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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/m² 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/m² 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The formation of chasmothecia by the strawberry powdery mildew pathogen (Podosphuera aphanis) is widespread but often sporadic throughout the range of strawberry cultivation. In some production regions, notably in wanner climates, chasmothecia are reportedly rare. We confirmed that the pathogen is heterothallic, and that initiation of chas-molhecia 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 pyro-sequencing 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. [ABSTRACT FROM AUTHOR]

Growth and development of Erysiphe necator (syn. Uncinula necator) has been extensively studied under controlled conditions, primarily with a locus 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°C) on pathogen development or host resistance. Pretreatment of susceptible Vitis vinifera leaf tissue by exposure to cold temperatur (2 to ≤8°C for 2 to 8 h) reduced infection efficiency and colony expansion when tissues were subsequently inoculated. Furthennore, 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. [ABSTRACT FROM AUTHOR]

A collection of four clonal isolates of Podosphaera aphanis was heterothallic and was composed of two mutually exclusive mating types. Cleistothecial initials ≈20 to 30 µm 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. [ABSTRACT FROM AUTHOR]

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 ≈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 FROM AUTHOR]

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 ≦10°C. The most suppressive waveband was in the range from 400 to 450 nm, although a lesser effect of wavebands from 450 to 500 and 500 to 550 nm could be detected. At 15°C, near the lower threshold for suppression of [ABSTRACT FROM AUTHOR]

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°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°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 FROM AUTHOR]

Clusters of Vitis vinifera and V. labrusca are reported to become resistant to Plasmopara viticola at stages of development ranging from l 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. [ABSTRACT FROM AUTHOR]

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. Auto fluorescent 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 paplillae 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The genomics era brought unprecedented opportunities for genetic analysis of host resistance, but it came with the challenge that accurate and reproducible phenotypes are needed so that genomic results appropriately reflect biology. Phenotyping host resistance by natural infection in the field can produce variable results due to the uncontrolled environment, uneven distribution and genetics of the pathogen, and developmentally regulated resistance among other factors. To address these challenges, we developed highly controlled, standardized methodologies for phenotyping powdery mildew resistance in the context of a phenotyping center, receiving samples of up to 140 grapevine progeny per F1 family. We applied these methodologies to F1 families segregating for REN1- or REN2-mediated resistance and validated that some but not all bioassays identified the REN1 or REN2 locus. A point-intercept method (hyphal transects) to quantify colony density objectively at 8 or 9 days postinoculation proved to be the phenotypic response most reproducibly predicted by these resistance loci. Quantitative trait locus (QTL) mapping with genotyping-by-sequencing maps defined the REN1 and REN2 loci at relatively high resolution. In the reference PN40024 genome under each QTL, nucleotide-binding site−leucine-rich repeat candidate resistance genes were identified—one gene for REN1 and two genes for REN2. The methods described here for centralized resistance phenotyping and high-resolution genetic mapping can inform strategies for breeding resistance to powdery mildews and other pathogens on diverse, highly heterozygous hosts. [ABSTRACT FROM AUTHOR]

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 &2le; 0.05) improved control in some spray regimes. Although this sanitation practice did not affect inoculum dynamics through bloom, very few spores were recovered thereafter from mummies collected from the ground, whereas abundant ascospores and conidia were recovered from mummies in the trellis for an additional 6 to 8 weeks. [ABSTRACT FROM AUTHOR]

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 weatherdriven models describing maturation and release of P. macularis ascospores, models that can eventually be incorporated into hop disease management programs. [ABSTRACT FROM AUTHOR]

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 macula, is) 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 highand 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 54 amplicon sequencing markers, validated across a panel of 391 U.S. samples and 123 international samples. The results suggest that niacu[£iris from U.S. commercial hop yards form one population closely related to P macularis of the United Kingdom, while P m Licul(iris from U.S. feral hop locations grouped with P macul(iris of Eastern Europe. Included in this multiplex was a marker that successfully tracked V6-vii·ulence in 65 of 66 samples with a confirmed V6-phenotype. A new qPCR assay for high-throughput genotyping of P macuknis mating type generated the highest resolution distribution map of' P mcicularis mating type to date. Together, these genotyping strategies enable the highand 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. [ABSTRACT FROM AUTHOR]

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 RUN], 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 RUN] 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.I 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, RUN!-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. [ABSTRACT FROM AUTHOR]

The putative center of origin of Plasmopara viticola, the causal agent of grape downy mildew, is eastern North America, where it has been described on several members of the family Vitaceae (e.g., Vitis spp., Parthenocissus spp., and Ampélopsis spp.). We have completed the first large-scale sampling of P. viticola isolates across a range of wild and cultivated host species distributed throughout the above region. Sequencing results of four partial genes indicated the presence of a new P. viticola species on Vitis vulpina in Virginia, adding to the four cryptic species of P. viticola recently recorded. The phylogenetic analysis also indicated that the P. viticola species found on Parthenocissus quinquefolia in North America is identical to Plasmopara muralis in Europe. The geographicdistribution and host range of five pathogen species was determined through analysis of the internal transcribed spacer polymorphism of 896 isolates of P. viticola. Among three P. viticola species found on cultivated grape, one was restricted to Vitis interspecific hybrids within the northern part of eastern North America. A second species was recovered from V. vinifera and V. labrusca. and was distributed across most of the sampled region. A third species, although less abundant, was distributed across a larger geographical range, including the southern part of eastern North America. P. viticola clade aestivalis predominated (83% of isolates) in vineyards of the European winegrape V. vinifera within the sampled area, indicating that a single pathogen species may represent the primary threat to the European host species within eastern North America. [ABSTRACT FROM AUTHOR]

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 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 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.