Sierra N. Wolfenbarger

Powdery Mildew Outbreaks caused by Podosphaera macularis on Hop Cultivars Possessing the Resistance Gene R6 in the Pacific Northwestern United States

Resistant cultivars of hop (Humulus lupulus) have been grown, with the aim of helping to manage powdery mildew in the Pacific Northwest since the first report of the disease in the field in 1997 (4). A major objective of many breeding programs is development of resistance to powdery mildew, and this has generally been achieved by single resistance genes (qualitative resistance). One such gene, R6 (3), has been utilized extensively in new cultivars and has prevented epidemics of the disease in those cultivars across the Pacific Northwestern United States for approximately 15 years. In 2011, a grower in Washington State reported outbreaks of powdery mildew on cv. Apollo, which is thought to possess powdery mildew resistance derived from R6. Fungicides and cultural control measures were applied, and the grower reported no substantial crop damage from the disease. During the winter of 2012, the same grower planted rhizomes of cv. Apollo in a greenhouse in the Yakima Valley of Washington State and later found the plants to be affected by powdery mildew. Affected leaves from plants of cvs. Apollo, Newport, and Nugget (all reported [3] or assumed to possess R6 based on pedigree) grown in the same greenhouse were later provided to the authors. Conidia obtained from each affected plants were transferred to plants of the highly susceptible cv. Symphony, which is not known to contain any resistance genes. After 10 to 14 days of incubation, resultant conidia from each cultivar above (total of three isolates) were transferred to greenhouse grown plants of cvs. Nugget and Symphony and incubated at 18°C. Within 7 days, all three isolates produced powdery mildew colonies characteristic of P. macularis (2) on both cultivars. Cleistothecia did not develop in any colonies. In addition, Nugget and Symphony plants were inoculated with a field population of P. macularis originating from cultivars lacking R6 in Oregon. These inoculations on Nugget did not develop powdery mildew whereas Symphony plants did. Non-inoculated controls remained free of powdery mildew. Results were identical in two additional experiments. The sequence of the mating type idiomorph, MAT1-1, was obtained to confirm identity of the pathogen as P. macularis as described previously (1). The sequences were identical among the three isolates obtained from the greenhouse in Washington and isolates of P. macularis obtained previously from Oregon and Washington. MAT1-2 idiomorph was not detected in the isolates collected. While R6-virulent strains have been detected previously in race characterization experiments, these strains have not caused widespread epidemics of powdery mildew. The increasing prevalence of virulent strains of P. macularis and outbreaks of powdery mildew on formerly resistant cultivars necessitates changes in breeding strategies and disease management efforts to minimize damage resulting from the disease. The distribution of virulent strains of the pathogen and susceptibility of formerly resistance cultivars to powdery mildew are currently under investigation. References: (1) B. Asalfet et al. Phytopathology 103:717, 2013. (2) R. Bélanger et al. The Powdery Mildews: a Comprehensive Treatise. APS Press, St. Paul, MN, 2002. (3) P. Darby. Brew Hist. 121:94, 2005. (4) C. Ocamb et al. Plant Dis. 83:1072, 1999.

Development of partial ontogenic resistance to powdery mildew in hop cones and its management implications.

Knowledge of processes leading to crop damage is central to devising rational approaches to disease management. Multiple experiments established that infection of hop cones by Podosphaera macularis was most severe if inoculation occurred within 15 to 21 days after bloom. This period of infection was associated with the most pronounced reductions in alpha acids, cone color, and accelerated maturation of cones. Susceptibility of cones to powdery mildew decreased progressively after the transition from bloom to cone development, although complete immunity to the disease failed to develop. Maturation of cone tissues was associated with multiple significant affects on the pathogen manifested as reduced germination of conidia, diminished frequency of penetration of bracts, lengthening of the latent period, and decreased sporulation. Cones challenged with P. macularis in juvenile developmental stages also led to greater frequency of colonization by a complex of saprophytic, secondary fungi. Since no developmental stage of cones was immune to powdery mildew, the incidence of powdery mildew continued to increase over time and exceeded 86% by late summer. In field experiments with a moderately susceptible cultivar, the incidence of cones with powdery mildew was statistically similar when fungicide applications were made season-long or targeted only to the juvenile stages of cone development. These studies establish that partial ontogenic resistance develops in hop cones and may influence multiple phases of the infection process and pathogen reproduction. The results further reinforce the concept that the efficacy of a fungicide program may depend largely on timing of a small number of sprays during a relatively brief period of cone development. However in practice, targeting fungicide and other management tactics to periods of enhanced juvenile susceptibility may be complicated by a high degree of asynchrony in cone development and other factors that are situation-dependent.

