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A powdery mildew disease of variegated ivy (Hedera canariensis L. var. azorica) was observed on the Tyrrhenian coast in Tuscany (Italy) in spring 1998. Symptoms began as small, nearly circular reddish spots that later enlarged and coalesced. The hyaline mycelium produced abundant, ellipsoid conidia in long chains that ranged from 20 to 40 μm in length and from 12 to 25 μm in width. Cleistothecia were globose (100 to 120 μm diameter), dark brown (when mature) with a basal ring of mycelioid appendages, and contained several (up to 20) ovate asci, each generally containing two ascospores. Ascospores were hyaline, one-celled, ellipsoid (20 to 35 μm in length and 10 to 20 μm in width). The morphological characteristics of this fungus were those given for Erysiphe cichoracearum DC. Infection also was found on English ivy (Hedera helix L.). It is reported that this species is, occasionally, subject to powdery mildew caused by E. cichoracearum (1). Conidia from infected leaves were shaken onto leaves of melon (Cucumis melo L.), cucumber (Cucumis sativus L.), watermelon (Citrullus lanatus [Thunb.] Matsum. & Nakai), lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), tobacco (Nicotiana tabacum L.) and variegated and English ivy. After 7 days, the disease was observed on cucumber, melon, watermelon, tobacco, and variegated ivy. Examination confirmed that test plants were infected with E. cichoracearum. This is the first report of E. cichoracearum on variegated ivy in Italy. Reference: (1) P. P. Pirone. 1970. Diseases and Pests of Ornamental Plants. The Ronald Press, New York.

In 1995, powdery mildew was observed on commercial greenhousegrown Aster ericoides L. from La Plata, Buenos Aires Province. The disease affected about 95% of the growers. Mildew first appeared as white circular patches on the adaxial surface of leaves. As disease progressed, the abaxial surface of leaves, petioles, stems, and calyces were covered by cottony masses of mycelium and conidia, and basal leaves later wilted and died. Young plants (4 to 5 leaves) through flowering plants were affected. Conidia were ovoid-cylindrical, often slightly constricted at the ends, and were produced in chains on unbranched conidiophores. Conidia lacked fibrosin bodies and ranged from 30 to 41 μm × 10 to 19 μm. Long unbranched germ tubes were formed from the ends of conidia. The morphological characteristics of the fungus fit those described for Erysiphe cichoracearum DC (1). In addition, the perfect stage was found on older tissues. Subglobose, dark brown cleistothecia (105 to 210 μm in diameter) with a basal ring of myceloid appendages were observed. Cleistothecia contained multiple ellipsoid asci (48 to 69 μm × 30 to 37 μm) with two hyaline, one-celled, ellipsoid ascospores (22 to 26 μm × 11 to 15 μm). Pathogenicity was tested by pressing diseased leaves onto healthy leaves of aster cv. Sunset and incubating plants in humidity chambers for 48 h. The powdery mildew that developed was morphologically identical to the original isolate. This is the first report of E. cichoracearum on heath aster in Argentina. Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.

Creeping or Canada thistle (Cirsium arvense (L.) Scop.) is a perennial weed of Eurasian origin that arrived in North America as early as the 1700s (3). Spreading by seeds and rhizomes, it is now widely distributed in Canada, Alaska, and 40 other states. It is apparently absent from Texas, Oklahoma, Louisiana, Mississippi, Alabama, Georgia, Florida, and South Carolina (1). Powdery mildew is common on C. arvense in Europe, but it has never been observed in North America (4). In Europe and Asia, powdery mildew of C. arvense is caused by any one of the following fungi: Leveillula taurica, two species of Sphaerotheca, and varieties of Erysiphe cichoracearum and E. mayorii. Specimens of C. arvense infected with powdery mildew (deposited in the U.S. National Fungus Collections as BPI 843471) were collected in the fall of 2003 near Moscow, ID and in two areas in Oregon (the canyon of the Grande Ronde River and near the base of the Wallowa Mountains). Mycelium and cleistothecia were observed on stems and upper and lower surfaces of leaves. The mean diameter of the cleistothecia was 122 (±11.6) μm. Basally inserted, mycelioid appendages were hyaline or brown and varied considerably in length, but most were in the range of 80 to 120 μm. Asci averaged 58 (±5.5) μm × 35 (±4.1) μm in length and width, respectively. Each ascus bore two ascospores averaging 23 (±1.4) μm × 14 (±1.7) μm. Conidia averaged 30 (±3.0) μm × 14 (±0.8) μm. The specimens fit the description of E. cichoracearum DC. (2). Because the length/breadth ratio of conidia is greater than 2, the specimens could be further diagnosed as E. cichoracearum var. cichoracearum (2). Also noteworthy was the presence of the hyperparasitic Ampelomyces quisqualis Ces. ex Schlechtend. E. cichoracearum is thought to be a cosmopolitan powdery mildew of broad host range, but this concept is difficult to reconcile with the absence of mildew on North American populations of C. arvense for more than 200 years. References: (1) Anonymous. USDA Natural Resources Conservation Service Plants Profile for Cirsium arvense. On-line publication, 2003. (2) U. Braun. A monograph of the Erysiphales (powdery mildews), J. Cramer, Berlin-Stuttgart, 1987. (3) G. Cox. Alien Species in North America and Hawaii, Island Press, Washington, D.C., 1999. (4) D. F. Farr et al. Fungal Databases, Systematic Botany and Mycology Laboratory, ARS, USDA. On-line publication, 2003.

In October 2003, potato plants in three fields (cv. Desiree, Satina, Midas, and Mondial) in Lancaster, California exhibited symptoms and signs of powdery mildew. Disease symptoms were most severe on cvs. Desiree and Santina. Disease expression was greater along sprinkler lines and in localized areas from which the disease spread to surrounding plants. Severely affected plants began collapsing just prior to water cutoff. Early symptoms comprise small dark areas on the adaxial surface of leaves, along the veins, and at the petioles. Dark lesions consisting of mycelia and conidiophores were also visible on the main stems of affected plants. As the disease progressed, leaves were covered by a gray powdery fungal mass, and older leaves became necrotic. Conidial chains arising from the hyaline, epiphytic mycelia consisted of two to eight conidia. The cylindric to doliform conidia measured 16.8 to 22.8 μm wide (mean = 19.2, standard error = 0.36, N = 30) × 28.8 to 45.6 μm long (mean = 32.4, standard error = 0.75, N = 30). No cleistothecia were observed. Identification of the causal agent as Golovinomyces cichoracearum (synonyms G. orontii and Erysiphe cichoracearum) based on morphology was confirmed by internal transcribed spacer (ITS)-polymerase chain reaction (PCR). Conidia were washed off the affected leaves, concentrated by filtration and centrifugation, and sonicated to release genomic DNA. PCR was performed on the sonicated conidia with primers ITS4 and ITS5 (2), and the resulting amplicon was purified and sequenced. BLAST analysis of the ITS sequence revealed a 99% homology to E. cichoracearum from an Ambrosia sp. (GenBank Accession No. AF011292). Pathogenicity was confirmed on potato seedlings cv. Red La Soda. Inoculations were performed twice on six plants (three pots) each time. A sterile brush was used to transfer conidia from the affected leaves to seedlings consisting of two to three fully expanded leaves. A plastic bag was placed around each pot containing two seedlings for 1 to 2 days and then removed. Noninoculated controls were stroked with a sterile brush, placed in a plastic bag for 1 to 2 days, and kept in the greenhouse on a separate bench. Two control plants were included for each inoculation. Plants were maintained in a greenhouse at approximately 25 to 28°C and 40 to 60% relative humidity. After 7 days, dark spots were visible on the leaves of all inoculated plants, and conidiophores with conidia identical to those of the isolate used as the inoculum source were apparent after 10 days. The controls showed no disease symptoms or signs. To our knowledge, this is the first report of powdery mildew caused by G. cichoracearum on potato in California. The first field report of the disease was from Washington in 1950 (1), with subsequent reports from Utah and Ohio. References: (1) J. D. Menzies. Plant Dis. Rep. 34:140, 1950. (2) T. J. White et al. PCR Protocols. Academic Press, New York, 1990.

