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 Dr. 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.
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 suggest the presence of multiple, race-specific resistance genes segregating independently in rotundifolia and aestivalis progeny and suggest that some of the resistance genes would be rapidly overcome if inappropriately deployed. However, some progeny were resistant regardless of the pathogen source, suggesting the presence of all parental resistance alleles as a resistance gene pyramid. The stability of resistance in these individuals and the pathogen-dependent resistance of other individuals were confirmed in 2007. The rotundifolia and aestivalis breeding populations underscore one critical application of marker assisted selection â€“ monitoring and pyramiding all functional resistance genes using a simple molecular assay rather than assaying resistance and durability by complex inoculation studies with multiple pathogen sources.
We also confirmed in 2007 that either of the two putative resistance genes from the davidii resistance source is sufficient for resistance regardless of pathogen source; these genes have 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, and this appears to hold true with davidii.
To address the second objective, we require molecular markers that are polymorphic (appear different between the two parents) to track regions of the genome that were contributed to progeny by the resistant parent. We have identified 157 Simple Sequence Repeat markers (SSRs) that are polymorphic in these populations. Thus far, we have developed multiplexes for 39 SSRs and used them to screen all progeny in the three populations. In addition, we have identified amplified fragment length polymorphism (AFLP) markers associated with resistance in each of the populations. Our preliminary results support the two-gene models suggested by phenotypic data for the davidii and rotundifolia populations. Marker-trait associations in the aestivalis population will require QTL analysis.
Upon confirmation of which polymorphic markers predict disease resistance, we will focus on providing tightly-linked markers flanking disease resistance genes. From crosses representing each resistance source, we have germinated at least 600 seed and will test the utility of our markers for MAS, while using recombinants to more precisely track resistance genes.