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Staying one step ahead of a pathogen: Hop powdery mildew in the Pacific Northwest.
S. WOLFENBARGER (1), M. C. Twomey (1), D. M. Gadoury (2), B. J. Knaus (3), N. J. Grunwald (4), D. H. Gent (5). (1) Oregon State University, Department of Botany and Plant Pathology, Corvallis, OR, U.S.A.; (2) Cornell University, Department of Plant Pathology and Plant-Microbe Biology, Geneva, NY, U.S.A.; (3) US Department of Agriculture-Agricultural Research Servi

In the Pacific Northwestern region (PNW) of the United States, hop powdery mildew, caused by <i>Podosphaera macularis,</i> is not known to produce ascocarps despite forming ascocarps in Europe and the eastern U.S. Sexual reproduction in <i>P. macularis</i> and other Ascomycetes is regulated by the mating type locus <i>MAT1</i>. Pairing of the two locus idiomorphs (<i>MAT1-1</i> and <i>MAT1-2) </i>is required for sexual reproduction. Thus, presence of a single mating type idiomorph in <i>P.macularis</i> populations of the PNW would explain the absence of ascocarps. We developed a PCR assay to identify both <i>MAT1-1</i> and <i>MAT1-2</i> in <i>P. macularis</i>. Matings with <i>MAT1-2</i> isolates and PCR assays confirmed heterothallism and exclusive presence of <i>MAT1-1</i> within 317 PNW isolates of <i>P. macularis</i> collected in 2012 and 2013. In contrast, the frequency of both idiomorphs among 56 isolates gathered from the eastern United States and Europe approximated a 1:1 ratio (Chi square <i>P</i>=0.806). Controlled environment studies indicated that late season temperatures in the PNW are conducive to ascocarp formation. Microscopy and differential staining revealed a process of ascocarp development and ascospore maturation similar to that reported for other powdery mildews. Our findings can explain the absence of ascocarps in PNW, and have important implications for management of hop powdery mildew. Quarantine regulations in the PNW have been revised to delay, if not prevent the introduction of the second mating type.

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