What explains hop growers’ fungicide use intensity and management costs in response to powdery mildew?

Authors: Hwang, Jae Young; BHATTACHARYYA, SHARMODEEP - Oregon State University; CHATTERJEE, SHIRHSHENDU - City University Of New York; MARSH, THOMAS - Washington State University; PEDRO, JOSHUA - City University Of New York; Gent, David

Submitted to: American Phytopathology Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2024
Publication Date: 10/15/2024
Citation: Hwang, J., Bhattacharyya, S., Chatterjee, S., Marsh, T.L., Pedro, J.F., Gent, D.H. 2024. What explains hop growers’ fungicide use intensity and management costs in response to powdery mildew? American Phytopathology Society. 114(10):2287-2299. https://doi.org/10.1094/PHYTO-04-24-0127-R.
DOI: https://doi.org/10.1094/PHYTO-04-24-0127-R

Interpretive Summary: Observational data are commonly collected in scientific studies because of impracticalities or ethical considerations that prevent conduct of randomized control studies. Interpretation of observational data requires special care during data analysis to understand and make inferences on the causes of observed responses. In this research, we analyzed an extensive data collected from commercial hop yards in Oregon to understand correlates and causes of pesticide use intensity and the costs growers incur when managing the disease powdery mildew. The correlates of pesticide use intensity are multifaceted, but we find that ultimately biological and production factors moderate the incidence of powdery mildew, which has a direct relationship on the number of pesticide active constituents that growers apply and their costs. Our analyses point to several potential strategies for reducing pesticide use and costs for powdery mildew on hop. We also highlight the utility of our methods for related analyses in other observational studies.

Technical Abstract: Methods for causal inference from observational data are common in human disease epidemiology and social sciences but are used relatively little is plant pathology contexts. We draw upon an extensive data set of the incidence of hop plants with powdery mildew (caused by Podosphaera macularis) collected from yards in Oregon during 2014 to 2017 and associated metadata on grower cultural practices, cultivar susceptibility to powdery mildew, and pesticide application records to understand variation in and causes of growers’ fungicide use and their associate costs. An instrumental causal forest model identified growers’ spring pruning thoroughness, cultivar susceptibility to two of the dominant pathogenic races of P. macularis, network centrality of a yards during May-June and June-July time transitions, and the initial strain of the fungus detected as important variables determining the number of pesticide active constituents applied by growers and the associated costs they incurred in response to powdery mildew. Exposure-response function models fit after covariate weighting indicated both the number of pesticide active constituents applied and their associated costs scaled linearly with the seasonal mean incidence of plants with powdery mildew. While the causes of pesticide use intensity are multifaceted, biological and production factors collectively influence the incidence of powdery mildew, which has a direct exposure-response relationship on the number of pesticide active constituents that growers apply and their costs. Our analyses point to several potential strategies for reducing pesticide use and costs for powdery mildew on hop. We also highlight the utility of these methods for causal inference in observational studies.