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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #338901
Title: Infestation of hop seed (Humulus lupulus) by chasmothecia of the powdery mildew fungus, Podosphaera macularis

Author
item CLAASSEN, BRIANA - Oregon State University
item WOLFENBARGER, SIERRA - Oregon State University
item HAVILL, JOSHUA - University Of Minnesota
item ORSHINSKY, ANGELA - University Of Minnesota
item Gent, David - Dave

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2017
Publication Date: 5/5/2017
Citation: Claassen, B.J., Wolfenbarger, S.N., Havill, J.S., Orshinsky, A.M., Gent, D.H. 2017. Infestation of hop seed (Humulus lupulus) by chasmothecia of the powdery mildew fungus, Podosphaera macularis. Plant Disease. 101(7):1323.

Interpretive Summary: Powdery mildew is an important disease of hop in the primary production regions of the crop in the Pacific Northwestern U.S. The fungus requires two distinct mating types to complete its life cycle and reproduce sexually. One of the two mating types is known to occur in the Pacific Northwest at present. In the autumn of 2015, seed was collected from wild hop plants at 7 locations in Minnesota for grow out and evaluation of various traits. Prior to planting, seeds were examined and found to be infested with the sexual stage of the powdery mildew fungus. Microscopic examination and genetic evidence indicated the organism present was indeed the hop powdery mildew fungus. To our knowledge, this is the first report of infestation of hop seed by the sexual, survival structures of the pathogen. Current quarantine laws that restrict import of planting materials for hop into Idaho, Oregon, and Washington explicitly exempt seed. However, seed infested with the pathogen potentially could introducing novel isolates and mating types of the pathogen. Seed transmission of powdery mildew organisms is scarcely reported, and studies are needed to determine the risk of disseminating the pathogen on infested seed. Until such information is available, caution is advised when moving seed from regions where powdery mildew occurs.

Technical Abstract: Powdery mildew is responsible for large economic losses in hop in the primary production regions of the crop in the Pacific Northwestern U.S. (Gent et al. 2008). Podosphaera macularis is heterothallic, but to date only the MAT1-1 mating type has been confirmed in the Pacific Northwest (Wolfenbarger et al. 2015) and ascocarps of the fungus have not been observed in this region (Gent et al. 2006). In the autumn of 2015, seed was collected from wild hop plants at 7 locations in Minnesota for grow out and evaluation of various traits. Prior to planting, seeds were examined under low magnification (30-50×) and in 9 of the 11 seedlots, representing 4 of 7 locations, the seed was found to be externally infested with spherical to flattened, black chasmothecia. In infested lots, the number of seed bearing chasmothecia averaged 45% (range 5 to 89%; n = 107 to 200 seeds per lot). Scanning electron microscopy indicated chasmothecia had a mean diameter of 82µm and were shriveled with a concaved base. Chasmothecia were easily dislodged from the seed coat despite the appendages being embedded in a mat of pannose mycelium. Conidiophores and conidia were not observed. The morphological characters were consistent with the genus Podosphaera (Braun et al. 2002; Wolfenbarger et al. 2015). Chasmothecia were confirmed as P. macularis by extracting DNA from 10 to 15 seeds from each of 6 seedlots using a DNeasy PowerSoil Kit (Qiagen, Carlsbad, CA) and amplifying and sequencing the MAT1-1 and MAT1-2 idiomorphs as described by Wolfenbarger et al. (2015). Standard nucleotide BLAST searches in GenBank indicated that the sequences were 97% similar to MAT1-1 (accession KJ922755.1) and 100% similar to MAT1-2 (accession KJ741396.1) sequences of P. macularis. To our knowledge, this is the first report of infestation of hop seed by chasmothecia of P. macularis. Current quarantine laws that restrict import of planting materials for hop into Idaho, Oregon, and Washington explicitly exempt seed. However, seed infested with chasmothecia potentially may spread the pathogen.