Downy Mildew on Hops (Humulus lupulus) is a disease caused by the fungal pathogen, Pseudoperonospora humuli. It is a common pest in many hop growing regions of the world and can cause direct yield loss if cones are infected. In susceptible varieties or with severe infections, the disease can also cause crow rot which can affect plant growth for future seasons, or can even cause plant death.
There are several fungicide options available for control, including basic sulfur applications, but these fungicides typically work best when applied as a preventative fungicide. Applying fungicides at regular intervals, along with sanitation practices such as crown pruning, are the most common management tactics to avoid Downy Mildew Infection on hops. Researchers at Oregon State University and the USDA have developed a growing degree day model that can help predict the emergence of basal spikes from overwintered mycelia in infected crowns. This growing degree model, available on the Pest Prophet app, can be used as a tool for deciding when to begin preventative fungicide program.
Calculating Growing Degree Days
To calculate amount of growing degree day unit accumulation for a given field, a lower threshold of 43F is used for Downy Mildew on Hops basal spike emergence. The GDD amount can be calculated from daily high and low temperatures using a sine model, and a horizontal cut-off method, or this can be done on the Pest Prophet app using localized weather data. The recommended default date to start accumulating Growing Degree Day units (biofix) is February 1st (for the Northern Hemisphere).
Predicting Downy Mildew Basal Spike Emergence
For Downy Mildew basal spike emergence, the output of the growing degree model is a probability of spike emergence, which can be used as a tool, along with variety susceptibility data, for beginning fungicide spray programs. The graph below is from research performed in the Pacific Northwest (Gent et al, 2010.)
As a general rule for highly susceptible varieties, a probability of 1% should be used to begin scouting (93 GDD-F), and 25% (103 GDD-F) for spraying .
For moderately susceptible varieties: 25% probability (103 GDD-F) is recommended for beginning to scout and 50% probability (118 GDD-F) is recommend for beginning spray program.
For resistant varieties, 50% probability (118 GDD-F) is recommend for scouting and 75% probability (144 GDD-F) is recommended for beginning spray program.
Probability of emergence will also depend on amount of infected crowns the previous season. Growers might want to adjust scouting and spraying dates earlier following a high infection season, or later following a season without much Downy Mildew.
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