Accelerating the Development of Powdery Mildew Resistant Grapevines through Marker Assisted Selection

Summary of year one results: The aim of this project is to harness molecular biology in the selection and advancement of improved cultivars having resistance to powdery mildew. Segregating populations from three sources of significant powdery mildew resistance (Vitis davidii, V. rotundifolia, and V. aestivalis), each backcrossed to V. vinifera, were previously generated by David Ramming. The first objective of this proposal is to characterize the plant-pathogen interactions, in terms of race-specificity and microscopic analysis, for each of the three resistance sources in order to inform the second objective, which is the development of molecular markers that co-segregate with powdery mildew resistance in each of these populations for use by grape breeding programs.

In 2006, powdery mildew resistance was assessed in 182 progeny from the three populations using three separate pathogen sources in California and New York. The resulting data suggests the presence of multiple race-specific resistance genes segregating in two of the populations. A subset of rotundifolia and aestivalis progeny resistant in the vineyard were susceptible to one or both pathogen sources. Therefore, we hypothesize and have data suggesting that these resistant parents contribute two (or more) resistance genes that segregate independently in their progeny and that some of the resistance genes would be rapidly overcome if inappropriately deployed. In contrast, some progeny were resistant regardless of the pathogen source, suggesting the presence of all parental resistance alleles (as a resistance gene pyramid). These results will be confirmed using powdery mildew isolates with differential interactions with the resistance genes. These two populations underscore the value of marker assisted selection, with which we will be able to monitor and pyramid all functional resistance genes using a simple molecular assay rather than assaying resistance and durability by complex inoculation studies with multiple pathogen sources.
We have not identified race-specificity in the davidii resistance source, which has the added intrigue of providing resistance against the penetration of the fungus (i.e., the pathogen is unable to access the epidermal cells where it must obtain sustenance to survive). Most powdery mildew penetration resistance genes are effective against all races of powdery mildew.

To address the second objective, we require molecular markers that are polymorphic (appear different between the two parents), so that we can track groups of genes, or regions of the genome, that were contributed by the resistant or the susceptible parent to individual progeny. For the first phase of the project, we are using Simple Sequence Repeat markers, or SSRs. In the first year, we were able to test 160 SSRs against each of the parents in these three populations, and we successfully identified polymorphic markers for each linkage group in each population. In each population over 100 SSRs were informative for mapping. We will initiate a second round of marker exploration to enhance marker coverage and will identify which polymorphic markers predict disease resistance to target further marker saturation for each population.