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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #336996

Title: Homothallism in Pseudoperonospora humuli

item Gent, David - Dave
item COHEN, YIGAL - Bar-Ilan University
item RUNGE, FABIAN - University Of Hohenheim

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2017
Publication Date: 3/21/2017
Citation: Gent, D.H., Cohen, Y., Runge, F. 2017. Homothallism in Pseudoperonospora humuli. Plant Pathology. doi: 10.1111/ppa.12689.

Interpretive Summary: Downy mildew is an important disease of hop worldwide. The pathogen produces a specialized survival spored called an oospore.In some pathogens, this spore structure can be produced by a single individual while in other organisms two, compatible mating types are required. We found that oospores could be produced by individual cultures of the hop downy mildew pathogen. This is what is referred to as homothallism. However, multiple attempts to generate disease with the oospores failed. This finding is important because it will inform future studies on the genetics and ecology of the pathogen.

Technical Abstract: The hop downy mildew pathogen, Pseudoperonospora humuli, forms oospores abundantly in diseased hop tissue. Diverse monosporangial isolates of P. humuli collected from Japan, Germany, and five states in the USA readily formed oospores within hop leaves when inoculated singly, suggesting homothallism. Single zoospore isolates also readily formed oospores within hop leaves, further supporting the homothallic nature of this oomycete. The majority of oospores were deemed viable based morphological characteristics (densely organized and uniformly granular cytoplasm) and plasmolysis when exposed to 4 M sodium chloride. However, downy mildew failed to develop when hop leaves were inoculated with newly formed oospores. Attempts to condition oospores by treatment with potassium permanganate or glusulase, brief exposure to freezing temperature, or passage through an earthworm did not lead to infection of hop leaves. Oospores derived from a monosporangial isolate of P. humuli and overwintered outdoors in infested leaves buried in soil also failed to infect. In contrast to the heterothallic sister species Pseudoperonospora cubensis, P. humuli is homothallic and oospores of the organism appear to require as yet unknown conditions to stimulate their germination and/or infection.