEDs nullified the suppressive effect of red light. Our results indicate that brief exposure to red light during the dark interval may be as effective as continuous illumination in suppressing powdery mildew in greenhouse rose plant (Rosa × hybrida).

A collection of four clonal isolates of Podosphaera aphanis was heterothallic and was composed of two mutually exclusive mating types. Cleistothecial initials approximately 20 to 30 microm in diameter were observed within 7 to 14 days after pairing of compatible isolates and developed into morphologically mature ascocarps within 4 weeks after initiation on both potted plants maintained in isolation and in field plantings in New York State and southern Norway. Ascospores progressed through a lengthy maturation process over winter, during which (i) the conspicuous epiplasm of the ascus was absorbed; (ii) the osmotic potential of the ascospore cytoplasm increased, resulting in bursting of prematurely freed spores in water; and, finally, (iii) resulting in the development of physiologically mature, germinable, and infectious ascospores. Release of overwintered ascospores from field collections was coincident with renewed plant growth in spring. Overwintered cleistothecia readily dehisced when wetted and released ascospores onto glass slides, detached strawberry leaves, and leaves of potted plants. Plant material exposed to discharged ascospores developed macroscopically visible mildew colonies within 7 to 10 days while noninoculated controls remained mildew free. Scanning electron and light microscopy revealed that cleistothecia of P. aphanis were enmeshed within a dense mat of hyphae on the persistent leaves of field-grown strawberry plants and were highly resistant to removal by rain while these leaves remained alive. In contrast, morphologically mature cleistothecia on leaves of nine deciduous perennial plant species were readily detached by simulated rain and seemed adapted for passive dispersal by rain to other substrates. Contrary to many previous reports, cleistothecia appear to be a functional source of primary inoculum for strawberry powdery mildew. Furthermore, they differ substantially from cleistothecia of powdery mildews of many deciduous perennial plants in their propensity to remain attached to the persistent leaves of their host during the intercrop period.

The effect of day length on production and germinability of conidia and severity of disease caused by Podosphaera pannosa, the causal agent of rose powdery mildew, was studied. Whole potted plants or detached leaves of Rosa interspecific hybrid 'Mistral' were inoculated with P. pannosa and exposed to 0, 12, 18, 20, 22, or 24 h of artificial light per day in growth chambers equipped with mercury lamps. Increasing duration of illumination from 18 to 20 to 24 h per day reduced production of conidia by 22 to 62%. Exposure to 24 h of illumination per day also strongly reduced disease severity compared with 18 h. Our results suggest that increasing day lengths from 18 h per day to 20 to 24 h may suppress the disease significantly and, thereby, reduce the need for fungicide applications against powdery mildew.

Berries of grapevine (Vitis spp.) have a limited window of susceptibility to Uncinula necator, the causal agent of grapevine powdery mildew, until the onset of ontogenic resistance approximately 2 to 3 weeks postbloom. This phenomenon has been demonstrated in several cultivars of V. vinifera and in V. labruscana 'Concord', which all exhibited a similar duration of susceptibility. To identify genetic variation for ontogenic resistance, we screened a diverse collection of Vitis species and interspecific hybrids maintained in the USDA-ARS cold-hardy Vitis germplasm collection in Geneva, NY. Of the 79 genotypes whose fruit clusters were screened for susceptibility to powdery mildew under field conditions, 50 exhibited a high level of constitutive resistance to powdery mildew and did not develop more than trace levels of disease when inoculated, irrespective of the stage of berry development at inoculation. Twenty-four genotypes exhibited a significant gain of resistance as berries aged. This ontogenic resistance was conserved across four species and several interspecific hybrids of Vitis spp., although the timing of the onset of ontogenic resistance varied by genotype. The mechanism of ontogenic resistance was examined for four genotypes. Similar to previous studies, ontogenic resistance greatly reduced the incidence of successful penetration. Despite the broad conservation of ontogenic resistance across species, one genotype (V. rupestris 'R-65-44') remained susceptible past the onset of ripening, over 1 month later than reported previously for V. vinifera and V. labruscana. Variation in the resistance phenotype was observed among the rachis, pedicels, and berries within clusters of the majority of genotypes studied. The genetic variation in ontogenic resistance, in particular the discovery of a genotype in which berries remain susceptible as they mature, will facilitate further study of the inheritance and molecular basis of ontogenic resistance.

