Characterization of Resistance to Powdery Mildew in the Hop Cultivars Newport and Comet

Hop powdery mildew, caused by Podosphaera macularis, has become an important disease in the Pacific Northwest since introduction of the pathogen in 1997. Resistant cultivars of hop (Humulus lupulus) have been developed and typically utilize monogenic resistance. One such gene, R6, provided field immunity to the disease until virulent strains of P. macularis emerged and became widespread in 2012 (3). Due to the increase of powdery mildew on formerly resistant cultivars, studies were conducted to characterize potential sources of host resistance. The genetic basis for resistance in Comet (4) and in Newport (2) is unknown despite these cultivars being frequently utilized in breeding programs. Comet is derived from a powdery mildewresistant mother (Sunshine) and a susceptible male. Newport is derived from two fully susceptible parents, but has been suggested to possess R6-based resistance (1). Field observations suggest that Comet and Newport may possess quantitative and/or qualitative resistance against R6-virulent strains of the pathogen. To characterize the resistance in these cultivars, 13 clonal isolates of P. macularis with known virulence were inoculated onto leaves of greenhouse-grown Comet and Newport plants. After 21 days of incubation, powdery mildew developed on Newport only when challenged with isolates of P. macularis possessing R4 and R6 virulence. It was not possible to differentiate between R4and R6-based resistance because an isolate of the pathogen possessing only one of these virulence factors is not extant in culture collections. On Comet, only minute powdery mildew lesions developed when challenged with R3virulent isolates of the fungus. Results were identical in two additional runs of the experiment. To determine if Newport may possess quantitative resistance to powdery mildew, 12 detached leaves were inoculated with a suspension containing a mixture of three isolates of P. macularis possessing R6 virulence at a total concentration of 10 conidia/mL in 0.05% Tween (Sigma-Aldrich, St. Louis, MO) plus water. Infection severity, lesion diameter, and conidia production were quantified 10 days later to observe quantitative responses from the cultivars. Cultivars Nugget (moderately susceptible and possessing R6 [1]) and Symphony (no known R genes) were included for comparison. The experiment was conducted three times. All measures of quantitative resistance were reduced on Newport as compared to the other cultivars (Fig. 1). Powdery mildew had not been reported on Newport prior to the emergence of R6-virulent strains of the of P. macularis. These findings suggest that Newport possesses R6and/or R4-based resistance, as well as some degree of quantitative resistance. Comet appears to possess R3. That only small, sparse colonies develop on Comet when challenged with R3-virulent isolates suggests that Comet may also possess quantitative resistance. This finding is supported by observations that powdery mildew rarely develops on Comet under field conditions in the Pacific Northwest even though R3-virulent strains are thought to be widespread. Both cultivars appear to have useful levels of quantitative resistance to powdery mildew that could be utilized in breeding programs against emerging virulent strains of P. macularis. Inheritance studies are warranted to verify these findings.