Characterization of Podosphaera macularis Derived from the Hop Cultivar ‘Strata’ and Strata’s Resistance to Powdery Mildew in Oregon

Abstract

Powdery mildew is one of the most destructive diseases of hop. Host resistance is a most efficient means of managing the disease, but resistance may not be durable. The cultivar Strata was developed by Oregon State University, and the patent application stated the plant was resistant to powdery mildew based on initial greenhouse screening and 8 years of observations at three locations in western Oregon. In 2019, powdery mildew colonies were reported in a commercial yard of Strata near Woodburn, Oregon. Two clonal isolates of Podosphaera macularis were isolated via single-chain transfers from colonies and characterized by inoculating each of seven differential cultivars. Cultivars with Rb, R3, and R5 were susceptible to the isolates. The susceptibility of Strata to powdery mildew was characterized by inoculating seven isolates of P. macularis with known virulences onto leaves of Strata. Sporulation appeared on Strata leaves with all of the differential isolates independent of their specific virulence, but only at trace levels with isolates possessing V6-virulence. Strata-derived isolates were phenotypically indistinguishable from Cascade-adapted isolates when inoculated onto the cultivars Cascade, Strata, and Symphony, as only Strata-derived and Cascade-adapted isolates could cause severe disease on Cascade and Strata. Like Cascade-adapted isolates, Strata-derived isolates also lack virulence on hop plants possessing the R-gene R6. These findings should inform both disease risk assessment in the field and future breeding tactics and strategies involving Strata. Further characterization of Strata is warranted to determine if its resistance is under the same genetic control as Cascade.