Graduate Thesis Or Dissertation

Identification and distribution of the mating-type locus and development of cleistothecia of Podosphaera macularis

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  • Podosphaera macularis, causal agent of hop powdery mildew, is known to produce cleistothecia (syn. chasmothecia) in eastern North America and Europe, but ascocarps are not reported from the Pacific Northwestern region of North America. Reasons for the apparent absence of cleistothecia in the Pacific Northwest are unknown. We established that P. macularis is heterothallic and that ascocarp ontogeny, maturation, dehiscence, and ascopore infection proceeds similarly to other powdery mildew fungi. Genome sequencing of a MAT1-1 isolate revealed the structure of the MAT1 locus and presence of MAT1-1-3 demonstrating further similarities to other powdery mildew fungi. PCR assays with primers designed from conserved domains of the MAT1 idiomorphs were developed to characterise the frequency of idiomorphs in populations of P. macularis. Amongst 317 samples of P. macularis collected during 2012 and 2013 from the Pacific Northwest only the MAT1-1 idiomorph was found. In contrast, among 56 isolates from the eastern United States and Europe, MAT1-1 and MAT1-2 idiomorphs were detected at equivalent frequencies. At temperatures representative of late season conditions in the Pacific Northwest, cleistothecia formed readily when a MAT1-1 isolate from the Pacific Northwest was paired with a MAT1-2 isolate collected from outside the region. Although these findings do not encompass all climatic, geographic, or temporal barriers that could inhibit the formation of cleistothecia, the current absence of the ascigerious stage of P. macularis in the Pacific Northwest could be explained by the absence of the MAT1-2 mating type idiomorph.
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