Metalaxyl is translocated from roots to leaves to control a number of oomycete pathogens, but systemic movement from vegetative organs into fruit and vapor activity against Plasmopara viticola, the causal agent of grapevine downy mildew, has not been examined experimentally. We inoculated fruit clusters of grapevines with P. viticola at prebloom, bloom, or 1 week postbloom. We then selectively applied mefenoxam (288 mg/liter), the active enantiomer of metalaxyl, to the leaves or stem tissue 12 to 48 h after inoculation. Little to no downy mildew developed on fruit when mefenoxam was applied to leaf tissue, stem tissue, or both. In contrast, downy mildew symptoms were severe on inoculated clusters on untreated shoots. When potential vapor activity was blocked, we observed fungicidal activity on seedling foliage in response to apparent systemic movement from treated stems and soil, but not from leaves. However, when vapor activity was permitted, mefenoxam residues on treated leaves controlled disease on other, untreated leaves. In subsequent vineyard experiments, vapor and systemic activity provided equivalent and near-complete suppression of downy mildew on clusters 48 h post inoculation. Furthermore, inoculated grape seedlings that were placed near mefenoxam-treated seedlings in open and closed systems developed nil to trace levels of downy mildew compared with controls, further indicating that the material has strong vapor activity.

ABSTRACT Production of grape (principally cultivars of Vitis vinifera) for high-quality wines requires a high level of suppression of powdery mildew (Uncinula necator syn. Erysiphe necator). Severe infection of either fruit or foliage has well-documented and deleterious effects upon crop and wine quality. We found that berries nearly immune to infection by U. necator due to the development of ontogenic resistance may still support diffuse and inconspicuous mildew colonies when inoculated approximately 3 weeks post-bloom. Fruit with diffuse mildew colonies appear to be healthy and free of powdery mildew in late-season vineyard assessments with the naked eye. Nonetheless, presence of these colonies on berries was associated with (i) elevated populations of spoilage microorganisms; (ii) increased evolution of volatile ethyl acetate, acetic acid, and ethanol; (iii) increased infestation by insects known to be attracted to the aforementioned volatiles; (iv) increased rotting by Botrytis cinerea; and (v) increased frequency of perceived defects in wines prepared from fruit supporting diffuse powdery mildew colonies. Prevention of diffuse infection requires extending fungicidal protection until fruit are fully resistant to infection. Despite a perceived lack of improvement in disease control due to the insidious nature of diffuse powdery mildew, potential deleterious effects upon crop and wine quality thereby would be avoided.

ABSTRACT We evaluated direct and interactive effects of light quality and intensity, temperature and light, diurnal rhythms, and timing of high relative humidity during long day lengths on sporulation of Bremia lactucae, the causal agent of lettuce downy mildew, using inoculated lettuce seedlings and detached cotyledons. Suppression of sporulation by light was strongly dependent upon temperature and there was little suppression at

ABSTRACT Several aspects of grapevine downy mildew epidemiology that are fundamental to model predictions were investigated. Simple rainfall-, temperature-, and phenology-based thresholds (rain > 2.5 mm; temperature > 11 degrees C; and phenology > Eichorn and Lorenz [E&L] growth stage 12) were evaluated to forecast primary (oosporic) infection by Plasmopara viticola. The threshold was consistent across 15 years of historical data on the highly susceptible cv. Chancellor at one site, and successfully predicted the initial outbreak of downy mildew for 2 of 3 years at three additional sites. Field inoculations demonstrated that shoot tissue was susceptible to infection as early as E&L stage 5, suggesting that initial germination of oospores, rather than acquisition of host susceptibility, was probably the limiting factor in the initiation of disease outbreaks. We also found that oospores may continue to germinate and cause infections throughout the growing season, in contrast to the widely-held assumption that the supply of oospores is depleted shortly after bloom. Lesion productivity (sporangia/lesion) did not decline with age of a lesion in the absence of suitable weather to induce sporulation. However, the productivity of all lesions declined rapidly through repeated cycles of sporulation. Extremely high temperatures (i.e., one day reaching 42.8 degrees C) had an eradicative effect under vineyard conditions, and permanently reduced sporulation from existing (but not incubating) lesions to trace levels, despite a later return to weather conducive to sporulation. In fair weather, most sporangia died sometime during the daylight period immediately following their production. However, over 50% of sporangia still released zoospores after 12 to 24 h of exposure to overcast conditions.

