Powdery mildew on a cone and leaves.  Image courtesy of Walter F. Mahaffee.

Helping Hop Growers Predict Powdery Mildew

Agricultural Research Service scientists and cooperators have created an Internet-based model to help farmers determine when fungicides should be applied to help control hop powdery mildew in the Pacific Northwest.

Since hop powdery mildew's emergence in the Pacific Northwest in 1996, the disease has had a major impact on the hop industry, resulting in the loss of the entire crop on thousands of acres in some years. The disease can be controlled with fungicides, but annual control costs have been as high as $400 per acre.

To economically manage the disease while protecting the environment, growers need methods to help them decide when fungicides should be used. So ARS plant pathologist Walter F. Mahaffee of the Horticultural Crops Research Unit, Corvallis, Ore., and Carla Thomas of the University of California, Davis, developed the model.

The hops model is a modification of the Gubler/Thomas grape powdery mildew risk infection model that is used in all grape-growing regions of the world. Mahaffee's team demonstrated that brief exposure to high temperatures inhibited disease development. They then used this information to develop model rules to more accurately predict the risk of infection.

In conjunction with FoxWeather, Mahaffee's team further developed the model to predict the infection risk five days into the future. In 2002 and 2003, this forecast was 80 percent accurate for one day into the future and 60 percent accurate for five days into the future. Trials in grower fields and small plots in 2002 and 2003 showed that growers could reduce fungicide use by one to three applications and still have less disease.

Since powdery mildew affects a wide variety of crops, Mahaffee hopes to further modify the model so it can predict the infection risk of all powdery mildews, regardless of region.

ARS is the U.S. Department of Agriculture's chief scientific research agency.