By Shouan Zhang

Downy mildew is one of the most economically important diseases of cucurbits, especially in areas with high humidity and rainfall such as South Florida. This disease occurs on cucurbits, including cantaloupe, cucumber, pumpkin, squash and watermelon.

Downy mildew is an annual disease problem on pumpkin and squash in the eastern and central United States for many years.

A tremendous breeding effort in the mid-20th century resulted in adequate control of this disease in cucumber production without fungicide application.

Since 2004, the resurgence of the devastating disease has caused substantial economic losses to cucumber growers, and it continues to be an important disease problem in other cucurbit crops that significantly impacts production.

Because the production of cucurbits exists year-round throughout Florida, downy mildew is endemic and occurs nearly every growing season to some degree.

Look out for the symptoms
Downy mildew begins as small watersoaked lesions on the underside of leaves.

The lesions are typically limited by small leaf veins, giving the lesions an angular appearance. It is caused by an oomycetous pathogen called Pseudoperonospora cubensis.

Under high humidity, a layer of gray-brown to purplish-black fluffy stuff appears on the underside of the infectedleaves. Using a microscrope, look for acutely and dichotomously branched sporangiophores—tree-like structure—bearing lemon-shaped sporangia. These are characteristic of the downy mildew pathogen.

Leaves will turn necrotic and curl upward within days under favorable weather conditions. Therefore, scouting for the disease is critical.

Symptoms of downy mildew vary on watermelon and cantaloupe, and the lesions are not always angular and often not associated with an upward curling of the leaves. The downy mildew pathogen doesn’t affect cucurbit fruit.

But it can result in significantly reduced yields and fruit deformation, especially in cucumber. In addition, early defoliation may increase exposure to direct sunlight, increasing sun-scalded fruit in watermelon and winter squash.

The mystery behind downy mildew

Pseudoperonospora cubensis is an obligate parasite that requires live host tissue to survive and reproduce. The pathogen must overwinter in an area that has mild weather—such as south Florida—and where cucurbits are present.

The spores are dispersed via wind or splash water to neighboring plants and to other fields by wind or irrigation water.

Symptoms appear several days to nearly two weeks after infection.

The pathogen favors cool and moist weather. Therefore, downy mildew is common during south Florida’s winter growing season.

Optimum conditions for sporulation are 59 to 77 degrees Fahrenheit (15 to 25 degrees Celsius) with six to 12 hours of leaf wetting, such as from morning dew. Sometimes temperatures during daytime don’t favor this pathogen, but nighttime temperatures may be ideal.

Downy mildew management
1. Resistant cultivars Host resistance is the most economically important and effective component in integrated disease management and should be used whenever possible.

Resistant cultivars have been developed for cucumbers and cantaloupe, and to a lesser extent for squash and pumpkin.

Although downy mildew has been severe on resistant cucumber cultivars in recent years, they are more effective than susceptible cultivars in delaying infection.

2. Early detection Early detection of downy mildew is critical to prevent damage from this disease due to its rapid and destructive nature. Many cucurbit growers suffered huge losses from downy mildew as they waited until disease symptoms were clearly visible before they began to spray.

Following early detection, immediately apply preventative fungicides to control this destructive foliar disease. A downy mildew forecasting system—http://cdm.ipmpipe.org—has been established to help cucurbit growers to time fungicide applications for maximum benefit. The system documents the disease outbreaks and provides a risk assessment for future outbreaks in regions where cucurbits are grown.

3. Chemical control Most likely, you won’t be able to satisfactorily control downy mildew without fungicides. Both protectant and systemic products should be applied.

Fungicides are effective when applied before infection and continually sprayed at five- to seven-day intervals.

Previcur Flex, Tanos, Ranman, Reason, Presidio, Aliette and Quadris are examples of fungicides that control downy mildew in Florida. These products should be applied in a resistance-management program that rotates products with different modes of action.

Protective fungicides, such as chlorothalonil and mancozeb, should be incorporated as tankmix partners.

Shouan Zhang is a vegetable plant pathologist based at the Institute of Food and Agricultural Sciences’ Tropical Research and Education Center, University of Florida, in Homestead. He can be reached at szhang0007@ufl.edu or (305) 246-7001, ext. 213.