North Carolina State system predicts downy mildew infestations

In 2004, North Carolina cucumber growers noticed yellow, angular spots on the leaves of their crop. By the time they had diagnosed the disease and treated the plants, it was too late.

By the end of the growing season, downy mildew had caused an estimated $20 million in damage to the cucumber crop.

The disease quickly spread northward to Virginia, Maryland and Delaware, causing a similar level of devastation. The damage was unexpected.

While downy mildew is a common disease of melons and squash in the South, in the last 30 to 40 years it has not been a serious problem for cucumbers. Over the last couple of years, however, the disease has progressed as far north as New York, Michigan and Ontario, Canada, in each case causing similar levels of devastation to cucumbers. The disease has been so severe that many growers considered leaving the business.

Now a North Carolina State University plant pathologist is planning a counter attack. With a $140,000 research and Extension grant from the U.S. Department of Agriculture's Cooperative State Research, Education and Extension Service, Gerald Holmes, associate professor of plant pathology and North Carolina Cooperative Extension specialist in the College of Agriculture and Life Sciences at North Carolina State plans to study downy mildew's fungicide resistance and improve a current national online tracking system for downy mildew.

The grant was made available through the Southern Regional Integrated Pest Management Grants competition.

Unlike melons and squash, which are susceptible to the downy mildew fungus, disease-resistant cucumbers managed to avoid it for decades. But in 2004, new strains of downy mildew appeared that did not respond to several standard fungicides. The new strains attacked even disease-resistant plants.

Holmes was able to identify effective fungicide programs in 2004 by conducting on-farm research in the heart of the epidemic. Many growers adopted the programs Holmes developed in subsequent years, drastically reducing the impact of the disease.

In 1998, collaborating with Charles Main, professor emeritus of plant pathology, Holmes began the Cucurbit Downy Mildew Forecast, part of the North American Plant Disease Forecast Center developed by Main in 1995 for tobacco blue mold.

The forecast includes a map detailing the path of the disease agent (Pseudoperonospora cubensis) in addition to a forecast that indicates weather conditions and their impact on where the disease is likely to strike next.

Eighteen states within the eastern and central regions of the U.S. contribute to the tracking system, located at

Weather conditions play a pivotal role in spreading a disease like downy mildew. The disease is spread by tiny spores that are carried on the wind, so without certain weather conditions, it is unlikely that crops will be infected.

Forecasting systems like the one developed by Holmes and Main allow growers to spray their fields with fungicide only when there is a high likelihood of infection.

Holmes plans to use the CSREES grant make the system better. In addition to increasing the frequency of forecast updates, he will incorporate 3-D and GIS technology to improve the quality and accuracy of the forecasts.

The Regional Integrated Pest Management Grants competition for the southern region is managed by the Southern Region Integrated Pest Management Center, which is located in the College of Agriculture and Life Sciences at North Carolina State University.

For a copy of Holmes' proposal or for other funded proposals, see

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