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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #356988
Title: Risk factors for bud perennation of Podosphaera macularis on hop

Author
item Gent, David - Dave
item Mahaffee, Walter - Walt
item Turechek, William
item OCAMB, CYNTHIA - Oregon State University
item TWOMEY, MEGAN - Oregon State University
item WOODS, JOANNA - Oregon State University
item PROBST, CLAUDIA - Washington State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2018
Publication Date: 7/31/2018
Citation: Gent, D.H., Mahaffee, W.F., Turechek, W., Ocamb, C.M., Twomey, M.C., Woods, J.L., Probst, C. 2018. Risk factors for bud perennation of Podosphaera macularis on hop. Phytopathology. https://doi.org/10.1094/PHYTO-04-18-0127-R.
DOI: https://doi.org/10.1094/PHYTO-04-18-0127-R

Interpretive Summary: The hop powdery mildew fungus persists from season-to-season in the Pacific Northwestern U.S. through infection of crown buds. Historical data of on this process were analyzed to identify their association with the incidence of powdery mildew, growers’ use of fungicides, autumn and winter temperature, and other production factors. In Oregon, the incidence of powdery mildew on leaves was most severe and the number of fungicide applications made by growers greatest in yards where flag shoots were found in spring. Similarly, the incidence of plants with powdery mildew in Washington was significantly associated with the number of flag shoots present in early spring, although the number of fungicide applications made was independent of flag shoot occurrence. The occurrence of flag shoots was associated with prior occurrence of flag shoots in a yard, the incidence of foliar powdery mildew in the previous year, grower pruning method, and, in Washington, winter temperature. A census of hop yards in the eastern extent of the Oregon production region during 2014 to 2017 found flag shoots in 27 of 489 yards evaluated. Of the 27 yards with flag shoots, 22 were either chemically pruned or not pruned and 4 were mechanically pruned in mid-April, well after the initial emergence of flag shoots. The prevalence of yards with flag shoots also was related to thoroughness of pruning in spring (8.1% of yards with incomplete pruning versus 1.9% of yards with thorough pruning). A model was fit to the data from the intensively assessed yards in Oregon, with risk factors for occurrence of a flag shoot in the previous year, occurrence of foliar mildew in the previous year, and thoroughness of pruning in spring. The entirety of this research indicates that P. macularis appears to persist in a subset of chronically affected hop yards, particularly yards where spring pruning is conducted poorly. Targeted management of the disease in a subset of fields most at risk for producing flag shoots could potentially influence powdery mildew development region-wide.

Technical Abstract: The hop powdery mildew fungus, Podosphaera macularis, persists from season-to-season in the Pacific Northwestern U.S. through infection of crown buds because the ascigerous stage of the pathogen is absent in this region. Historical data of flag shoot occurrence and incidence in Oregon and Washington during 2000 to 2017 were analyzed to identify their association with the incidence of powdery mildew, growers’ use of fungicides, autumn and winter temperature, and other production factors. In Oregon, the incidence of powdery mildew on leaves was most severe and the number of fungicide applications made by growers greatest in yards where flag shoots were found in spring. The occurrence of flag shoots was associated with prior occurrence of flag shoots in a yard, the incidence of foliar powdery mildew in the previous year, grower pruning method, and, in Washington, winter temperature. A census of hop yards in the eastern extent of the Oregon production region during 2014 to 2017 found flag shoots in 27 of 489 yards evaluated. Of the 27 yards with flag shoots, 22 were either chemically pruned or not pruned and 4 were mechanically pruned in mid-April, well after the initial emergence of flag shoots. The prevalence of yards with flag shoots also was related to thoroughness of pruning in spring (8.1% of yards with incomplete pruning versus 1.9% of yards with thorough pruning). A Bayesian logistic regression model was fit to the data from the intensively assessed yards in Oregon, with binary risk factors for occurrence of a flag shoot in the previous year, occurrence of foliar mildew in the previous year, and thoroughness of pruning in spring. The model indicated that the median and 95% highest posterior density interval of the probability of flag shoot occurrence was 0.0008 (0.0000 – 0.0053) when a yard had no risk factors, but risk increased to 0.0065 (0.0000 – 0.0283) to 0.43 (0.175 – 0.709) when one to all three of the risk factors were present.