|Ocamb, Cynthia -|
|Farnsworth, Joanna -|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 2009
Publication Date: March 1, 2010
Citation: Gent, D.H., Ocamb, C.M., Farnsworth, J.L. 2010. Forecasting and management of hop downy mildew. Plant Disease. 94:425-431. Interpretive Summary: Downy mildew, caused by Pseudoperonospora humuli, is one of the most destructive diseases of hop. In this study, we developed and validated mathematical models to predict when the disease would first appear in a hop yard and estimate when control measures are needed to manage the disease. Predictions of when the disease would first appear provided on average 5 days of advanced warning when evaluated with data from 34 hop yards in western Oregon. We also discovered that disease appearance was related to disease severity in that hop yard in the previous season. By using these models, four less fungicide applications were needed to manage the disease as compared to routine fungicide applications. These studies indicate that downy mildew can be managed effectively with fewer fungicide applications than currently made by hop growers in this region if control measures timed appropriately using these disease risk forecast tools.
Technical Abstract: Downy mildew of hop, caused by Pseudoperonospora humuli, is managed in the Pacific Northwestern U.S. by regular application of fungicides. A degree-day model that forecasts the first emergence of shoots systemically infection with P. humuli (termed basal spikes) and a risk index for secondary spread of the disease were evaluated over four seasons in western Oregon. In surveys conducted in 34 hop yards, the predicted first spike emergence occurred on average 11.6 days (median 12 days) after spike emergence using a simple average degree-day model (base temperature 6.5ºC) developed for Washington State. Predictions based on a single sine model (base temperature 6ºC) provided on average 4.9 days (median -0.5 days) of advanced warning before the first spike emerged. Downy mildew severity in a previous season was negatively correlated with the degree-day emergence date of spikes the following year (r = -0.391). In experimental plots, disease severity was significantly greater where fungicide applications were timed using a risk index compared to routine fungicide applications in 2005 and 2007, but statistically similar between these treatments in 2006 and 2008. However, for treatments initiated using a degree-day threshold the area under the disease progress curve was 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. These studies indicate that downy mildew can be managed effectively with fewer fungicide applications than currently made by hop growers in this region if fungicide applications are timed to coincide with the predicted emergence of basal spikes and subsequent disease risk forecasts.