Submitted to: HortScience
Publication Type: Abstract Only
Publication Acceptance Date: 7/20/2005
Publication Date: 7/20/2005
Citation: Smith, J.M., Hummer, K.E., Mahaffee, W.F. 2005. Atypical disease response to hop powdery mildew [abstract]. Hortscience. 40(4):1034. Interpretive Summary: Open-pollinated hop seeds were collected from the United States, Canada, and Kazakhstan and screened for resistance to hop powdery mildew. More than 2,000 seedlings were tested multiple times with high levels of mildew. In the greenhouse, most seedlings were very susceptible and had white powdery mildew on both sides of their leaves. Three seedlings from Kazakhstan, remained uninfected. After further experimentation in growth chambers, these seedlings were unusually resistant. Six days after exposure to mildew, small dead regions of the upper leaf surface could be seen. Microscopic examination showed that these brown regions started at the center of the infection and radiated out. This is opposite the usual pattern in resistant plants where a dead zone develops around the fungus preventing its spread. This other kind of resistance could be useful in broadening the approach for developing new hop cultivars. The infection level of Kazakhstani genotypes was in between two other well known hop varieties.
Technical Abstract: Open-pollinated hop seeds (Humulus lupulus var. lupuloides E. Small, H. l. var. pubescens E. Small , H. l. var. neomexicanus Nelson and Cockerell and H. l. var. lupulus L.) were collected from the United States, Canada, and Kazakhstan and screened for resistance to hop powdery mildew (Podosphaera macularis Braun & Takamatus). A total of 2,108 seedlings were repeatedly inoculated with high levels of P. macularis. Under greenhouse conditions, most seedlings exhibited a compatible disease response typical of susceptible hosts with sporulating colonies covering both leaf surfaces. Three genotypes from hop seed native to Emba, Kazakhstan, remained uninfected in greenhouse assays. Further experimentation of those genotypes in growth chambers at 18°C revealed that they demonstrated an atypical disease response. Six days after inoculation, necrotic lesions on the adaxial leaf surfaces were visually apparent. Microscopic examination showed areas of collapsed epidermal cells, collapsed hyphae, and golden-brown discolorations extending out from the center of the infection. These symptoms contrasted those of a hypersensitive response in which fungal growth is restricted by the collapse of epidermal cells in advance of the fungus. The infection frequency of Kazakhstani genotypes was intermediate compared to H. l. cvs. Symphony (susceptible), and Nugget (resistant) after exposure to 3 pre-inoculation temperature regimes. These genotypes could represent a new source of polygenic resistance to hop powdery mildew, and could broaden hop germplasm available to global breeding programs.