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United States Department of Agriculture

Agricultural Research Service

Research Project: GENOMICS, GERMPLASM DEVELOPMENT AND IPM OF HOP Title: Systems Approaches to Mangement of Hop Downy Mildew

Authors
item Gent, David
item Nelson, M -
item Farnsworth, J -
item Ocamb, C -
item Grove, G -

Submitted to: Proceedings of the International Hop Growers Scientific Commission
Publication Type: Proceedings
Publication Acceptance Date: May 31, 2009
Publication Date: June 30, 2009
Citation: Gent, D.H., Nelson, M.E., Farnsworth, J.L., Ocamb, C.M., Grove, G.G. SYSTEMS APPROACHES TO MANGEMENT OF HOP DOWNY MILDEW. Proceedings of the International Hop Growers Scientific Commission. 2009. p. 80-83.

Interpretive Summary: A multi-faceted, systems approach to downy mildew management continues to be developed in the U.S. Components of this system are tools to detect the pathogen in the air of hop yards and weather-driven prediction systems for disease appearance and development. During 9 location-years of validation of a molecular test for detection the pathogen, detection occurred no later than 8 days after the appearance of trace levels of disease or airborne spores. During evaluation in commercial yards, use of the test results to initiate first fungicide applications led to enhanced disease control or fungicide use reduction in four of six yards compared to growers’ standard practices. During four growing seasons in western Oregon, a temperature model provided on average 4.9 days (median -0.5 days) of advanced warning before the first diseased hop shoot emerged. Surveys of 34 commercial hop yards in Oregon indicated the downy mildew severity during the previous season was related to when disease appeared the following year. When the first fungicide application was timed with this model, we found that hop plants had disease severity similar to or less than that of plots that received routine fungicide applications, but required four less fungicide applications. Collectively these results indicate that the severity of downy mildew is influenced substantially by the timing of control measures applied early in the growing season, and that growers have new tools to better manage the disease.

Technical Abstract: A multi-faceted, systems approach to downy mildew management continues to be developed in the U.S., and includes sporangia detection methods, epidemic modeling, and modified cultural practices to suppress the disease. During 9 location-years of validation of a PCR assay for detection of airborne Pseudoperonospora humuli, pathogen detection occurred no later than 8 days after the appearance of trace levels of disease or detection of airborne sporangia in a volumetric spore sampler. During evaluation in commercial yards, use of the PCR results to initiate the first fungicide application led to enhanced disease control or fungicide use reduction in four of six yards compared to growers’ standard practices. We also have developed and evaluated a degree-day model that forecasts the first emergence of shoots systemically infected with P. humuli and a risk index for secondary spread of the disease. During four growing seasons in western Oregon, the degree-day model provided on average 4.9 days (median -0.5 days) of advanced warning before the first spike emerged. Surveys of 34 commercial hop yards in Oregon indicated the downy mildew severity during the previous season was negatively correlated with the degree-day emergence date of spikes the following year (r = -0.391). In experimental plots, treatments initiated using a degree-day threshold had disease severity similar to or less than that of plots that received routine fungicide applications during three seasons of evaluation. Model-aided treatments required four less fungicide applications compared to routine fungicide applications. Collectively these results indicate that the severity of downy mildew is influenced substantially by the efficacy and timing of control measures applied early in the growing season.

Last Modified: 12/19/2014
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