Daniel Pap, Summaira Riaz, Alan Tenscher, and M. Andrew Walker*
*Department of Viticulture and Enology, University of California, Davis, CA 95616 (walker@ucdavis.edu)

Virtually all Vitis vinifera cultivars are highly susceptible to grape powdery mildew, which is caused by the biotrophic fungal pathogen Erysiphe necator. Discovery of new and unique resistance loci and their use in breeding programs will lead to sustainable and economic control of powdery mildew. Breeding populations can be used to generate genetic maps and to associate traits such as resistance with genetic markers, which can then be used as rapid and powerful tools for selection. While genetic mapping and DNA analysis can be semi-automated and generalized among crops, measuring or describing the trait of interest (phenotyping) is crop- or disease-specific. Developing reliable and rapid phenotyping systems is critically important for breeding and for developing accurate genetic markers. We compared three screening assays for grape powdery mildew resistance: field evaluations, a greenhouse assay with natural and supplemental powdery mildew inoculum, and a detached leaf in vitro assay. All three systems were measured using the one to six categorical scale devised by the International Organization of Vine and Wine (OIV). Data from the in vitro leaf assay was also compared with qRT-PCR by measuring the relative cDNA of the fungal tissue. All methods were assessed on a F₁ breeding population from a cross between susceptible V. vinifera F2-35 and resistant V. piasezkii DVIT2027. The variance and correlation among phenotype scores and the impact of different methods on detecting a resistance locus on a genetic map were evaluated. We concluded that scoring powdery mildew symptoms on detached leaves under a microscope is the most rapid and effective way to evaluate a segregating population.

Funding Support: Comparison of Different Screening Techniques for Grape Powdery Mildew Resistance