Common sage, Salvia officinalis L., is produced primarily in greenhouses for the culinary herb market in Argentina. Since 2003 during autumn and winter, powdery mildew symptoms have been repeatedly observed on potted common sage plants in commercial greenhouses located on the outskirts of Buenos Aires. The average disease incidence during this period was 85 to 90%. Circular, white, powdery patches developed on leaf surfaces and stems. Heavily infected leaves turned brown and died. Hyaline mycelium and nonlobed appressoria were observed. Conidiophores were simple with straight foot cells measuring 53.0 to 80.0 × 10.0 to 12.3 μm. Conidia were aseptate, hyaline, cylindrical to ovoid, measured 33.0 to 40.5 × 15.0 to 18.5 μm, did not contain fibrosine bodies, and were produced in chains. Cleistothecia were not observed. The pathogen was identified as Erysiphe cichoracearum DC (1). Pathogenicity was confirmed by gently pressing leaves displaying abundant sporulation onto the adaxial surface of healthy leaves. After 10 to 12 days, typical signs and symptoms of powdery mildew appeared on all inoculated plants. Pathogenicity tests were conducted in a greenhouse at 20 to 23°C and included 10 sage plants (five inoculated and five noninoculated). The experiment was performed twice, each time with the same result. Control plants did not show any signs or symptoms. E. cichoracearum DC was previously reported in the United States on Salvia sp. (2).To our knowledge, this is the first report of an outbreak of powdery mildew caused by E. cichoracearun on potted common sage plants produced in greenhouses in Argentina. References: (1) H. J. Boesewinkel. Rev. Mycol. Tome 41:493, 1977. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.

During July 2011, a severe powdery mildew outbreak was recorded in pumpkin (Cucurbita moschata Duch.) fields in economically poor areas in the Neelum Valley (Leswa, Mir Pura, Jura, Kundal Shahi, and Bela). Disease symptoms included grayish white circular to irregular patches consisting of epiphytic mycelia and conidia on both surfaces of the leaves of infected vines. Fungal mycelia and conidiophores were also visible in white powdery patches on the main stems, leaves, and petioles of affected plants. Leaves became necrotic with age. Powdery mildew symptoms were more severe on pumpkin vines located in the shade, and severely affected vines collapsed. A voucher specimen (IR00027) has been deposited in First Fungal Culture Bank of the Pakistan (FCBP), Institute of Agriculture Sciences, University of the Punjab, Lahore, Pakistan. For morphological observation, a clear tape strip was used to remove fungal tissue from infected leaves. The tape was mounted on microscope slides with water and examined with a light microscope (2). Conidiophores were unbranched, cylindrical, erect and arose singly on hyphal cells, 110 to 200 × 6 to 12 μm, composed of a foot cell and three to eight barrel-shaped conidia formed in chains with a sinuate edge, followed with a basal septum at the branching point or slightly displaced from the mycelium. Foot cells were 50 to 80 × 6 to 12 μm, and slightly swollen with constriction at the base. Conidia were cylindrical to doliiform, 30 to 50 × 14 to 20 μm and produced in chains. The length/breadth ratio of the conidia was 1.8 to 2.6. Fibrosin bodies were absent in both conidiophores and conidia. No cleistothecia were observed. Identification of the causal agent as Golovinomyces cichoracearum (DC.) V.P. Heluta (synonym Erysiphe cichoracearum) was based on morphology (1). Pathogenicity was determined using field-infected plant leaves transported to a greenhouse. Fresh conidia of field isolates of G. cichoracearum were transferred by a sterile brush from the affected leaves to fully expanded four to five healthy leaves of three 40-day-old vines. For the control, three vines were stroked with a clean sterile brush and control plants were placed 100 m away from the inoculated plants. For disease development 20°C and 80% relative humidity was maintained in the greenhouse. A plastic sheet was placed around each vine for a week and removed later. Inoculated vines developed visible white spots of powdery mildew on the leaves after 10 days in the greenhouse, whereas control plants remained asymptomatic. Fungal colony and conidial morphology on the leaves of inoculated plants were as described above. Previously, G. cichoracearum has been reported on other cucurbits (3) in the plains of Pakistan. To our knowledge, this is the first report of occurrence of G. cichoracearum as a causal agent of powdery mildew on pumpkin in Neelum Valley, Azad Kashmir. References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) J. C. Correll et al. Plant Dis. 71:248, 1987. (3) A. Wahid et al. Pak. J. Agric. Res. 9:209, 1988.

In this study, we cloned a new chitinase gene, EuCHIT2, from Eucommia ulmoides Oliver (E. ulmoides) using rapid amplification of cDNA ends (RACE) technology and constructed an overexpression vector, pSH-35S-EuCHIT2, to introduce it into tobacco (Nicotiana tabacum cv. Xanthi). Resistance to Erysiphe cichoracearum de Candolle (E. cichoracearum DC.) and molecular mechanisms in the transgenic tobacco were determined by drop inoculation, spore counting, determination of physicochemical indicators, and analysis of gene expression. The chitinase activity and resistance to E. cichoracearum DC. were significantly higher in the transgenic tobacco than in wild-type tobacco (p < 0.05). The activities of peroxidase (POD) and catalase (CAT), after inoculation with E. cichoracearum DC., were higher in the transgenic tobacco than in the wild-type. Conversely, the malondialdehyde (MDA) content was significantly lower in the transgenic tobacco than the wild-type before and after inoculation. In addition, our study also indicated that the resistance to E. cichoracearum DC. might involve the salicylic acid (SA) and jasmonic acid (JA) pathways, because the expression levels of pathogenesis-related gene 1 (PR-1a) and coronatine-insensitive 1 (COI1) were significantly increased and decreased, respectively, after inoculation with E. cichoracearum DC. The present study supports the notion that PR-1a and POD participate in resistance to E. cichoracearum DC. in the transgenic tobacco plants.

The Robinia pseudoacacia plant extract has been reported to have protective effect against cucumber powdery mildew (Sphaerotheca fuliginea). In this study, the active compound of R. pseudoacacia was isolated from the plant extract, and the structure was identified to be d -Pinitol by nuclear magnetic resonance. d -Pinitol, as a known compound, has important physiological activity on plant osmotic tolerance and medicinal activities on human health. This study is the first to report the antifungal activity of d -pinitol on an obligate parasite S. fuliginea and tried to develop a phytochemical fungicide. The evaluation in the growth chamber showed that the control effect of d -pinitol and its formulation against cucumber powdery mildew disease was 80.7% and 94.4%, at a concentration of 0.5 mg/ml. This formulation was further tested in a greenhouse to evaluate its control effect against cucumber (S. fuliginea) and tobacco (Erysiphe cichoracearum) powdery mildew under natural condition. The efficacy of d -pinitol in both trials was better than that of positive control at the recommended concentration. The content of d -pinitol in this plant was 25 mg/g (w/w, dry leaves weight) as determined by gas chromatography. The abundant biomass of d -pinitol in R. pseudoacacia plant extract makes it more economical in practical use. Our study provides a base for the future exploitation of d -pinitol as a natural phytochemical fungicide.

In the years 1974-79 in Skierniewice and in 12 other places Erysiphe cichoracearum DC (imperfect stage) was observed on cucumbers in 1979 both A. cichoraceum and Sphaerotheca fuliginea were found in Skierniewice, Zielonka and Gołębiewo. Then were distinguished in the conidial stage mainly on the basis of differences in conidial germination and the presence or absence of fibrous bodies, and in the perfect stage on the basis of the number of asci in peridia and the number of spores per ascus.