ABSTRACT Clusters of Vitis vinifera and V. labrusca are reported to become resistant to Plasmopara viticola at stages of development ranging from 1 to 6 weeks postbloom. It has been suggested that resistance is associated with loss of the infection court as stomata are converted to lenticels, but the time of onset, cultivar variation, and seasonal variation in ontogenic resistance has remained uncertain, as has the comparative susceptibility of stem tissue within the fruit cluster. In New York, we inoculated clusters of V. vinifera cvs. Chardonnay and Riesling and V. labrusca cvs. Concord and Niagara at stages from prebloom until 5 to 6 weeks postbloom. Berries were infected and supported profuse sporulation until 2 weeks postbloom, and pedicel tissue remained susceptible until 4 weeks postbloom. Although berries on later-inoculated clusters failed to support sporulation, discoloration and necrosis of berry tissues was often noted, and necrosis of the pedicel within such clusters often led to further discoloration, shriveling, reduced size, or loss of berries. When the epidermis of discolored berries that initially failed to support sporulation was cut, the pathogen emerged and sporulated through incisions, indicating that lack of sporulation on older symptomatic berries was due to infection at an early stage of berry development followed by conversion of functional stomata to lenticels during latency. We repeated the study on Chardonnay and Riesling vines in South Australia and found that the period of berry and rachis susceptibility was greatly increased. The protracted susceptibility of the host was related to the increased duration and phenological heterogeneity of bloom and berry development in the warmer climate of South Australia. The time of onset and subsequent expression of ontogenic resistance to P. viticola may thus be modified by climate and should be weighed in transposing results from one climatic area to another. Our results can be used to refine forecast models for grapevine downy mildew to account for changes in berry and rachis susceptibility, and to focus fungicide application schedules upon the most critical periods for protection of fruit.

Orthotydeus lambi reduced the severity of grape powdery mildew (Uncinula necator) on fruit and foliage of Vitis vinifera 'Chardonnay' and 'Riesling' in repeated field and laboratory trials. Vines were infested with O. lambi at two densities (5 or 30 mites per leaf) at each of two times (2 to 3 weeks prebloom and 1 week postbloom). Overall, powdery mildew on the berries and foliage was suppressed by early (prebloom) mite releases at both densities, but only by the higher density in late (postbloom) releases. In a separate trial, when foliage was infested at 30 mites per leaf but mites were excluded from certain fruit clusters, severity of powdery mildew was significantly reduced on the mite-free clusters of mite-infested shoots. Thus, O. lambi may suppress powdery mildew on the fruit by reducing inoculum from foliar infections. In laboratory studies, both immature and mature mites reduced infection efficiency, colony expansion, and sporulation of the mildew colonies; but immature mites were more voracious feeders, consuming more pathogen biomass per unit of mite biomass. Mites tore at the mycelium and conidia with their palps during feeding, leading to leakage, rapid loss of hyphal turgor, and collapse of hyphae.

Estimates of ascospore maturity generated by a model developed previously in New Hampshire, United States, were compared with the cumulative release of ascospores in southern Norway as monitored by volumetric spore traps at one site for 12 years, and at two additional sites for 2 years. In locations and years with frequent rain events, model-estimated ascospore maturity closely approximated observed ascospore release. However, in years with protracted dry periods of 1 to 3 weeks with no or little rain, not only was spore release delayed, but release continued to lag behind predicted maturity even after several rain events subsequent to the dry interval. By halting degree-day (base = 0°C) accumulation if 7 consecutive days without rain occurred, accuracy of the model during "dry" years was greatly improved, without substantially affecting accuracy in "wet" seasons. With minimal additional effort on the part of the user, this simple modification increases the accuracy of model-derived estimates of ascospore maturity when lack of rain slows ascospore maturation.