Distribution and Characterization of Podosphaera macularis Virulent on Hop Cultivars Possessing R6-Based Resistance to Powdery Mildew

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 with 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, and R5 but not R1 or R2. In addition, R6-virulent isolates were shown to infect differential cultivars reported to possess the R6 gene and also the R4 gene, although R4 has not yet been broadly deployed in the United States. R6-virulent isolates were not detected from the eastern United States 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 with plants lacking R6, indicating a fitness cost to the fungus. R6-virulent isolates also produced fewer conidia when compared with 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 virulence occurs regardless of host genotype. In field studies, powdery mildew was suppressed by at least 50% on plants possessing R6 as compared with those without R6 when coinoculated with R6-virulent and avirulent isolates. 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 U.S. Pacific Northwest.

Pre- and Postinfection Activity of Fungicides in Control of Hop Downy Mildew

Optimum timing and use of fungicides for disease control are improved by an understanding of the characteristics of fungicide physical mode of action. Greenhouse and field experiments were conducted to quantify and model the duration of pre- and postinfection activity of fungicides most commonly used for control of hop downy mildew (caused by Pseudoperonospora humuli). In greenhouse experiments, control of downy mildew on leaves was similar among fungicides tested when applied preventatively but varied depending on both the fungicide and the timing of application postinfection. Disease control decreased as applications of copper were made later after inoculation. In contrast, cymoxanil, trifloxystrobin, and dimethomorph reduced disease with similar efficacy when applied 48 h after inoculation compared with preventative applications of these fungicides. When fungicides were applied 72 h after inoculation, only dimethomorph reduced the sporulating leaf area similarly to preinoculation application timing. Adaxial chlorosis, necrosis, and water soaking of inoculated leaves, indicative of infection by P. humuli, were more severe when plants were treated with cymoxanil, trifloxystrobin, and dimethomorph 48 to 72 h after inoculation, even though sporulation was suppressed. Trifloxystrobin and dimethomorph applied 72 h after inoculation suppressed formation of sporangia on sporangiophores as compared with all other treatments. In field studies, dimethomorph, fosetyl-Al, and trifloxystrobin suppressed development of shoots with systemic downy mildew to the greatest extent when applied near the timing of inoculation, although the duration of preventative and postinfection activity varied among the fungicides. There was a small reduction in efficacy of disease control when fosetyl-Al was applied 6 to 7 days after inoculation as compared with protective applications. Trifloxystrobin had 4 to 5 days of preinfection activity and limited postinfection activity. Dimethomorph had the longest duration of protective activity. Percent disease control was reduced progressively with increasing time between inoculation and application of dimethomorph. These findings provide guidance to the use of fungicides when applications are timed with forecasted or post hoc disease hazard warnings, as well as guidance on tank-mixes of fungicides that may be suitable both for resistance management considerations and extending intervals between applications.

Adaptation to Partial Resistance to Powdery Mildew in the Hop Cultivar Cascade by Podosphaera macularis

The hop cultivar Cascade has been grown in the Pacific Northwestern U.S. and elsewhere with minimal input for management of powdery mildew (Podosphaera macularis) for nearly 15 years due to the putatively quantitative resistance in this cultivar. While partial resistance is generally thought to be more durable than qualitative resistance, in 2012, powdery mildew was reported on Cascade in Washington State. Field surveys conducted during 2013 to 2016 indicated increasing prevalence of powdery mildew on Cascade, as well as an increasing number of fungicide applications applied to this cultivar in Washington State. Nearly all isolates of P. macularis tested were able to infect Cascade in laboratory inoculations. However, the greatest number of colonies, most conidia produced, and the shortest latent period was only observed with isolates derived originally from Cascade, as compared with other isolates derived from other cultivars. Further, the enhanced aggressiveness of these isolates was only manifested on Cascade and not six other susceptible cultivars, further indicating a specific adaptation to Cascade by the isolates. There was no evidence of a known major R-gene in Cascade, as seven isolates of P. macularis with contrasting virulence all infected Cascade. Among 158 isolates obtained from hop yards planted to Cascade, only two (1.3%) were able to infect the cultivar Nugget, which possesses the resistance factor termed R6, indicating that isolates of P. macularis virulent on Nugget are largely distinct from those adapted to Cascade. Further, race characterization indicated Cascade-adapted isolates of P. macularis were able to overcome R-genes Rb, R3, and R5, but not other known R-genes. Therefore, multiple R-genes and other sources of partial resistance are expected to provide resistance to Cascade-adapted strains of the fungus. Given the plasticity of the powdery mildew fungus, breeding strategies for powdery mildew need to consider the potential for adaptation to both qualitative and partial resistance in the host.