In 2002, the occurrence of fungi of the order Erysiphales on plants of the Słowiański Park located in Goorzów Wielkopolski was investigated. Plant samples were collected once a month, from August to November. The samples examined were above ground plant parts colonized by powdery mildew fungi. A total of 78 samples were collected. Apart from 14 species of the order Erysiphales, Ampelomyces quisqualis parasitizing on Erysiphe cichoracearum var. cichoracearum, Phyllactinia mali and Podosphaera tridactyl var. tridactyla was found. Ampelomyces quisqualis affected hyphae, oidia, and young cleistothecia of P. mali. In contrast, in E. cichoracearum var. cichoracearum, Po. tridactyle var. tridactyla, this hyperparasite colonized only hyphae and oidia. This paper for the first trime informs of A. quisqualis parasitizing on P. mali and Po. tridactyla var. tridactyla.

Certain plant-derived agents (PDA) which could induce resistance in okra leaves against powdery mildew (Erysiphe cichoracearum) and biochemical changes in treated leaves were investigated during 2011 and 2012 growing seasons. At the two-leaf old, four- and eight-week leaves of the greenhouse-grown okra plants were sprayed with either PDA two days before inoculation with conidia. Among all tested PDA, neem seed oil (NSO) caused the highest protection, followed by jojoba oil (JO) and extract of Reynoutria sachalinensis. Moreover, levels of total protein, activity of peroxidase, polyphenyloxidase and chitinase and total phenols content of treated two-leaf old highly increased after application of NSO and JO, but not by the pathogen (untreated control). Finally, based on conidia germination, lack of the fungitoxic effect of JO and RE and also the slight effect of NSO (9–13.75%) may demonstrate the main role of these agents for inducing resistance through the activation of biochemical defence in treated leaves.

Resveratrol is a stilbene compound that is synthesized by plants in response to biotic stress and has been linked to health benefits associated with the consumption of certain foods and food products, such as grapes and wine. The final step in the biosynthesis of resveratrol is catalyzed by the enzyme stilbene synthase (STS). Here, we assessed the expression of two STS genes (VqSTS36 and VpSTS36) from the wild grape species Vitis quinquangularis (accession 'Shang-24'; powdery mildew (PM) resistant) and Vitis pseudoreticulata (accession 'Hunan-1'; PM susceptible) following infection by Uncinula necator (Schw.) Burr, the causal agent of PM disease. Some correlation was observed between the relative levels of STS36 transcript and disease resistance. We also cloned the 5' upstream sequence of both VpSTS36 and VqSTS36 and generated a series of 5' VqSTS36 promoter deletions fused to the GUS reporter gene in order to analyze expression in response to wounding, the application of exogenous stress-associated hormones, and biotic stress in tobacco leaves. The promoter was shown to be induced by the hormone salicylic acid (SA), inoculation with the fungal pathogen Erysiphe cichoracearum, and by wounding. These results suggest that VqSTS36 is regulated by biotic stresses and that it plays an important role in mediating disease resistance in grape.

NAC (for NAM, ATAF1, 2, and CUC2) family genes encode plant-specific transcription factors that play important roles in plant development regulation and in abiotic and biotic stresses. However, the function of NAC genes in grapevines is not clear. A novel NAC transcription factor, designated as VpNAC1, was isolated from Chinese wild Vitis pseudoreticulata. It belongs to the TERN subgroup and is a nuclear-targeting protein and functions as a transcriptional activator. Moreover, VpNAC1 was induced by the fungus Erysiphe necator and the exogenous hormones, particularly salicylic acid, methyl jasmonate and ethylene. Over-expression of VpNAC1 in tobacco plants enhanced their resistance to Erysiphe cichoracearum and Phytophthora parasitica var. nicotianae Tucker. These results suggest that VpNAC1 acts as a positive regulator in biotic stresses.

Rheum emodi, vernacularly known as Archu, is one of the important high altitude medicinal plants widely distributed in Himalayan regions. Though widely used in Ayurveda for curing various human diseases, its use in plant diseases is limited. Ethanolic extract of Rheum rhizome was assayed against spore germination of Alternaria solani, Heliminthosporium penniseti and Curvularia palliscens. The inhibition of spore germination was concentration dependent. Maximum inhibition was obtained at 4000 and 5000 ppm followed by 3000, 2000 and 1000 ppm. However, the extract was highly effective in the pre-inoculation treatment against powdery mildew (Erysiphe cichoracearum) of balsam (Impatiens balsamania) under field conditions. High performance liquid chromatographic (HPLC) analysis of balsam leaves showed increased synthesis of phenolic acids, which has been correlated with induced resistance in inhibiting the disease intensity of balsam powdery mildew.

In this study, two WRKY genes were isolated from Erysiphe necator-resistant Chinese wild Vitis pseudoreticulata W. T. Wang 'Baihe-35-1', and designated as VpWRKY1 (GenBank accession no. GQ884198) and VpWRKY2 (GenBank accession no. GU565706). Nuclear localization of the two proteins was demonstrated in onion epidermal cells, while trans-activation function was confirmed in the leaves of 'Baihe-35-1'. Expression of VpWRKY1 and VpWRKY2 was induced rapidly by salicylic acid treatment in 'Baihe-35-1'. Expression of VpWRKY1 and VpWRKY2 was also induced rapidly by E. necator infection in 11 grapevine genotypes; the maximum induction of VpWRKY1 was greater in E. necator-resistant grapevine genotypes than in susceptible ones post E. necator inoculation. Furthermore, ectopic expression of VpWRKY1 or VpWRKY2 in Arabidopsis enhanced resistance to powdery mildew Erysiphe cichoracearum, and enhanced salt tolerance of transgenic plants. VpWRKY2 also enhanced cold tolerance of transgenic plants. In addition, the two proteins were shown to regulate the expression of some defense marker genes in Arabidopsis and grapevine. The data suggest that VpWRKY1 and VpWRKY2 may underlie the resistance in transgenic grapevine to E. necator and tolerance to salt and cold stresses.

The anti-fungal activity of two alkaloids isolated from bulbs of Zephyranthes citrina was observed against 10 fungi, viz, Aternaria solani, A. triticina, Curvularia lunata, C. maculuns, Cercospora malvacearum, Erysiphe sp., Fusarium udum, Helminthosporium pisi, H. speciferum and Ustilago cynodontis. Different concentrations (200, 400, 600, 800, 1000 µg/ml of alkaloids (A and B) were used. Spore germination was inhibited at 600, 800, 1000 µg/ml. B alkaloid was used against Erysiphe cichoracearum causing powdery mildew in balsam (Impatiens balsamina) in the field as pre- and post-inoculation treatments at 1000, 1500, 2000 µg/ml doses. The extract was effective in both pre- and post-inoculation treatments. Foliar application of this alkaloid resulted in inducing synthesis of phenolic acid in the leaves of balsam. Maximum phenolics were detected in the leaves treated with 1500 µg/ml in both pre- and post-inoculation treatments. The increase in the production of phenolics in treated leaves of balsam can be cor...

The occurrence and geographic distribution of powdery mildew on cucurbits was studied in the Czech Republic, Austria, France, Germany, Great Britain, Italy, Slovakia, Slovenia, Spain, Switzerland, the Netherlands, as well as in Turkey and Israel, during the period 1995–2000. In total, 599 leaf samples with powdery mildew symptoms were collected from cucurbits from 166 locations within the Czech Republic; an additional 69 samples were collected from 42 locations elsewhere. Two powdery mildews (Golovinomyces cichoracearum and Podosphaera xanthii) were identified. The host range included the cultivated cucurbits (Cucumis melo, C. sativus, Cucurbita pepo, C. maxima, and Citrullus lanatus) and several other species. P. xanthii was the only powdery mildew pathogen found in Spain, Israel, and Turkey. P. xanthii and G. cichoracearum were detected in the other surveyed countries, occurring in both mixed infections and separately. In the more northerly latitudes and higher elevations, G. cichoracearum is more often the single species. G. cichoracearum was the dominant powdery mildew species in the Czech Republic (detected in 98.8% of the locations there); P. xanthii was found as the lone species in 1.2% of locations. At 28.4% of locations, G. cichoracearum was found with P. xanthii as mixed infections. The hyperparasitic fungus Ampelomyces quisqualis was found in 30% of the samples from the Czech Republic and was also recorded in Austria, Italy, Slovenia, Switzerland, and Great Britain.