Maturation and release of ascospores of Venturia inaequalis were assessed at Geneva and Highland, NY, and at Durham, NH, by microscopic examination of crushed pseudothecia excised from infected apple leaves that were collected weekly from orchards (squash mounts) in 14 siteyear combinations. Airborne ascospore dose was monitored at each location in each year of the study by volumetric spore traps. Additional laboratory assessments were made at Geneva to quantify release from infected leaf segments upon wetting (discharge tests). Finally, ascospore maturity was estimated for each location using a degree-day model developed in an earlier study. Ascospore maturation and release determined by squash mounts and discharge tests lagged significantly behind cumulative ascospore release as measured by volumetric spore traps in the field. The mean date of 98% ascospore discharge as determined by squash mounts or discharge tests occurred from 23 to 28 days after the mean date on which 98% cumulative ascospore release had been detected by volumetric traps. In contrast, cumulative ascospore maturity estimated by the degree-day model was highly correlated (r 2 = 0.82) with observed cumulative ascospore release as monitored by the volumetric traps. Although large differences between predicted maturity and observed discharge were common during the exponential phase of ascospore development, the date of 98% cumulative ascospore maturity predicted by the model was generally within 1 to 9 calendar days of the date of 98% cumulative ascospore recovery in the volumetric traps. Cumulative ascospore discharge as monitored by the volumetric traps always exceeded 98% at 600 degree days (base = 0°C) after green tip. Estimating the relative quantity of primary inoculum indirectly by means of a degree-day model was more closely aligned with observed ascospore release, as measured by volumetric traps, than actual assessments of ascospore maturity and discharge obtained through squash mounts and discharge tests. The degree-day model, therefore, may be a more accurate predictor of ascospore depletion than squash mounts or discharged tests, and has the added advantage that it can be widely applied to generate site-specific estimates of ascospore maturity for any location where daily temperature data are available.

ABSTRACT The epidemiology and control of black rot (Guignardia bidwellii) was studied from 1995 to 1999 in vineyards in Dresden and Naples, NY, where disease pressure was moderate and extreme, respectively. The efficacy of serial applications of myclobutanil, provided at 2-week intervals and varied with respect to their number and time of initiation, was examined within the context of host phenology, inoculum availability, and sanitation. At Dresden, sprays applied over 4 weeks through the immediate prebloom stage provided only 13 to 91% control of diseased clusters, despite the release of 95% of the season's ascosporic inoculum during the period of fungicidal protection. However, applications immediately prior to bloom plus 2 and 4 weeks later, which afforded protection while fruit are highly susceptible to infection, provided virtually complete control. At Naples, where mummified berries were retained in the canopy after mechanical pruning, this same regime provided only approximately 80% disease control, but applying a fourth spray 2 weeks prebloom generally improved control. Hand-pruning mummies to the ground in selected plots significantly (P

ABSTRACT Grape berries are highly susceptible to powdery mildew 1 week after bloom but acquire ontogenic resistance 2 to 3 weeks later. We recently demonstrated that germinating conidia of the grape powdery mildew pathogen (Uncinula necator) cease development before penetration of the cuticle on older resistant berries. The mechanism that halts U. necator at that particular stage was not known. Several previous studies investigated potential host barriers or cell responses to powdery mildew in berries and leaves, but none included observation of the direct effect of these factors on pathogen development. We found that cuticle thickness increased with berry age, but that ingress by the pathogen halted before formation of a visible penetration pore. Cell wall thickness remained unchanged over the first 4 weeks after bloom, the time during which berries progressed from highly susceptible to nearly immune. Autofluorescent polyphenolic compounds accumulated at a higher frequency beneath appressoria on highly susceptible berries than on highly resistant berries; and oxidation of the above phenolics, indicated by cell discoloration, developed at a significantly higher frequency on susceptible berries. Beneath the first-formed appressoria of all germinated conidia, papillae occurred at a significantly higher frequency on 2- to 5-day-old berries than on 30- to 31-day-old fruit. The relatively few papillae observed on older berries were, in most cases (82.8 to 97.3%), found beneath appressoria of conidia that had failed to produce secondary hyphae. This contrasted with the more abundantly produced papillae on younger berries, where only 35.4 to 41.0% were located beneath appressoria of conidia that had failed to produce secondary hyphae. A pathogenesis-related gene (VvPR-1) was much more highly induced in susceptible berries than in resistant berries after inoculation with U. necator. In contrast, a germin-like protein (VvGLP3) was expressed within 16 h of inoculation in resistant, but not in susceptible berries. Our results suggest that several putative barriers to infection, i.e., cuticle and cell wall thickness, antimicrobial phenolics, and two previously described pathogenesis-related proteins, are not principal causes in halting pathogen ingress on ontogenically resistant berries, but rather that infection is halted by one or more of the following: (i) a preformed physical or biochemical barrier near the cuticle surface, or (ii) the rapid synthesis of an antifungal compound in older berries during the first few hours of the infection process.

