Downy mildew, caused by the fungus Pseudoperonospora cubensis, is found annually on squash, cucumbers, pumpkins, and muskmelons grown in all areas. Although we consider downy mildew of all cucurbits to be caused by the same species, strains of P. cubensis exist. For example, it is not uncommon to see squash, cantaloupe, and cucumber severely diseased by downy mildew whereas watermelons, nearby, show no signs of this disease. Downy mildew can reduce yield, fruit quality, and harvesting time. Downy mildew can kill plants if plants are severely infected early. It does so by leaf infections which impair necessary food production in the plant. Late season infections often signal an excuse to buyers for reduced prices because of alleged reduced sugar content in the fruit.

Leaf symptoms can be used to diagnose downy mildew in the field in most cases. On cucurbits other than watermelon, small yellowish areas occur on the upper leaf surface. Later, a more brilliant yellow coloration occurs with the internal part of the lesion turning brown.

Usually the spots will be angular as they are somewhat restricted by the small leaf veins. When the leaves are wet, a downy white-gray-light blue fungus growth can be seen on the underside of individual spots (lesions). On watermelons, yellow leaf spots may be angular or non-angular, and they will later turn brown to black in color. Often on watermelons, an exaggerated upward leaf curling will occur.