Crop losses caused by pests and weeds have been estimated at 42% worldwide, with plant pathogens responsible for almost $10 billion worth of damage in the USA in 1994 alone. Elevated carbon dioxide [ECO(2)] and associated climate change have the potential to accelerate plant pathogen evolution, which may, in turn, affect virulence. Plant-pathogen interactions under increasing CO(2) concentrations have the potential to disrupt both agricultural and natural systems severely, yet the lack of experimental data and the subsequent ability to predict future outcomes constitutes a fundamental knowledge gap. Furthermore, nothing is known about the mechanistic bases of increasing pathogen agressiveness. In the absence of information on crop species, it is shown here that plant pathogen (Erysiphe cichoracearum) aggressiveness is increased under ECO(2), together with changes in the leaf epidermal characteristics of the model plant Arabidopsis thaliana L. Stomatal density, guard cell length, and trichome numbers on leaves developing post-infection are increased under ECO(2) in direct contrast to non-infected responses. As many plant pathogens utilize epidermal features for successful infection, these responses provide a positive feedback mechanism facilitating an enhanced susceptibility of newly developed leaves to further pathogen attack. Furthermore, a screen of resistant and susceptible ecotypes suggest inherent differences in epidermal responses to ECO(2).

Exudates from sclerotia of two Sclerotium rolfsii isolates (one causing collar rot in Cicer arietinum, isolate VC971, and the other leaf spots in Rauvolfia serpentina, isolate VL016) were assayed for their antifungal activity against 26 fungi consisting of plant parasites as well as saprophytes. Spore germination of all the test fungi was affected by the exudates reaching 100% in some cases. Foliar spray with exudates of isolate VL016 significantly reduced disease incidence of balsam (Impatiens balsamina) powdery mildew caused by Erysiphe cichoracearum and pea (Pisum sativum) powdery mildew caused by Erysiphe pisi, under field conditions. Characterization of exudates from 25 isolates of S. rolfsii revealed pH ranging from 3.8 to 5.3 and colour from light yellow to deep yellow. Among the phenolic acids found in the exudates were tannic, gallic, caffeic, vanillic, ferulic, chlorogenic and cinnamic acids. Oxalic acid was also found in varied amounts. Among the phenolic acids, ferulic acid was found to be present at high concentration in exudates of most isolates (3.9–153.4 μg/ml). The antioxidant properties of phenolics, which generally inhibit fungal morphogenesis including spore germination along with the antifungal nature of some phenolics are chiefly attributed to the inhibitory effect of sclerotial exudates of S. rolfsii. Additionally, both the isolates VC971 and VL016 showed almost similar antifungal activities despite they are of different origin and thereby demonstrate the antifungal nature of sclerotial exudates.

Podosphaera ( Sphaerotheca ) euphorbiae on Euphorbia epithymoides (= E. polychroma , cushion spurge) is reported for the first time in North America. Neoerysiphe ( Erysiphe ) galeopsidis on Lamium amplexicaule (henbit), Blumeria ( Erysiphe ) graminis on Festuca idahoenis (Idaho fescue), Taphrina johansonii on Populus tremuloides (trembling aspen), and Taphrina occidentalis on Alnus rubra (red alder) are reported for the first time from Idaho. Reports of Erysiphe cichoracearum on henbit may reflect misdetermined Neoerysiphe galeopsidis .

Cross-talk between signal transduction pathways is a central feature of the tightly regulated plant defense signaling network. The potential synergism or antagonism between defense pathways is determined by recognition of the type of pathogen or pathogen-derived elicitor. Our studies have identified WRKY70 as a node of convergence for integrating salicylic acid (SA)- and jasmonic acid (JA)-mediated signaling events during plant response to bacterial pathogens. Here, we challenged transgenic plants altered in WRKY70 expression as well as WRKY70 knockout mutants of Arabidopsis with the fungal pathogens Alternaria brassicicola and Erysiphe cichoracearum to elucidate the role of WRKY70 in modulating the balance between distinct defense responses. Gain or loss of WRKY70 function causes opposite effects on JA-mediated resistance to A. brassicicola and the SA-mediated resistance to E. cichoracearum. While the up-regulation of WRKY70 caused enhanced resistance to E. cichoracearum, it compromised plant resistance to A. brassicicola. Conversely, down-regulation or insertional inactivation of WRKY70 impaired plant resistance to E. cichoracearum. Over-expression of WRKY70 resulted in the suppression of several JA responses including expression of a subset of JA- and A. brassicicola-responsive genes. We show that this WRKY70-controlled suppression of JA-signaling is partly executed by NPR1. The results indicate that WRKY70 has a pivotal role in determining the balance between SA-dependent and JA-dependent defense pathways.

Arabidopsis thaliana is a host to the powdery mildew Erysiphe cichoracearum and nonhost to Blumeria graminis f. sp hordei, the powdery mildew pathogenic on barley (Hordeum vulgare). Screening for Arabidopsis mutants deficient in resistance to barley powdery mildew identified PENETRATION3 (PEN3). pen3 plants permitted both increased invasion into epidermal cells and initiation of hyphae by B. g. hordei, suggesting that PEN3 contributes to defenses at the cell wall and intracellularly. pen3 mutants were compromised in resistance to the necrotroph Plectosphaerella cucumerina and to two additional inappropriate biotrophs, pea powdery mildew (Erysiphe pisi) and potato late blight (Phytophthora infestans). Unexpectedly, pen3 mutants were resistant to E. cichoracearum. This resistance was salicylic acid–dependent and correlated with chlorotic patches. Consistent with this observation, salicylic acid pathway genes were hyperinduced in pen3 relative to the wild type. The phenotypes conferred by pen3 result from the loss of function of PLEIOTROPIC DRUG RESISTANCE8 (PDR8), a highly expressed putative ATP binding cassette transporter. PEN3/PDR8 tagged with green fluorescent protein localized to the plasma membrane in uninfected cells. In infected leaves, the protein concentrated at infection sites. PEN3/PDR8 may be involved in exporting toxic materials to attempted invasion sites, and intracellular accumulation of these toxins in pen3 may secondarily activate the salicylic acid pathway.

Powdery mildew, caused by Oidium lini, is an important disease of flax (Linum usitatissimum) in western Canada. Investigations on the genetics of resistance to powdery mildew in flax resulted in the identification of several resistant Canadian and introduced cultivars. A single dominant gene for resistance to powdery mildew, designated PM1, was identified in Canadian flax cultivars (‘AC Watson’, ‘AC McDuff’, and ‘AC Emerson’) and introduced flax cultivars (‘Atalante’ and ‘Linda’). In addition, two putative dominant genes for resistance to powdery mildew were identified in ‘Linda’.

