Downy Mildew of hop (Humulus lupulus) caused by Pseudoperonospora humuli can be a very challenging disease to control in certain geographies. Growers commonly use a program of regular fungicide applications along with some cultural practices such as pruning crowns in spring. Fungicide applications are much more effective if used as a preventative measure, before disease symptoms are visible. Disease outbreaks are more likely to occur under high humidity or following rain events. 46F (8C) to 73F (23C) is the optimum temperature range for the disease to spread. Under these conditions the infection can spread from leaves and shoots and into the hop crown and root system.
Hops Downy Mildew Forecasting Models
Researchers have developed forecasting models to help predict when disease outbreaks may occur, so that growers can time preventative fungicide sprays to maximize their effectiveness. The Downy Mildew Basal Spike Emergence Model is very useful to help predict the emergence of overwintering Pseudoperonospora humuli and can be used be as a tool for determining when to start a fungicide application program.
For use during the season, once shoots reach 15 cm, the Downy Mildew Risk Index model is available. This model was developed in England in the 1970s but has also been tested in the pacific northwest region of the United States, as well as some other areas. (Any disease models should be tested and validated locally before being replied upon.) The Royle model estimates disease risk based on a rolling 2 day average regression using humidity, rainfall and temperature data as inputs.
Calculate Hops Downy Mildew Risk Index Model
The following rules are used to calculate the daily Hops Downy Mildew Risk Index Model (Royle, 1979):
(i) If no rain was recorded during a 48-h period, then the index = 0, else;
(ii) If mean temperature was less than 8°C during a period of rain, then the index= 0, else;
(iii) Index = –63 + (22*RH) +(84*Rain), where “RH” is the number of hours of relative humidity ≥90% in the previous 48 h, and “Rain” is millimeters of rain in the previous 48 h. The model was calculated daily with weather data collected from 4 P.M. to 4 P.M. for a 48-h period. An empirical threshold of 500 “risk units” was considered to be a severe infection event that warranted a fungicide application.
The Pest Prophet app can be used to calculate this automatically using weather data for any specific field, even without a weather station.
How to use the Hops Downy Mildew Risk Index Model
As highlighted above a threshold of 500 “risk units” is recommended as an action point. At this risk level, preventative fungicide applications should be made at the highest label rate, and minimum interval between sprays.
If the risk is below this threshold, preventative fungicides might still be needed depending on the susceptibility of the variety, and location of the field. The interval between fungicide sprays can be adjusted to longer to reflect the lower risk level. Based on experience, or variety susceptibility, growers may find that a threshold lower than 500 “risk units” is more suitable for their situation. (This number is meant to be empirical for all areas.)
Source:
Royle, D. J. 1979. Prediction of hop downy mildew to rationalize fungicide use. Rep. Dep. Hop Res. Wye Coll. 1978:49-57.
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