Downy mildew of cucurbits, Pseudoperonospora cubensis, is found annually on squash, cucumbers, pumpkins, muskmelons, and other cucurbits in Florida. The disease occurs on watermelons yearly in the South Florida, but in northern Florida, it may be present in some years but not others. Although downy mildew of all cucurbits is caused by the same species, strains within the species seem to exist. It is not uncommon to see squash, cantaloupe, and cucumber severely diseased by downy mildew whereas nearby watermelons show no signs of the disease.
Although downy mildew can kill plants severely infected at an early stage, downy mildew typically reduces yield, fruit quality, and harvesting time by causing leaf infections that impair photosynthesis in the plant.
Leaf symptoms can be used to diagnose downy mildew in the field in some cases. On cucurbits, other than watermelon, small yellowish spots occur on the upper leaf surface initially away from the leaf margin. Later, a more brilliant yellow coloration occurs with the internal part of the lesion turning brown. Lesions are usually angular as leaf veins restrict their expansion. When the leaves are moist, a downy grayish fungal growth may be seen on the underside of lesions.
On watermelons, yellow leaf spots may or may not be angular and later turn brown to black in color. On watermelons, an exaggerated upward leaf curling occurs that growers sometimes liken to a dead man’s hand.
Survival And Spread
Spores are easily dispersed by wind from one leaf spot to another leaf in a field or to another nearby planting. Under ideal conditions, spores may be transported for many miles (sometimes hundreds of miles) from one field to another. When a spore contacts a leaf and the leaf is wet, the spore germinates and penetrates the leaf tissue. Within four to seven days, new lesions capable of producing spores are produced. As this cycle continues, an epidemic situation occurs and control becomes increasingly difficult.
Since nighttime temperatures between 55°F and 75°F and relative humidity above 90% provide ideal conditions for infection, cucurbits planted in South Florida always are at risk from downy mildew. In North Florida, cooler nighttime temperatures in the spring often delay the onset of downy mildew epidemics until flowering on squash and cucumbers.
Control of downy mildew on cucurbits is achieved primarily by the use of resistant varieties and/or fungicide spray programs. Fungicide sprays are recommended for all cucurbits. Resistant varieties are currently available and can help reduce fungicide applications. For the past several years, downy mildew-resistant cucumbers have been attacked in Florida as well as up and down the East Coast signaling a possible race shift overcoming varietal resistance.
Squash, pumpkin, cantaloupe, and non-resistant cucumber varieties are very susceptible and should be sprayed every five to seven days. If cucurbits are planted close to established fields infected with downy mildew, a spray program should be initiated as soon as the first true leaves are present.
Gene McAvoy is Regional Vegetable Extension Agent IV with the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)