As plants mature it has been observed that some become more resistant to normally virulent pathogens. The ability to manifest the Age-Related Resistance (ARR) response in Arabidopsis to Pseudomonas syringae pathovars tomato ( Pst) coincided with the transition to flowering in plants both delayed and accelerated in the transition to flowering. ARR was also associated with a change in PR-1 gene expression, such that young plants expressed PR-1 abundantly at 3 days post inoculation (dpi) while mature plants expressed much less. The Arabidopsis ARR response requires SA accumulation via isochorismate synthase ( ICS1 ) [24] . ICS1 was expressed one dpi with virulent and avirulent Pst in both young and mature plants. The ARR response was also effective versus avirulent Pst providing an additional 4-fold limitation in bacterial growth. Arabidopsis ARR was found to be ineffective against two necrotrophs, Erwinia carotovora subspecies carotovora (bacterium) and Botrytis cinerea (fungus) and one obligate biotroph, Erysiphe cichoracearum (fungus). However, mature wild type, SA-deficient sid2 and NahG plants supported little growth of the obligate biotrophic oomycete, Peronospora parasitica . Therefore ARR to P. parasitica appears to be SA-independent, however the level of ARR resistance was somewhat reduced in these mutants in some experiments. Thus, there may be numerous defence pathways that contribute to adult plant resistance in Arabidopsis.

Nine hundred and seventy-seven (977) cucumber (Cucumis sativus L.) accessions from the U.S. National Plant Germplasm System (NPGS) collection were tested for resis- tance to powdery mildew, caused by the fungus Podosphaera xanthii (Castagne) Braun and Shishkoff, formerly known as Sphaerotheca fuliginea (Schlecht.) Poll. Plants from each accession were evaluated in the greenhouse following inoculation with fi eld isolates of P. xanthii. Each plant was placed into one of three susceptibility classes based on the amount of fungal growth and sporulation on the hypocotyl, stem, petioles and leaves - susceptible (S), intermediate (I) or resistant (R). Of the 977 accessions (9.6%), 94 contained at least one I or R-type plant. Seventeen of the 20 most-resistant accessions came from Asian sources, including China (PIs 418962, 418964, 432860, and 432870), India (PIs 197085, 197088, and 605930), Japan (PIs 279465, 288238, 390258, and 390266), Pakistan (PI 330628), the Philip- pines (PIs 426169 and 426170), and Taiwan (PIs 321006, 321009, and 321011). A quantitative study was conducted to compare mildew reproduction on S, I, and R-type cucumbers in the greenhouse and under fi eld conditions in Ames, Iowa. Leaf disks were removed weekly and microscopic counts made of spore populations. The leaf disk method was superior to visual rating for ranking and differentiating intermediate from resistant accessions. Both the intermediate (moderately resistant) and highly resistant accessions provided excellent protection against powdery mildew in the fi eld. Powdery mildew is one of the world's most widespread and damaging diseases of green- house and fi eld-grown cucumbers (Cucumis sativus L.). The most frequently cited causal agents are Golovinomyces cichoracearum Gelyuta (syn. Erysiphe cichoracearum DC) and Podosphaera xanthii (Castagne) Braun and Shishkoff (syn. Sphaerotheca fuliginea (Schlecht.) Poll.). In early reports (Barnes and Epps, 1956; Whitaker and Davis, 1962), G. cichoracearum was identifi ed as the primary causal agent in the United States, but more recent studies indicate that P. xanthii is the more common pathogen in the U.S. (Ballantyne, 1975; McCreight et al., 1987). Both species were observed on Cucumis melo L. in France, with P. xanthii isolated alone in 79% of the cases, G. cichoracearum isolated alone in 18% of the cases, and the remainder a mixture of both fungi (Pitrat et al., 1998). The primary causal agent in the U.S. may have shifted toward P. xanthii over time, but there were probable misidentifi cations in some of the older literature. The two species can be diffi cult to differentiate in the absence of the perfect stage, but useful identifi cation methods are now available. Podosphaera xanthii can be distinguished from G. cichoracearum by the presence of well-defi ned fi brosin bodies in the conidia and the occasional production of forked germination tubes (Boesewinkel, 1980; Kable and Ballantyne, 1963). Fibrosin bodies are visible with a light microscope, appearing as conical, disc and rod-shaped structures when the conidia are mounted in 3% aqueous KOH (Kable and Ballantyne, 1963). No races of P. xanthii have been reported from cucumber, although multiple races have been reported from muskmelon (C. melo) (Hosoya et al., 2000; Mc- Creight et al., 1987; Pitrat et al., 1998). Disease resistance from introduced germ- plasm has long been a valuable tool for control- ling powdery mildew in cucumber. Much of the early breeding work in the U.S. and sources of resistance was summarized by Peterson (1975) and was recently updated by Jahn et al. (2002). Two of the early examples of cucumber powdery mildew resistance from introduced germplasm

Silicon (Si) has been used in agriculture to protect plants against disease for hundreds of years and its prophylactic effects in monocots and dicots are well documented. The mechanisms by which Si exerts its protective effects in planta , however, are uncertain and presently the subject of debate. In this study, we sought to determine if Arabidopsis thaliana could be used to clarify the role of Si in plant–pathogen interactions. Accordingly, X-ray microanalysis mapping, light microscopy, scanning and transmission electron microscope techniques were used to examine the leaves of Si− fed A. thaliana plants inoculated with the powdery mildew fungus, Erysiphe cichoracearum . The results of this study demonstrate for the first time, that A. thaliana is a species that absorbs Si and that the incidence of powdery mildew disease for Si− fed plants is significantly lower compared to control plants. In particular, treatment with Si appeared to induce the production of an electron-dense, fungitoxic substance that accumulated within and around the collapsed fungal haustoria of infected epidermal cells within the leaves of disease-resistant plants. These results with Arabidopsis corroborate recent observations in other non-related species and support the emerging theory that the mechanisms by which Si imparts resistance to plants are complex and are not entirely explained by the traditionally proposed role of Si as a reinforcer of mechanical resistance. Collectively, the findings of the present study have established the Arabidopsis thaliana - Erysiphe cichoracearum pathosystem as a valid model to investigate the role of Si in plant–microbe interactions.

Harpin is a proteinaceous bacterial elicitor inducing a hypersensitive response (HR) in non-host plants. To engineer disease resistant plants using this elicitor, the gene (hrpZpss) encoding a harpin from Pseudomonas syringae pv. syringae (strain LOB2-1, causal agent of bacterial blight of lilac) was cloned. The hrpZpss gene was fused to a cauliflower mosaic virus 35S promoter for constitutive expression and a pea phenyl alanine ammonia-lyase, PAL, promoter for pathogen-inducible expression and the resultant constructs, 35S-hrpZ and PAL-hrpZ, were introduced into tobacco (cv. Petite Havana, SR1), a non-host plant. Transgenic tobacco expressing harpinpss at a high level exhibited neither phenotypic change nor accumulation of pathogenesis-related (PR) proteins in the absence of pathogens. However, when the transformants were inoculated with conidia of powdery mildew fungus, Erysiphe cichoracearum, HR-like local lesions were produced on the lower leaves of the plants harboring 35S-hrpZ as well as PAL-hrpZ. Three kinds of PR proteins (PR-1a, Q and S) accumulated in the transgenic plant much more than in the control plant. Many transgenic plants that produced local lesions showed an enhanced resistance to E. cichoracearum. These results suggest that the resistance reaction observed in the harpin transformants was induced by the pathogen. The possible mode of action of harpinpss is discussed.

The responses of Cucumis melo genotypes MR-1 and PI 124112 to 20 isolates of powdery mildew pathogens (Erysiphe cichoracearum, Sphaerotheca fuliginea) that originated from Cucurbita spp. were evaluated on leaf discs as part of a pathotype and race determination programme in the Czech Republic. One isolate of S. fuliginea produced limited mycelia on MR-1. One isolate of E. cichoracearum was virulent on MR-1, while six other isolates infected MR-1, which reacted showing symptoms of incomplete resistance. In the set of 10 mixed ( E. cichoracearum+S. fuliginea ) isolates only one was virulent on MR-1 and another one on PI 124112. These genotypes expressed incomplete resistance after inoculation with eight mixed isolates. This is the first experimental evidence of differential responses, i.e., incomplete resistance or susceptibility, by C. melo genotypes MR-1 and PI 124112 to cucurbit powdery mildew isolates at the true leaf stage. Expression of resistance or incomplete resistance responses of C. melo Iran H to E. cichoracearum and two mixed isolates was recorded. Until now, Iran H has been considered a universal susceptible host. These reactions indicate that in this genotype could be located unknown race-specific resistance factor(s) to one or both species of powdery mildew.

