A Comprehensive Characterization of Ecological and Epidemiological Factors Driving Perennation of Chasmothecia on Hop ().

Hop powdery mildew, caused by the ascomycete fungus , is a consistent threat to sustainable hop production. The pathogen utilizes two reproductive strategies for overwintering and perennation: (i) asexual vegetative hyphae on dormant buds that emerge the following season as infected shoots; and (ii) sexual ascocarps (chasmothecia), which are discharged during spring rain events. We demonstrate that chasmothecia, in the absence of any asexual growth forms, are a viable overwintering source capable of causing early season infection two to three orders of magnitude greater than that reported for perennation via asexual growth. Two epidemiological models were defined that describe (i) temperature-driven maturation of chasmothecia; and (ii) ascosporic discharge in response to duration of leaf wetness and prevailing temperatures. ascospores were confirmed to be infectious at temperatures ranging from 5 to 20°C. The organism's chasmothecia were also found to adhere tightly to the host tissue on which they formed, suggesting that these structures likely overwinter wherever hop tissue senesces within a hop yard. These observations suggest that existing early season disease management practices are especially crucial to controlling hop powdery mildew in the presence of chasmothecia. Furthermore, these insights provide a baseline for the validation of weather-driven models describing maturation and release of ascospores, models that can eventually be incorporated into hop disease management programs.