ABSTRACT Berries of Vitis vinifera are reported to be susceptible to infection by Uncinula necator until soluble solids levels (brix) reach 8%, and established colonies are reported to sporulate until brix reach 15%. However, our analysis of disease progress on fruit of selected V. vinifera cultivars indicated that severity became asymptotic several weeks earlier in fruit development. When mildew-free fruit clusters of V. vinifera 'Chardonnay', 'Riesling', 'Gewürztraminer', and 'Pinot Noir' were inoculated at stages ranging from prebloom to 6 weeks postbloom, only fruit inoculated within 2 weeks of bloom developed severe powdery mildew. Substantial ontogenic resistance to infection was expressed in fruit nearly 6 weeks before fruit brix reached 8% and over 2 months before they reached 15%. Rachises of 'Chardonnay' and 'Riesling' fruit clusters developed severe powdery mildew when inoculated at bloom, and disease increased steadily over the next 60 days. The rachis of fruit clusters inoculated 31 days after bloom developed only trace levels of powdery mildew. Berry weight of all four cultivars at harvest was reduced when fruit clusters were inoculated at bloom or 16 days postbloom, primarily by splitting, rotting, and dehydration of mildewed berries, but the weight of later-inoculated berries was not reduced. Inoculation of berries just as ontogenic resistance increased markedly, approximately 3 to 4 weeks postbloom, resulted in the development of inconspicuous, diffuse, non-sporulating mildew colonies on berries, sometimes associated with a network of necrotic epidermal cells. Rather than a protracted and relatively static period of berry susceptibility lasting 3 months, fruit of V. vinifera appear to acquire ontogenic resistance rapidly after fruit set. A refocusing of disease management on this critical period of high fruit susceptibility should greatly improve the efficacy of fungicides directed against powdery mildew.

ABSTRACT Grape berries become resistant to powdery mildew early in development and are nearly immune to infection within 4 weeks after bloom. In this study, ontogenic resistance did not reduce attachment, germination, or appressorium formation of Uncinula necator on 3- to 4-week-old berries of Vitis vinifera 'Chardonnay' or 3-week-old berries of V. labruscana 'Concord'. Pathogen ingress halted at the cuticle before formation of a penetration pore. As berries aged, hyphal elongation and colony growth slowed until finally no secondary hyphae formed on fully resistant berries. More appressoria formed per unit of hyphal length as berries aged, indicating that failure to penetrate older berries led to increased attempts to penetrate resistant fruit. Additionally, hyphae within the colonies began to die as berries aged. Finally, the number of degree-hours between germination and sporulation of the colony (latent period) increased and sporophore density decreased with berry age at time of inoculation. Thus, ontogenic resistance both slows, and eventually halts disease development on grape berries, and limits the likelihood of spread by reducing absolute supply of conidia and delaying their formation. It furthermore has a consistent, stable, and predictable impact on grape powdery mildew and operates in a similar fashion and to a similar degree in both V. labruscana and V. vinifera, although at a slightly earlier phenological stage in V. labruscana.

ABSTRACT The period of fruit susceptibility to Guignardia bidwellii (anamorph Phyllosticta ampelicida), the causal agent of grape black rot, was determined in the field. Intact fruit were inoculated weekly from bloom until 8 weeks later with a suspension containing 2 x 10(5) conidia per ml. Disease progress was monitored approximately every 2 days until 3 to 5 weeks after inoculation, depending on the year and variety. Fruit of Vitis x labruscana 'Concord' exhibited a period of maximum susceptibility from midbloom until 2 to 4 weeks later, although some berries became symptomatic when inoculated 4 to 5 weeks postbloom. Fruit of V. vinifera 'Chardonnay' and 'Riesling' exhibited a period of maximum susceptibility from midbloom until 3 to 5 weeks later, although some berries retained their susceptibility until 6 to 7 weeks postbloom. These susceptible periods were approximately 2 to 4 weeks shorter than previously assumed. Fruit age at the time of inoculation affected the length of the incubation period (time from inoculation until symptom appearance). When the incubation period was defined in terms of degree hours (base = 0 degrees C) accumulated after inoculation, DH(50) values (the number of degree hours required to reach 50% of final disease severity) increased by at least 50% as berries neared the end of their susceptible period. Newly symptomatic berries continued to appear for over 1 month after inoculation of older fruit. Thus, age-related or ontogenic, host resistance was manifested as both a decline in susceptibility and a significant increase in incubation period length. The control of black rot is likely to be improved by tailoring the intensity of fungicidal protection to the abbreviated period of fruit susceptibility defined in this study. Furthermore, the efficacy of management programs and the results of epidemiological studies are likely to be misinterpreted unless the variable effect of fruit age on incubation period length is recognized.