For obligate parasites like Erysiphe cichoracearum, manipulation of nitrogen (N) and potassium (K) nutrition may provide an alternative control method for powdery mildew in greenhouse crops. Subsequently, Hiemalis begonias “Hilda” and “Renaissance” were grown in separate experiments with a factorial combination of three N (30, 120, 240 mg L−1) and three K (30, 120, 240 mg L−1) rates applied at each irrigation. One‐half the plants in each treatment were inoculated with E. cichoracearum, while the remaining plants were sprayed regularly with the fungicide triadimefon to prevent disease development. Plant growth and powdery mildew incidence in both cultivars were significantly influenced by N fertilization and pathogen inoculation, but relatively unaffected by K fertility. Shoot dry mass in both cultivars increased with increasing N rate up to 120 mg N/L, but decreased from pathogen inoculation. This latter response increased in magnitude with increasing N rate in “Hilda”, but not in “Renaissance”. ...

In Arabidopsis spp., the jasmonate (JA) response pathway generally is required for defenses against necrotrophic pathogens and chewing insects, while the salicylic acid (SA) response pathway is generally required for specific, resistance (R) gene-mediated defenses against both biotrophic and necrotrophic pathogens. For example, SA-dependent defenses are required for resistance to the biotrophic fungal pathogen Erysiphe cichoracearum UCSC1 and the bacterial pathogen Pseudomonas syringae pv. maculicola, and also are expressed during response to the green peach aphid Myzus persicae. However, recent evidence indicates that the expression of JA-dependent defenses also may confer resistance to E. cichoracearum. To confirm and to extend this observation, we have compared the disease and pest resistance of wild-type Arabidopsis plants with that of the mutants coi1, which is insensitive to JA, and cev1, which has constitutive JA signaling. Measurements of the colonization of these plants by E. cichoracearum, P. syringae pv. maculicola, and M. persicae indicated that activation of the JA signal pathway enhanced resistance, and was associated with the activation of JA-dependent defense genes and the suppression of SA-dependent defense genes. We conclude that JA and SA induce alternative defense pathways that can confer resistance to the same pathogens and pests.

In order to evaluate the antifungal activity of saponins in extracs of quillay (Quillaja saponaria MOL.), against powdery mildew (Erysiphe cichoracearum DC. Ex Merat and Sphaerotheca fuliginea (Schlecht ex Fr.) Poll.) of cucurbit crops under natural conditions, two experiments were done: one with cucumber (Cucumis sativus L.) under greenhouse conditions, and a second with squash (Cucurbita maxima Duch.) under field conditions. A split plot design with seven treatments and four replicates was used. Results in cucumber showed that QL 1000 reached a maximum of 51.8% of disease control with an average of 37.9% control; while QL Ultra only gave a 27.8% control with an average of 15.8%. Results in squash, showed QL 30B reached 52.2% control of the disease with an average of 41.7% while QL Ultra reached 54.6% control with an average of 34.7%. The lower and medium dosages gave 49 and 42% control respectively. Myclobutanil reached 90% control in cucumber, while sulphur gave 59.6 and 78% control in squash and cucumber respectively. High dosages of QL 1000 and QL 30B showed insuficient levels of control. Saponins dosages in the range of 32 to 400 ppm may be recomended to control powdery mildew. These results also suggest that some compounds in the extracts are lost through purification causing a decrease in their antifungal action.Se realizaron dos experimentos para evaluar la actividad antifungica de saponinas presentes en extractos de quillay sobre oidio de las cucurbitaceas (Erysiphe cichoracearum DC. Ex Merat y Sphaerotheca fuliginea (Schlecht ex Fr.) Poll. ) en condiciones de ataque natural. El primero con pepino (Cucumis sativus L.) bajo condiciones de invernadero, y el segundo con zapallo de guarda (Cucurbita maxima Duch) en condiciones de campo. Para ambos se utilizo el diseno de parcelas divididas con siete tratamientos y cuatro repeticiones. Los resultados en pepino demostraron que QL 1000 logro un maximo de 51,8% de control de la enfermedad con un promedio de 37,9%, mientras que QL Ultra solo alcanzo 27,8% de control con un promedio de 15,8%. En zapallo de guarda, se observo que QL 30B logro 52,2% de control con un promedio de 41,7%, mientras que QL ultra alcanzo un 54,6% de control con un promedio de 34,7%. Las dosis bajas y medias promediaron niveles de control de oidio de las cucurbitaceas del 42% y 49%, respectivamente. Myclobutanil en pepino alcanzo un 90% de control, mientras que azufre redujo la enfermedad en un 59,6 y 78% para zapallo y pepino, respectivamente. Dosis altas de QL 1000 y QL 30B ejercieron un nivel insuficiente de control. Se recomienda usar dosis entre 32 y 400 ppm de saponinas. Estos resultados sugieren que algunos compuestos presentes en los extractos se pierden a traves de la purificacion.

Plants of the discovered chlorophyll-deficient mutant of oak (ML) display enhanced disease resistance to the fungus Erysiphe cichoracearum, causal agent of powdery mildew. Quantitative imaging of chlorophyll (Chl) fluorescence revealed that the net photosynthetic rate (PN) declined progressively in both untreated and invaded ML leaves as well as in inoculated wild-type (WT) leaves. Images of non-photochemical fluorescence quenching (NPQ) in both untreated and infected mutant leaves suggested that the capacity of Calvin cycle had been reduced and that there was a complex metabolic heterogeneity within the ML leaf. The ML mutant accumulates reactive oxygen species, ROS (H2O2) from the oxidative burst followed by spontaneous cell death that mimic the hypersensitive response. Reduction in pathogen sporulation on ML leaves correlated with the accumulation of soluble saccharides and a more rapid induction of defence responses including expression of some defence proteins (β-1,3-glucanase and chitinase). Unlike to WT plants, ML- conferred phenotype activates and/or de-represses multiple defence responses, making them more easily induced by pathogens.

Attempts at breeding squash (Cucurbita pepo) resistant to powdery mildew (Sphaerotheca fuliginea and Erysiphe cichoracearum) have heretofore been unsuccessful due mainly to association of the resistance trait with reduction in yield. The goal of the present work was to determine if heterozygous hybrids expressing partial resistance could be potentially valuable for squash growers. One heterozygous hybrid each of zucchini, cocozelle, and vegetable-marrow squash was grown alongside a leading susceptible commercial hybrid cultivar of the same type, under standard field conditions. Although fungicides were applied at recommended rates every 6 to 13 days to suppress powdery mildew, the susceptible hybrids became heavily infested later in the season. The resistant hybrids were similar to their respective commercially available susceptible hybrids in fruit appearance and other horticulturally important traits. The resistant hybrids yielded at least as well as the commercial hybrids early in the season and outyielded them later in the season.

The allelopathic potential of Ageratum conyzoides was investigated under different environmental stress conditions, including nutrient deficiency, physical damage, 2,4-D treatment, competition with Bidens pilosa, infection with Erysiphe cichoracearum, and feeding by Aphiids gossypii. The inhibitory effects of A. conyzoides volatiles on peanut (Arachis hypogaea), redroot amaranth (Amaranthus retroflexus), cucumber (Cucumis sativus), and ryegrass (Lolium multiforum) increased when plants were grown under nutrient-deficient conditions or in competition with B. pilosa; however, there was no difference with physical damage or 2,4-D treatment. Phytoinhibitory effects decreased under fungal infection and aphid feeding. Volatiles from A. conyzoides plants infected with E. cichoracearum or exposed to A. gossypii feeding inhibited or killed fungi and insects. Precocenes and their derivatives, monoterpenes, and sesquiterpenes were the major volatile components of A. conyzoides.

