Yard age, cultivar susceptibility, and spring pruning practices as risk factors for overwintering of Podosphaera macularis on hop

Hop powdery mildew, caused by Podosphaera macularis, may be initiated by sexual or asexual spores of the pathogen, but only asexual reproduction has been reported in the Pacific Northwest United States. In this region, overwintering is via bud perennation, leading to “flag shoots” the following season. We expanded on previous research and analyzed 766 location-years of flag shoot data from Oregon hop yards collected during 2014 to 2020, examining potential risk factors of yard age, cultivar susceptibility to powdery mildew, pruning method (mechanical means or not), and pruning thoroughness. Bayesian logistic regression models were fitted to estimate the probability of flag shoot presence or absence. We found little evidence to support yard age as a significant risk factor for flag shoot presence, as the 95% credible interval for the parameter associated with yard age was consistently centered around 0 in each of our models. However, cultivar susceptibility to powdery mildew, spring pruning method, and spring pruning thoroughness were associated with increases in the probability of observing a flag shoot. Whereas yard age was not an important risk factor, there was an interrelationship between yard age, pruning method, and pruning thoroughness that suggests second-year yards may indirectly be at an elevated risk for flag shoot development because these yards are less often pruned using mechanical methods. Our research further identifies yards that are most at risk for overwintering of P. macularis, and reinforces the importance of thorough pruning in yards with an elevated risk for possessing a flag shoot.