ANR-877, New Oct 1994. Edward J. Sikora, Extension Plant Pathologist

---

Plant Disease Notes Foliar Diseases of Cucurbits

Common diseases that may appear on the foliage or the fruit of cucurbits include powdery mildew, downy mildew, Alternaria leaf spot, and angular leaf spot.

Powdery Mildew

Powdery mildew, caused by the fungus Erysiphe cichoracearum, affects cucumber, cantaloupe, pumpkin, and squash. The fungus produces a white powdery growth on leaves. Crown leaves are infected initially and may wilt and die. Disease development is favored by high temperatures and high relative humidity.

Downy Mildew

Downy mildew, caused by the fungus Pseudoperonospora cubensis, affects cucumber and cantaloupe. The fungus produces yellow to brown spots on the upper leaf surface. Under moist conditions, a fluffy, gray fungal growth appears on the underside of the leaf, corresponding to the leaf spots above. Infected leaves may die as the disease spreads from the crown outward. Moist conditions favor downy mildew development.

Alternaria Leaf Spot

Alternaria leaf spot, caused by the fungus Alternaria cucumerina, affects cantaloupe and cucumber as well as other cucurbits. The fungus causes tan to brown leaf spots (1/2 inch in diameter) that contain dark concentric rings within the lesions. Spots merge and defoliation occurs, beginning with the crown leaves. Weak plants are most susceptible to Alternaria leaf spot. The fungus is spread in wind and rain and is favored by warm, wet conditions.

Angular Leaf Spot

Angular leaf spot, caused by the bacterium Pseudomonas lachrymans, affects cucumber, squash, and pumpkin. Leaf spots are angular in appearance, being limited by major veins. They are water soaked initially and eventually turn grayish brown. Leaves take on a shot-holed appearance as spots drop out. The bacterium also produces small, water-soaked spots on fruit, which can lead to rotting of the fruit. The disease is favored by warm, wet conditions.

Control Of Foliar Diseases Of Cucurbits

Strategies for controlling foliar diseases of cucurbits include the following:

• Rotate crops.
• Plant certified, disease-free seed.
• Plant in well-prepared, fertile soil.
• Control weeds.
• Plant resistant varieties when available.
• Practice clean plowdown after harvest.
• Follow a fungicide spray program.

Recommended Fungicides And Bactericides. Pesticides are listed here by active ingredient with examples of brand names in parentheses. (These brands are usually available to commercial growers.) On fungicide labels, the active ingredient is usually listed below the brand name.

Chlorothalonil (Bravo, Evade, Echo); Mancozeb (Manzate 200 DF, Dithane DF, F- 45, Manex II); Maneb (Maneb 80, Manex); Copper (Kocide DF, 101)--Angular leaf spot; Sulfur (Wettable sulfur, sulfur dust)-- Powdery mildew; Benomyl (Benlate)--Powdery mildew.

Use chemicals only according to the directions on the label. Follow all directions, precautions, and restrictions that are listed.

Trade names are used only to give specific information. The Alabama Cooperative Extension Service does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.