We studied the effects of exogenous salicylic acid on the level of endogenous cytokinins and abscisic and salicylic acids in the tissues of leaves of phloxes contrasting in resistance against the powdery mildew causative agent: susceptible Phlox paniculata L. and resistant Ph. setacea L. Studies were carried out under the conditions of biotic stress. The initial level of salicylic and abscisic acids and cytokinins is the highest in the resistant phlox species. After treatment with salicylic species, the total level of cytokinins and endogenous salicylic acid increased in both species. When the treated phlox species were infected by the powdery mildew causative agent, the level of abscisic and salicylic acids increased in the susceptible Ph. paniculata, while that of cytokinins increased in the resistant Ph. setacea. The role of salicylic acid in the induction of plant defense reactions against phytopathogens is discussed.

To further the understanding of the natural genetic diversity for disease resistance to powdery mildew (Erysiphe cichoracearum) in Arabidopsis thaliana, quantitative trait loci analysis was undertaken on recombinant inbred lines derived from a cross between the resistant accession Warschau-1 and the susceptible Columbia-0. Powdery mildew grew less well on Warschau-1, but the resistance was not associated with a specific block in the infection sequence. Two potential powdery mildew disease-resistance loci were identified and mapped, one with a major effect and one with a minor effect on disease resistance. The two loci acted in an additive manner to confer resistance, and together they explained 65% of the variation in resistance. In addition, the major powdery mildew disease-resistance locus was genetically mapped to the bottom of chromosome III, a region containing the powdery mildew resistance loci RPW7, RPW8 and RPW10. Unlike resistance mediated by the RPW8 locus in the accession Moscow-1, resistance in Warschau-1 was not correlated with the hypersensitive response, highlighting the influence of genetic background or environmental factors on the expression of disease resistance. Together with the powdery mildew resistance loci described in other studies, these results suggest that A. thaliana is a useful source of natural powdery mildew disease resistance, which potentially can be utilized in fundamental studies and as a tool for applied studies.

When supplied via roots to tobacco host plants, aqueous solutions of allopurinol, a specific inhibitor of xanthine oxidase and dehydrogenase, markedly reduced or completely prevented the visible development of tobacco powdery mildew fungus Erysiphe cichoracearum(E. orontii) at concentrations as low as 25–50 μ M administered for periods as short as 2 days. The duration of this allopurinol-dependent protection from the pathogen was of at least 20 days after the last administration. If supplied as foliar sprays at concentrations up to 1600 μ M , allopurinol was not absorbed by the leaf tissues and did not affect the fungal infection. Histological observations revealed that on tobacco plants treated with allopurinol via roots, a sharp reduction inE. cichoracearum mycelial development started only 48 h after inoculation (after formation of the first haustoria) and was accompanied by a strong delay and reduction of sporulation. The proportion of haustoria partially or entirely embedded in thick translucent sheaths (possibly callosic in nature) increased significantly in allopurinol irrigated plants reaching about 41% of the total haustoria (compared to 9% in controls) 7 days post-inoculation. No effects of allopurinol supply was observed on Blumeria graminis and Erysiphe trifolii development on wheat and clover plants, respectively. The present results are consistent with the view that allopurinol affects pathogen growth and sporulation by inhibition of xanthine dehydrogenase in the plant, the pathogen or both organisms.

Fifty-seven cultivars of zinnia (Zinnia elegans Jacq.) were studied for 17 weeks to determine their resistance to alternaria blight (Alternaria zinniae Pape), powdery mildew (Erysiphe cichoracearum DC ex Merat) and bacterial leaf & flower spot [Xanthomonas campestris pv. zinniae (syn. X. nigromaculans f. sp. zinniae Hopkins & Dowson)]. A disease severity scale was used to determine acceptability for landscape use. At week 4, all cultivars were acceptable. At week 10, eleven cultivars were acceptable. At week 17, all cultivars were unacceptable. Ten cultivars had been killed by one or more pathogens by week 17. Only two cultivars showed any tolerance to any disease (powdery mildew) at week 17.

In previous work, UEA1 and UCSC1, two geographically distinct, powdery mildew isolates, were recognized for their ability to infect Arabidopsis thaliana. We have clarified the identity of these isolates by determining their host ranges, reexamining their morphology, and comparing their DNA sequences for the 5.8S ribosomal RNA and two flanking internal transcribed spacer sequences. These experiments confirm that UEA1 is a member of Erysiphe cruciferarum and that UCSC1 belongs to E. cichoracearum. Interactions of the two Erysiphe isolates with 360 A. thaliana accessions were examined to provide a comprehensive profile of naturally occurring powdery mildew resistance in this weedy species. The majority of A. thaliana accessions (213) were susceptible to both isolates. Among the accessions exhibiting some degree of resistance, most (84) responded differentially to UEA1 and UCSC1 and the remainder were resistant to both isolates. Notably, resistance to UCSC1 cosegregated with RPW7, a locus previously demonstrated to confer resistance to UEA1 in Ms-0 × Landsberg (erecta) crosses. With this large collection of resistant accessions, questions about species specificity, genetic diversity and the evolution of resistance to powdery mildews can be addressed.

International audience; The relative incidence of Erysiphe cichoracearum and Sphaerotheca fuliginea, both agents of powdery mildew of cucurbits, was determined from 275 samples of mildewed leaves of cucurbits collected in 1994 from five regions of France. E. cichoracearum was identified in 9 to 39% of the mildewed leaf samples from four of the regions but was not detected in samples from the Mediterranean island of Corsica. The genetic structure of the French population of E. cichoracearum was examined using RFLPs of the ribosomal internal transcribed spacers amplified by PCR, random amplified polymorphic DNA (RAPD) markers, pathogenicity and mating-type tests. Forty-one isolates, including one from England, were analysed. Cluster analysis from 147 RAPD fragments using 16 primers revealed the existence of three distinct genetic lineages corresponding to three rDNA haplotypes (designated groups A, B and C). Bootstrap, genetic diversity, gametic disequilibrium and private allele analyses supported this differentiation. The genetic differentiation observed in the French population was not related to the geographical origin of the isolates. Group A isolates may be more specialized on melon as, with one exception, they were of race 1 (growth on four of the five melon cultivars tested) in comparison with group B and C isolates, which were of race 0 (growth on IranH only). Thus, the genetic differentiation observed may indicate a host-specialized subdivision within the French population of E. cichoracearum from cucurbits. Gametic disequilibrium analysis among RAPD loci and biological observations suggest that the sexual stage is of minor importance for epidemics of E. cichoracearum on cucurbits.

A set of 31 Cucurbita pepo L. commercial cultivars with different fruit shapes (morphotypes) was evaluated for resistance to powdery mildew of cucurbits (Erysiphe cichoracearum and Sphaerotheca fuliginea) under field conditions of a natural infection. During vegetative growth, the infection degree (ID) was assessed four times on leaves (L) and twice on stems and petioles (S). The `Area below the curve' values of the disease infection progress were calculated separately for leaves (ABC-L) and stems and petioles (ABC-S). Data were subjected to one-way analyses of variance and mean separation was performed using LSD multiple range tests. Significant differences in field resistance were found between morphotypes. Cultivars with scallop and acorn fruit shapes were the most resistant. The straightneck, crookneck and ornamental gourd groups showed medium levels of field resistance. The zucchini, cocozelle, pumpkin and vegetable marrow groups of morphotypes were the most susceptible. Isozyme spectra of peroxidases for 11 selected C. pepo cultivars representing different groups of morphotypes with various levels of field resistance were analysed as well. They could be ranged into three basic groups which correspond with the cultivar's level of field resistance. This phenomenon is discussed as a potential biochemical marker in C. pepo selection.

