Infestation of hop seed (Humulus lupulus) by chasmothecia of the powdery mildew fungus, Podosphaera macularis

Powdery mildew of hop (Humulus lupulus) is caused by Podosphaera macularis. Since 1997, the disease has frequently caused severe economic loss in production regions in the Pacific Northwestern region of the U.S., the primary area where hops are grown in the country (^†). P. macularis is heterothallic, but to date only the MAT1-1 mating type has been confirmed in the Pacific Northwest (^†) and ascocarps of the fungus have not been observed in this region (^†). In the autumn of 2015, seed was collected from wild hop plants at seven locations in Minnesota for grow out and evaluation of various traits. Prior to planting, seeds were examined under low magnification (30 to 50×) and in nine of the 11 seedlots, representing four of seven locations, the seed was found to be externally infested with spherical to flattened, black chasmothecia (syn. cleistothecia). 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 82 µm 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 (^†;^†). Chasmothecia were confirmed as P. macularis by extracting DNA from 10 to 15 seeds from each of six seedlots using a DNeasy PowerSoil Kit (Qiagen, Carlsbad, CA) and amplifying and sequencing the MAT1-1 and MAT1-2 idiomorphs as described by^†. The sequences obtained for MAT1-1 and MAT1-2 were identical among the extractions of the six seedlots. 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 (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 may spread the pathogen, potentially 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 P. macularis on infested seed. Until such information is available, caution is advised when moving seed from regions where powdery mildew occurs.