Cercospora leaf spot occurs on all cucurbits but is most common on watermelon, cantaloupe, and cucumber. This disease is usually found only on the foliage, but if the environment is suitable, symptoms may also occur on petioles and stems. The fungus is not known to infect fruit. On watermelon, leaf spots manifest on young leaves as small grey or white spots with black margins. Larger leaf spots which are circular to irregularly circular develop on other cucurbits. The centers of these leaf spots are tan to light brown becoming transparent and brittle with time. Lesions with surrounding chlorotic halos may coalesce and turn leaves yellow. Although defoliation from the disease may reduce fruit size and quality, serious economic losses are rare.
CONDITIONS FOR DISEASE DEVELOPMENT
Conidia of Cercospora citrullina become airborne and may be carried great distances on moist winds. Infection requires free moisture and is favored by temperatures of 26–32°C (80–90°F). Cercospora citrullina survives on crop debris, volunteers and cucurbit weeds.
Incorporate cucurbit debris into the soil to hasten its breakdown and /or remove pruning debris entirely from the field. Rotate out of cucurbits for two to three years and establish a fungicide spray program to help control this disease.