A powdery mildew (Erysiphales) infection of Arabidopsis thaliana was found in the greenhouse of the Department of Molecular Biology at Massachu- setts General Hospital (MGH isolate). We studied the structure of the anamorph, the sequence and chro- nology of its development, and its virulence on A. thaliana and other hosts. The teleomorph of the fun- gus was not found on the material we examined. The MGH isolate caused severe powdery mildew symp- toms on some A. thaliana accessions, infecting ro- sette and cauline leaves, stems and siliques, but was practically avirulent on others. Previously, two Erysi- phales have been reported to infect A. thaliana. These were identified as Erysiphe cichoracearum (UCSC isolate) and E. cruciferarum (UEA isolate). The MGH isolate was found to be a member of the E. cichoracearum group in the section Golovinomyces. It can be distinguished from both of these previously described powdery mildews of A. thaliana based on the morphology, host range and virulence. It might be best referred to E. orontii following the system pro- posed by Braun.

Cold and hot water leaf extracts of papaya (melon tree, pawpaw) (Carica papaya L.) were effective in reducing the growth of powdery mildew fungi in vitro and in reducing the spread of powdery mildew disease on pepper plants. The toxicity of cold water leaf extracts of papaya to fungi was greater than that of hot water extracts, suggesting the bioactive extract could be heat sensitive. Leaf extracts of papaya have the potential for control of powdery mildew disease of pepper in the field.

Interactions can occur among insects and plant pathogens exploiting the same host plants, and induced changes in plant chemistry may mediate these interactions. Most studies have examined interactions from the perspective of individual insects or plant pathogens. We determined whether co-occurring insects and plant pathogens alter each other's exploitation patterns in late-season surveys of mature cucurbits. We made field observations of the densities of 3 herbivore species, percentage of leaf damage and powdery mildew coverage, and occurrence of 2 common disease symptoms on F2 individuals of domestic zucchini × wild gourd plants. We extracted bitter, toxic cucurbitacins and antipathogenic peroxidase enzymes from leaves and examined variation in chemistry based on the presence or absence of exploiters. We found that cucumber beetles were present in lower densities on plants with powdery mildew. No other interactions between insect occurrence and plant pathogen symptoms were found. Levels of insect infestation and plant pathogen symptom occurrence were not related to cucurbitacin content or peroxidase activity. Some exploiters do influence host use patterns by other pest insects and pathogens, but most insects and pathogens appear to interact idiosyncratically with the host plant late in the season in this system, perhaps because of low densities of exploiters and low induction of plant chemical resistance.

Hypocotyls, cotyledons and true leaves of in vitro seedlings of 10 cucumber and melon genotypes resistant to downy and powdery mildew were cultured on several combinations of initiation and multiplication media to produce callus and subsequently cell suspensions as suitable sources for isolation and culture of protoplasts. Cotyledons of both species were shown to be the most responsive to variation in culture media. However, calli and cell suspensions derived from hypocotyls generally provided higher number of protoplasts by treatment with several enzymatic solutions. The protoplasts formed new cell walls after 12 h of culture in liquid culture medium and first cell division was observed 2 d later with more frequent divisions after one week of culture.

True-breeding lines of Zinnia marylandica Spooner, Stimart & Boyle [allotetraploids of Z. angustifolia H.B.K. and Z. violacea Cav. (2n = 4x = 46)] were backcrossed with autotetraploid Z. angustifolia (2n = 4x = 44) and Z. violacea (2n = 4x = 48). Seed-generated, backcross (BC1) families were screened for resistance to alternaria blight (Alternaria zinniae Pape), bacterial leaf and flower spot [Xanthomonas campestris pv. zinniae (Hopkins and Dowson) Dye], and powdery mildew (Erysiphe cichoracearum DC. ex Merat). All BC1 families exhibited high levels of resistance to alternaria blight and powdery mildew. BC1 families derived from crossing Z. marylandica with autotetraploid Z. angustifolia were highly resistant to bacterial leaf and flower spot, whereas BC1 families derived from crossing Z. marylandica with autotetraploid Z. violacea were susceptible to this disease. Our results suggest that one Z. angustifolia genome in BC1 allotetraploids is sufficient to confer resistance to A. zinniae and E. cichoracearum, but at least two Z. angustifolia genomes are required in BC1 allotetraploids to provide resistance to X. campestris pv. zinniae.

True-breeding lines of Zinnia marylandica Spooner, Stimart and Boyle (allotetraploids of Z. angustifolia H.B.K. and Z. violacea Cav. (2n = 46)) were reciprocally backcrossed with diploid and autotetraploid forms of Z. angustifolia (2n =22 or 44) and Z. violacea (2n =24 or 48). In most cases, backcrosses were more successful with Z. angustifolia and Z. violacea as autotetraploi ds than as diploids. Seed-generated, backcross (BC 1) families were obtained by crossing Z. marylandica (as female) with autotetraploid Z. angustifolia or autotetraploid Z. violacea. BC1 plants were phenotypically intermediate between the two parental lines for most morphological characters. Crosses between Z. marylandica and autotetraploid Z. angustifolia yielded BC1 plants with 33% stainable pollen, whereas crosses between Z. marylandica and autotetraploid Z. violacea yielded BC1 plants that produced malformed, poorly-stained pollen. No embryos were observed in capitula collected from field-grown BC1 plants. BC1 hybrids of Z. marylandica and autotetraploid Z. violacea produced larger capitula and more ray florets than Z. marylandica, and exhibited novel combinations of floral pigments not observed in Z. marylandica ray florets. BC1 hybrids of Z. marylandica and Z. violacea have commercial potential as seed-propagated, bedding plants. The common zinnia (Zinnia violacea (McVaugh, 1984); formerly Z. elegans Jacq. (Tomes, 1963)) is a popular bedding plant and cut flower that exhibits considerable phenotypic diversity for plant habit and ray floret color and morphology. Commercial cultivars of Z. violacea, however, are subject to attack by three major pathogens: Altemaria zinniae Pape (alternaria blight), Erysiphe cichoracearum DC. ex Merat (powdery mildew), and Xanthomonas campestris pv. zinniae Hopkins and Dowson (bacterial leaf and flower spot) (Ander- sen, 1971; Jones and Strider, 1979; Lipschutz, 1965; Tomes, 1963). These pathogens incite moderate to severe epiphytotics within Z. violacea plantings, resulting in plant losses and/or decreased orn- amental value. The narrow-leaved zinnia (Z. angustifolia (McVaugh, 1984)) is highly resistant to these pathogens (Andersen, 1971; Jones and Strider, 1979; Lipschutz, 1965; Torres, 1963), and is a promising source of disease resistance for Zinnia breeding programs. Crosses between Z. angustifolia (2n = 2x = 22) and Z. violacea (2n = 2x = 24) yield infertile allodiploids (2n = 2x = 23) (Boyle and Stimart, 1982; Terry-Lewandowski et al., 1984). Colchicine-induced allotetraploids (2n = 4x = 46) exhibit preferential pairing of homolo- gous chromosomes and infrequent pairing of homoeologous chro- mosomes, and, as a consequence, are partially fertile and breed true from seed (Terry-Lewandowski et al., 1984). Allotetraploids of Z. angustifolia and Z. violacea, collectively known as Z. marylandica (Spooner et al., 1991), exhibit high levels of resistance to A. zinniae and E. cichoracearum and moderate to high levels of resistance to X. campestris pv. Zinniae (Terry-Lewandowski and Stimart, 1983). To obtain the expression of new characters, it would be desirable to backcross Z. marylandica to the parental species. The objectives of this study were to determine the pathways of backcrossing Z. marylandica to Z. angustifolia and Z. violacea, and to examine the morphology and fertility of the BC 1 plants and uniformity within BC1 families.