Identification and distribution of the mating-type idiomorphs in populations of Podosphaera macularis and development of chasmothecia of the fungus

Authors: WOLFENBARGER, SIERRA - Oregon State University; TWOMEY, MEGAN - Oregon State University; GADOURY, DAVID - Cornell University; Knaus, Brian; Grunwald, Niklaus - Nik; Gent, David - Dave

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2014
Publication Date: 9/15/2015
Citation: Wolfenbarger, S.N., Twomey, M.C., Gadoury, D., Knaus, B.J., Grunwald, N.J., Gent, D.H. 2015. Identification and distribution of the mating-type idiomorphs in populations of Podosphaera macularis and development of chasmothecia of the fungus. Plant Pathology. 64:1094-1102.

Interpretive Summary: The fungus that causes powdery mildew on hop is produces overwintering structures referred to as cleistothecia in eastern North America and Europe. The structures allow the pathogen to persist in the absence of living tissue of the host and increase its genetic diversity. Cleistothecia have not yet been reported from the Pacific Northwestern region of North America. We established that the fungus requires two distinct forms (mating types) for cleistothecia to form, but only one of the two mating types appears to exist in the Pacific Northwestern U.S. However, the strains of the powdery mildew fungus present in the Pacific Northwest are fully capable of producing cleisthothecia when paired with the necessary second mating type. The findings are important because they suggest that efforts should be directed to limiting the likelihood that the second mating type of the fungus is introduced to the region, which is predicted to make powdery mildew more difficult and costly to manage in both the near and long-term.

Technical Abstract: Podosphaera macularis, the causal agent of hop powdery mildew, is known to produce cleistothecia (syn. chasmothecia) in eastern North America and Europe. Ascocarps have not yet been reported from the Pacific Northwestern region of North America. Reasons for the apparent absence of cleistothecia in the Pacific Northwest were unknown. We established that P. macularis is heterothallic and ascocarp ontogeny, maturation, dehiscence, and ascospore infection proceed 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 Pacific Northwest MAT1-1 isolate 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.