Host resistance, both quantitative and qualitative, is the preferred long-term approach for disease management in many pathosystems, including powdery mildew of hop (Podosphaera macularis). In 2012, an epidemic of powdery mildew occurred in Washington and Idaho on previously resistant cultivars whose resistance was putatively based on the gene designated R6. In 2013, isolates capable of causing severe disease on cultivars with R6-based resistance were confirmed in Oregon and became widespread during 2014. Surveys of commercial hop yards during 2012 to 2014 documented that powdery mildew is now widespread on cultivars possessing R6 resistance in Washington and Oregon, and the incidence of disease is progressively increasing. Pathogenic fitness, race, and mating type of R6-virulent isolates were compared to isolates of P. macularis lacking R6-virulence. All isolates were positive for the mating type idiomorph MAT1-1 and were able to overcome resistance genes Rb, R3, R5, whereas virulent isolates also invariably could infect differential cultivars reported to possess R6 and R4, though R4 has not yet been broadly deployed in the U.S. R6-virulent isolates were not detected from the eastern U.S. during 2012 to 2015. In growth chamber studies, R6-virulent isolates of P. macularis had a significantly longer latent period and produced fewer lesions on plants with R6 as compared to plants lacking R6, indicating a fitness cost to the fungus. R6-virulent isolates also produced fewer conidia when compared to isolates lacking R6-virulence, independent of whether the isolates were grown on a plant with or without R6. Thus, it is possible that the fitness cost of R6-viruelence is exacted irrespective of the host genotype. In field studies, powdery mildew was suppressed by at least 50% on plants possessing R6 as compared to those without R6 when co-inoculated with R6-virulent and avirulent isolates in 2014. R6-virulence in P. macularis appears to be race-specific and, at this time, imposes a measurable fitness penalty on the fungus. Resistance genes R1 and R2 appear to remain effective against R6-virulent isolates of P. macularis in the Pacific Northwestern U.S.

The effect of variable temperature on the infection severity of Podosphaera macularis was investigated. Potted ‘Symphony’ hop plants were inoculated and exposed to different temperature regimes that included supraconducive temperatures (30 to 42°C) for varying periods of time (2 to 9 h). Infection severity (lesions per cm² of leaf area) was calculated 7 to 10 days after inoculation. Immediately exposing inoculated plants to 30°C for as little as 2 h significantly (P ≤ 0.05) reduced infection severity compared with exposure at a constant 18°C. However, exposure of inoculated plants to optimal conditions for 24 or 48 h prior to exposure to supraconducive conditions reduced this effect for plants exposed to 30, 33, or 36°C. Exposure to 39 or 42°C for 2 or more hours resulted in infection frequencies not significantly different from that of the uninoculated control regardless of prior exposure to favorable conditions. Exposure to simulated field temperatures programmed into growth chambers indicated that inoculation at 1700 or 2100 h resulted in significantly more disease than did inoculation at 0900 or 1300 h. Plants exposed to supraconducive or simulated field temperatures for 7 days prior to inoculation developed significantly lower disease severity than did plants maintained at 18°C for 7 days. The magnitude, length, and time of exposure to supraconducive temperatures in relation to time of inoculation plays an important role in the development of hop powdery mildew, and rules addressing these variables could be a useful addition to disease risk assessment models.

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The hop powdery mildew fungus Podosphaera macularis persists from season to season in the Pacific Northwestern United States through infection of crown buds because only one of the mating types needed to produce the ascigerous stage is presently found in this region. Bud infection and successful overwintering of the fungus leads to the emergence of heavily infected shoots in early spring (termed flag shoots). Historical data of flag shoot occurrence and incidence in Oregon and Washington State during 2000 to 2017 were analyzed to identify their association with the incidence of powdery mildew, growers’ use of fungicides, autumn and winter temperature, and other production factors. During this period, flag shoots were found on 0.05% of plants evaluated in Oregon and 0.57% in Washington. In Oregon, the incidence of powdery mildew on leaves was most severe and the number of fungicide applications made by growers greatest in yards where flag shoots were found in spring. Similarly, the incidence of plants with powdery mildew in Washington was significantly associated with the number of flag shoots present in early spring, although the number of fungicide applications made was independent of flag shoot occurrence. The occurrence of flag shoots was associated with prior occurrence of flag shoots in a yard, the incidence of foliar powdery mildew in the previous year, grower pruning method, and, in Washington, winter temperature. A census of hop yards in the eastern extent of the Oregon production region during 2014 to 2017 found flag shoots in 27 of 489 yards evaluated. In yards without flag shoots, 338 yards (73.2%) were chemically pruning or not pruned, whereas the remaining 124 (26.8%) were mechanically pruned. Of the 27 yards with flag shoots, 22 were either chemically pruned or not pruned and 4 were mechanically pruned in mid-April, well after the initial emergence of flag shoots. The prevalence of yards with flag shoots also was related to thoroughness of pruning in spring (8.1% of yards with incomplete pruning versus 1.9% of yards with thorough pruning). A Bayesian logistic regression model was fit to the data from the intensively assessed yards in Oregon, with binary risk factors for occurrence of a flag shoot in the previous year, occurrence of foliar mildew in the previous year, and thoroughness of pruning in spring. The model indicated that the median and 95% highest posterior density interval of the probability of flag shoot occurrence was 0.0008 (0.0000 to 0.0053) when a yard had no risk factors but risk increased to 0.0065 (0.0000 to 0.0283) to 0.43 (0.175 to 0.709) when one to all three of the risk factors were present. The entirety of this research indicates that P. macularis appears to persist in a subset of chronically affected hop yards, particularly yards where spring pruning is conducted poorly. Targeted management of the disease in a subset of fields most at risk for producing flag shoots could potentially influence powdery mildew development regionwide.

Black leg (caused by Plenodomus lingam and P. biglobosus) and chlorotic leaf spot (caused by Pyrenopeziza brassicae) are economically important fungal diseases of Brassicaceae crops. Surveys of seed fields and weed hosts were conducted to understand the distribution and prevalence of these diseases in Oregon after black leg and chlorotic leaf spot outbreaks occurred in Brassicaceae crops in 2014. Postharvest black leg ratings were conducted in seed fields of canola, forage rape, and turnip in 2015 and 2016. The incidence of black leg was greater for turnip (51%) than for canola (29%) and forage rape (25%). The overall average disease incidence was greater for seed crops harvested in 2015 (46%) than for crops harvested in 2016 (28%). A disease survey of wild Brassicaceae plants was conducted along Interstate 5 in Oregon. Brassicaceae weed population sites were identified and 40 sites were sampled for these diseases. Black leg and chlorotic leaf spot were present in 60 and 45%, respectively, of the sampled sites. Both species of Plenodomus were detected in weed populations, with P. lingam being the predominant species recovered (95%). The northernmost sample site with black leg was <32 km from the Oregon–Washington border, and the southernmost site with black leg was within 32 km of the Oregon–California border. Chlorotic leaf spot was detected <32 km from the Oregon–Washington border, whereas the southernmost site where it was detected was approximately 164 km from the Oregon–California border. Based on this study, infected crop residues and weed hosts may facilitate the persistence and spread of these pathogens.