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Poster: Biology & Disease Mgmt: Genetics of Resistance


Adaptation to quantitative resistance in the hop cultivar Cascade by Podosphaera macularis
D. GENT (1), D. Gent (1), M. Twomey (2), S. Wolfenbarger (2) (1) USDA Agricultural Research Service, U.S.A.; (2) Oregon State University Department of Botany and Plant Pathology, U.S.A.

In the hop powdery mildew pathosystem, the cultivar Cascade has not been substantially affected by powdery mildew despite being produced on a relatively broad scale for decades. Beginning in 2012, development of powdery mildew on this cultivar under field conditions has become increasingly common in Washington State. Surveys conducted during 2013 to 2015 indicated increasing occurrence and severity of powdery mildew on Cascade, concomitant with increases in the number of fungicide applications made growers. Nearly all isolates of Podosphaera macularis sporulated when inoculated to Cascade, although the latent period was shortest, infection frequency greatest, and the fungus most fecund on Cascade when inoculum was originally derived from this cultivar as compared to other cultivars. Only 4.9% of isolates obtained from Cascade were able to infect cv. Nugget, which possess the resistance gene termed R6, indicating that Cascade-adapted strains of the fungus are distinct from strains that attack cultivars possessing R6. In growth chamber experiments, powdery mildew levels on various cultivars were similar when inoculated with Cascade-adapted strains of P. macularis as compared to non-adapted strains, further supporting a specific adaptation by the fungus to the quantitative resistance in Cascade. Multiple R-genes are predicted to provide resistance to Cascade-adapted strains of the fungus, as is quantitative resistance found in other germplasm.