Reemergence of Hop Powdery Mildew (Podosphaera macularis) in Wisconsin

Hop (Humulus lupulus L.) is a perennial climbing bine grown worldwide in temperate regions that produces cones that are a crucial flavoring and aroma component of beer. Although the hop powdery mildew pathogen (Podosphaera macularis) is cosmopolitan and coincident with the range of hop, the disease has not been reported in Wisconsin for several decades, but has now reemerged as hop was widely planted to meet demands of an expanding craft brewing industry. In August of 2016 in Marathon County, hop plants of the cultivar Challenger exhibited signs of powdery mildew on cones and leaves. An estimated 75 to 100% of Challenger plants showed signs of the disease, though this cultivar only represented ∼13% of the total hop yard. Hop plants of the cultivars Galena, Southern Cross, and Chinook were found exhibiting similar signs on leaves in July of 2017 in a Dodge County hop yard. An estimated 5% of hop plants in this hop yard showed symptoms. Mildew colonies on adaxial leaf surfaces were discrete, roughly circular, white in color, and 2 to 3 mm in size. In advanced cases, fungal growth covered the entire leaf surface. Infected cones exhibited white, powdery growth externally and underneath bracts and bracteoles, and some cones exhibited brown discoloration and stunting. Hyphae were hyaline, flexuous or straight, and septate with diameters ranging from 5.0 to 7.5 μm (n = 17). Conidiophores stood erect from plant tissue, were 30.8 to 78.7 μm in length (n = 29) and bore conidia produced in chains. Conidia measured 24.1 to 31.6 × 15.5 to 22.4 μm (n = 50), were ovoid-ellipsoid or barrel-shaped, and had visible fibrosin bodies. No chasmothecia were observed. Based on fungal morphology and host, the causal agent was putatively identified as Podosphaera macularis (Wallr.) U. Braun & S. Takam (Mahaffee et al. 2009). To confirm this identification, the internal transcribed spacer (ITS) region of the rDNA was amplified using universal ITS1/ITS4 primers and sequenced (White et al. 1990). The resulting 525-bp sequence was deposited into GenBank (accession no. KY022479). A GenBank BLAST search showed 100% sequence identity to P. macularis on H. lupulus (AB525917). Mating type was determined via qPCR, using primer pairs in multiplex specific to regions of the mating loci reported in Wolfenbarger et al. (2015). All six isolates from 2017 were the alpha mating type, MAT1-1. In the Pacific Northwest, losses from powdery mildew have been estimated at 15%, with much greater impact in susceptible cultivars (Mahaffee et al. 2003). Establishment of overwintering populations of P. macularis in Wisconsin hop yards, particularly populations harboring both mating types or races virulent on cultivars currently resistant to the pathogen, would present a significant and new challenge to expansion of hop production in the state (Gent et al. 2008). Increased vigilance in sourcing nursery stock, careful selection of cultivars, and frequent inspections to detect disease early in the epidemic cycle is warranted to limit potential crop damage from the disease in Wisconsin.