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. 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 4-7 day interval. Copper-based fungicides may be used for protective sprays but they can be toxic to certain watermelon cultivars. 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. Rates and waiting intervals for fungicides labeled for the control of downy mildew.