Downy mildew is a disease that affects members of the cucurbit family, which includes cucumbers, melons, squash, and watermelons. It is caused by the fungus Pseudoperonospora cubensis. Host specificity is found in P. cubensis, with at least 5 pathotypes (isolates of the fungus that will infect some cucurbits but not others) found in the United States.The fungus must live on a living host plant, so it overwinters in areas that do not have a hard frost. The spores are spread by wind and can travel quite far. The disease thrives in high humidity, and can be found in NC nearly every summer. Downy mildew affects leaves, causing lesions such as those in the photo above. In severe cases, leaves curl upward and die.

 

This can result in reduced yields and sunscalded or misshapen fruits.

To control this disease, plant resistant cultivars in sunny plots with good airflow. Avoid late day irrigation. Follow a spray program if desired. Early detection of the disease is important. Downy mildew outbreaks are tracked and a forecasting system has been developed to help growers time their fungicide applications. The home grower can access this information as well. http://cdm.ipmpipe.org/

For more information and additional photos:

http://www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm

http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/Cucurbits.aspx