ABSTRACT A fundamental principle of integrated pest management is that actions taken to manage disease should be commensurate with the risk of infection and loss. One of the less-studied factors that determines this risk is ontogenic, or age-related resistance of the host. Ontogenic resistance may operate at the whole plant level or in specific organs or tissues. Until recently, grape berries were thought to remain susceptible to powdery mildew (Uncinula necator) until late in their development. However, the development of ontogenic resistance is actually quite rapid in berries, and fruit become nearly immune to infection within 4 weeks after fruit set. Our objective was to determine how and at what stage the pathogen was halted in the infection process on ontogenically resistant berries. Adhesion of conidia, germination, and appressorium formation were not impeded on older berries. However, once berries were approximately 3 weeks old and older, few germlings were able to form secondary hyphae. Ontogenically resistant berries responded rapidly to infection by synthesis of a germin-like protein that had been previously shown to play a role in host defense against barley powdery mildew. On susceptible berries, cell discoloration around penetration sites indicated the oxidation of phenolic compounds; a process that was followed by localized cell death. However, the pathogen was still able to infect such cells prior to their death, continue secondary growth, and thereby colonize young berries. Formation of papillae was not involved in the differential resistance mechanism of older berries. In susceptible berries, papillae formed frequently at infection sites but did not always contain the pathogen, whereas in resistant berries, the pathogen was always halted prior to the formation of papillae. The host defense, which conditions ontogenic resistance, operates in the earliest stages of the infection process, in the absence of gross anatomical barriers, prior to the formation of a functional haustorium and prior to the development of a conspicuous penetration pore. We also found that diffuse powdery mildew colonies that were not visible in the field predisposed berries to bunch rot by Botrytis cinerea, increased the levels of infestation by spoilage microorganisms, and substantially degraded wine quality. Our improved understanding of the nature, causes, and stability of ontogenic resistance in the grapevine/ powdery mildew system has supported substantial changes in how fungicides are used to control the disease. Present applications are more focused on the period of maximum fruit susceptibility instead of following a calendar-based schedule. This has improved control, reduced losses, and in many cases reduced the number of fungicide applications required to suppress the disease. Particularly where fungicides are deployed in a programmatic fashion and ontogenic resistance is dynamic, there may be equivalent improvements to be made in other hostpathogen systems through studies of how host susceptibility changes through time.

ABSTRACT Vitis labruscana 'Concord' is a grape cultivar widely grown in the United States for processing into juice and other grape products. Concord grapes are sporadically but sometimes severely damaged by the grape powdery mildew pathogen, Uncinula necator. Although the foliage is often reported to be moderately resistant to powdery mildew, severe fruit infection occurs in some years. We observed the seasonal development of powdery mildew on leaves, rachises, and berries of unsprayed Concord grapevines. Inoculations of flower and fruit clusters revealed a brief period of berry susceptibility and a protracted period of rachis susceptibility. The rachis remained highly susceptible to infection, and the severity of rachis infection increased throughout the growing season until the rachis formed a periderm shortly before harvest. In contrast, berries were nearly immune to infection within 2 weeks after fruit set. Rachis and berry infections were detected before the disease was observed on foliage, and the incidence of rachis and berry infection often exceeded disease incidence observed on foliage until after fruit acquired substantial ontogenic resistance. Excellent control of fruit infection, and adequate control of leaf infection, was achieved by two fungicide applications targeted at the peak period of fruit susceptibility. Although Concord is thought to be moderately resistant to powdery mildew, the rachis is highly susceptible, and may be the avenue by which prebloom infections make their way onto the developing fruit. Late-season infection of the rachis neither spread to the fruit, nor did it cause fruit to drop prematurely, and may be of little economic consequence on fruit destined for processing. Although fruit of V. vinifera cultivars have been reported to remain susceptible to infection until berry sugar levels reach 8 to 15%, Concord fruit become nearly immune to infection nearly 6 weeks before this stage of development. Because powdery mildew does not become conspicuous on foliage until late summer, it is generally regarded as a late-season problem on Concord grapes, and previous management programs have reflected this belief. However, the greatest contribution to control of fruit infection is due to fungicides applied during the peak period of fruit susceptibility, from bloom until shortly after fruit set, long before the disease is observed on foliage.