Variation Within and Between Vitis spp. for Foliar Resistance to the Downy Mildew Pathogen Plasmopara viticola

Vitis vinifera grapevines are highly susceptible to downy mildew caused by the host-specific oomycete Plasmopara viticola. In humid climates like New York, where the unchecked pathogen will defoliate vines and destroy young fruit clusters, growers depend upon multiple fungicide applications for disease management. However, management of P. viticola by fungicides is tenuous, because the pathogen has repeatedly overcome a broad range of previously effective fungicides (9). To complement chemical disease control (31) and predictive models (13), diseaseresistant interspecific hybrids have been developed to combine disease resistance with the production of high-quality fruit (1). Molecular markers associated with some downy mildew resistance genes have been identified to facilitate additional introgression of resistance (7,8,17,30). Numerous germplasm screens have been conducted in Vitis spp. to identify accessions resistant to downy mildew (3,5,6,18– 20,23,25,26,29). In those studies that screened multiple accessions per species, variation in resistance was identified not only between but also sometimes within species (10,26,29). In spite of these findings, Vitis spp. are often generalized as being more or less resistant without characterizing the resistance of accessions within each species (5,6,18,19,21,25). Some cultivars were screened in multiple studies and their resistance ratings frequently failed to match. For example, Vitis hybrid ‘Athens’ was rated resistant (19), moderately susceptible (5), and susceptible (3,25), and Vitis hybrid ‘Isabella’ was rated resistant (3,19), moderately resistant (6), moderately susceptible (5), and susceptible (25). These results highlight the need to control environment and pathogen source in grape downy mildew resistance screens and to ensure cultivar identity. However, one cultivar, Vitis hybrid ‘Concord,’ was consistently resistant (6,19) or moderately resistant (3) across studies and, thus, appears to have durable, broad-spectrum resistance. Given the long lifespan of grapevines in commercial vineyards and the high cost of their establishment, durable disease resistance is a desirable trait for cultivars. Race-specific resistance has been shown in other host-specific downy mildew pathosystems, including lettuce (32), Arabidopsis (11), and sunflower (24), and this form of resistance has been shown to be overcome by virulent isolates in each pathosystem. Although race-specific resistance has not previously been demonstrated in Vitis spp., isolates of P. viticola vary in their quantitative virulence on different cultivars of Vitis (12). Novel sources and mechanisms of disease resistance can be found among the genetically diverse accessions maintained by grapevine germplasm repositories (22). There are a number of repositories around the world and, in the United States, two United States Department of Agriculture– Agricultural Research Service (USDAARS) locations maintain a combined total of over 4,100 Vitis accessions. V. vinifera and cold-sensitive species are among the major holdings at the National Clonal Germplasm Repository near Davis, CA, whereas the cold-hardy species are maintained at the Plant Genetic Resources Unit (PGRU) in Geneva, NY. One limitation of screening for disease resistance in a Vitis germplasm repository is that the accessions are maintained as living grapevines, and the health status of the vines is critical to their long-term preservation. Therefore, screening for resistance in the repository vineyard is not routinely possible. In this study, two methods were tested for screening diverse accessions of Vitis spp. for resistance to downy mildew. In the first, a subset of the cold-hardy germplasm collection was vegetatively propagated, established in a vineyard, and natural epidemics of downy mildew were allowed to progress. In the second screen, leaves were directly collected from the germplasm repository, surface sterilized, and inoculated in petri dishes using a single isolate.