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

Authors: LAURIE, ROBERT - Oregon State University; RICHARDSON, BRIANA - Oregon State University; ROSS, CAMERON - Oregon State University; Gent, David - Dave

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2022
Publication Date: 12/6/2022
Citation: Laurie, R.W., Richardson, B.J., Ross, C.J., Gent, D.H. 2022. Yard age, cultivar susceptibility, and spring pruning practices as risk factors for overwintering of Podosphaera macularis on hop. PhytoFrontiers. https://doi.org/10.1094/PHYTOFR-10-22-0112-R.
DOI: https://doi.org/10.1094/PHYTOFR-10-22-0112-R

Interpretive Summary: Powdery mildew is one of the most important diseases affecting hop in major production region of the crop. An important aspect of disease management is reducing the amount of the fungus that survives the winter because this tends to delay development of the disease. In this research, we identified and mathematically modeled factors that influence the likelihood that the powdery mildew fungus will survive in a given hop yard overwinter. We found that how susceptible a given variety is to the disease and certain cultural practices used by hop producers have a large effect on survival of the fungus from year-to-year. The age of a hop yard did not directly influence the likelihood of survival of the fungus. However, yard age may have an indirect link to the risk of the fungus surviving because of how growers change their cultural practices in young versus established hop yards. Overall, this research is important because it helps growers understand when and where the powdery mildew is most likely to survive, and points them to cultural practices that can mitigate this risk.

Technical Abstract: Powdery mildew of hop, caused by the fungal pathogen Podosphaera macularis, is an economically important disease in the United States hop industry. Epidemics may be initiated by both sexual and asexual spores of the pathogen, but only asexual reproduction of the fungus has been reported in the major hop growing region of the United States, the Pacific Northwest. In the absence of formation of sexual reproduction, survival of the fungus requires overwintering in association with infected crown buds on hop plants, which subsequently emerge as colonized “flag shoots” the following season. Previous research identified risk factors for flag shoot emergence in established hop yards to include the prior occurrence of flag shoots, prior occurrence of powdery mildew, pruning thoroughness, and in some instances, winter temperature. We expanded on this work to analyze flag shoot data from 766 location-years of data from Oregon hop yards collected during 2014 to 2020, and examined the potential risk factors of age of the yard, cultivar susceptibility to powdery mildew, pruning method (mechanical means or not), and pruning thoroughness. We suspected that second-year yards were at greater risk of flag shoot occurrence than established yards due to environmental conditions and varied cultural practices applied to second-year yards. Within the 766 location-years, 46 yards (6%) harbored flag shoots at densities ranging between 0.001 and 0.141 flag shoots per plant. A series of Bayesian logistic regression models were fitted to estimate the probability of flag shoot presence or absence. We found little evidence to support age of yards as a significant risk factor for flag shoot presence, as the 95% credible interval for the beta 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 in our analyses, there was an interrelationship between age of the yard, 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 points to the fundamental importance of thorough pruning in yards that have an elevated risk for possessing a flag shoot.