Forecasting and Management of Hop Downy Mildew

Downy mildew of hop (Humulus lupulus), caused by Pseudoperonospora humuli, is managed in the Pacific Northwestern United States by regular application of fungicides. A degree-day model that forecasts the first emergence of shoots systemically infected 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 6.5°C) developed for Washington State. Predictions based on a single sine model (base temperature 6°C) 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 = –0.39). 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, in 2006, 2007, and 2008, treatments initiated using a degree-day threshold resulted in an area under the disease progress curve similar to or smaller than in treatments with routine fungicide applications. Model-aided treatments required four fewer 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.