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Figure 1.  Downy mildew on watermelon

SYMPTOMS

Cucumber and muskmelon are the most susceptible of the cucurbits to downy mildew, caused by  the fungus Pseudoperonospora cubensis, but the pathogen also attacks watermelon, squash, pumpkin, gourd, and other members of the Cucurbitaceae. Symptoms first appear as small, angular, yellow lesions on the upper side of the leaf. A white to purplish mildew may be observed on the lower side of the leaf during humid weather. As the lesions expand, their centers turn brown. Often the margins of diseased leaves curl upward. During favorable weather, leaf lesions coalesce and kill large areas of the leaf surface. This results in a stunting of the plant and a failure of the fruit to mature properly. Even those fruit which reach maturity may have an off-flavor. In severe cases, the entire plant will die.

DISEASE CYCLE

The fungus does not produce an overwintering spore and cannot survive the winter in Kansas. Thus, the fungus must be introduced into the state on infected transplants or by inoculum carried by air currents from the south. The main geographic  reservoirs of inoculum in the spring are the frost-free areas of the southern United States and Mexico where the fungus survives on living hosts. The infection process is favored by temperatures between 16 and 22 C and by a thin layer of water on the leaf surface. Unlike some downy mildews, Pseudoperonospora cubensis thrives in warm as well as cool temperatures provided there is sufficient moisture and a high relative humidity.

CONTROL

Downy mildew is an erratic disease in Kansas; it may cause significant damage one year and then not show up again for several more growing seasons. This is because the fungus does not overwinter in the state, and must be introduced into fields on infected transplants or by wind_blown inoculum carried in air currents from southern states. In addition, the fungus requires extended periods of wet weather and leaf wetness for the infection process. A grower may expect a downy mildew outbreak when inoculum is present and when weather conditions are favorable for infection.  Several cucumber varieties have resistance to downy mildew, but watermelon and muskmelon varieties do not.  Control of downy mildew is dependent on cultural practices, early detection and timely applications of fungicides. If you purchase transplants from another state or area, check them carefully for any symptoms of downy mildew. Do not plant any infected transplants! It's a good idea to inspect fields regularly for any evidence of downy mildew or other disease problems. Early detection is essential for the proper control of foliar pathogens. If weather conditions become favorable for the development of downy mildew, begin protective fungicide applications and continue on a 4 to 7 day interval. Protective fungicides such as mancozeb or chlorothalonil also will protect the plants from other foliar diseases (anthracnose). Copper-based fungicides may be used for protective sprays but they can be toxic to certain watermelon cultivars. If downy mildew is already present in the field, it is advisable to use a systemic fungicide (metalaxyl) to arrest disease development. This type of fungicide will stop lesion expansion once infection has occurred. A combination of chlorothalonil and metalaxyl has been shown to have a positive synergistic effect in controlling downy mildew.