Dew Diligence
The variation among pathogen strains can change from year to year. Young plants can be quite sensitive to the disease, which can lead to seedling death. If left unchecked, the disease will rapidly defoliate plants, causing large reductions in total yield, fruit quality, and even delay harvest time.
Field Diagnosis
Leaf symptoms can be used to diagnose this disease in the field. For most cucurbits, the lesions from the top will be small yellowish areas, with a corresponding watery lesion on the underside of the leaf. As lesions further develop, they become brighter yellow with the center of the lesion turning brown, until eventually the entire lesion is brown. For most cucurbits, these lesions are limited by the small leaf veins, giving them a characteristic angular appearance.
The exception is watermelon, where lesions of downy mildew can be angular or non-angular, and the disease often causes the leaves to curl upward. For all cucurbits, as the lesions mature spores will develop and appear as a brown-black fuzzy growth on the underside of the leaf that corresponds with the lesions on the top of the leaf. Upon closer inspection with a hand lens, the spores will look like a cluster of grapes. Spores are easily spread by wind from leaf to leaf and can move long distances to infect other cucurbit plantings.
The prime time for spore movement is during the late morning to midday. Nighttime temperatures between 55°F to 75°F and relative humidity more than 90% are ideal for downy mildew development, mostly coinciding with the cool, dewy mornings typical of fall and spring months in Florida. When a spore lands on a wet leaf, it will germinate and then penetrate the leaf tissue, with lesions appearing four to seven days later. This disease can spread so rapidly that it looks like the leaves have been scorched overnight, so routine scouting is very important.
Downy mildew spreads so easily from one cucurbit planting to another that a website, the Cucurbit Downy Mildew Forecast
(http://cdm.ipmpipe.org), has been developed to track and even help forecast the movement of the disease. This is used to track the occurrence of the disease across the whole country. You can sign up to receive an eMail alert or text if downy mildew is reported in an area near you so that you have an early alert to be watching for the disease.
Disease Management
To control downy mildew, use resistant varieties — which are available for many cucurbit crops. Try to adjust the planting dates if possible to avoid conditions favorable for the disease. Scout plants early and often to look for signs of the disease, especially in areas of the field that are shaded in the morning. If downy mildew has already been reported in your area on older plantings, begin application of preventive fungicides as soon as the first true leaves appear if the crop is direct seeded or immediately after setting transplants.
Spray programs for downy mildew are most effective when started prior to the first sign of disease because once downy mildew occurs it is difficult to control. Apply systemic fungicides immediately if symptoms are present or if downy mildew has been reported in the area, and reapply on a seven-day interval.
A fact sheet on downy mildew containing a complete list of current chemicals registered for downy mildew management has been compiled and can be found at http://gcrec.ifas.ufl.edu.
Be sure to alternate fungicides based on the FRAC (Fungicide Resistance Action Committee) grouping for fungicide mode of action for chemicals to prevent or delay chemical resistance from developing in the downy mildew population. Read and follow all label instructions.
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