Stay Alert to Anthracnose of Cucurbits
Anthracnose, caused by the fungus, Colletotrichum lagenarium, is a destructive disease of cucurbits occurring during warm and moist seasons. Significant damage can occur to cucumber, muskmelon, and watermelon unless resistant varieties are grown. Squash and pumpkins are rarely, if ever, infected.
All above-ground plant parts can be infected by Anthracnose. Symptoms vary among the three principal cucurbits infected. Leaf lesions begin as water-soaked and then become yellowish circular spots. On watermelon foliage, the spots are irregular and turn dark brown or black. On cucumber and muskmelon, the spots turn brown and can enlarge considerably. Stem lesions on muskmelon can girdle the stem and cause vines to wilt. Stem cankers are less obvious on cucumbers.
The most striking diagnostic symptoms are produced on the fruit, where circular, black, sunken cankers appear. On watermelon, the spots may measure ¼ inch to ½ inch in diameter and up to ¼-inch deep. When moisture is present, the black center of the lesion is covered with a gelatinous mass of salmon-colored spores. Cankers lined with this characteristic color can never be mistaken for any other disease. Similar lesions are produced on muskmelon and cucumber.
Survival And Spread
This disease is most common in the southern, mid-Atlantic, and midwestern states. The anthracnose fungus survives on diseased residue from the previous vine crop. The pathogen also may be carried on cucurbit seed. Under wet conditions, the fungus releases airborne spores (conidia) that infect vines and foliage.
The fungus depends on wetness and fairly high temperatures, 75°F being considered optimum. Conidia do not germinate below 40°F or above 86°F or if they are not supplied with a film of moisture.
In addition, the pathogen must have water to free the conidia from their sticky covering in the fruiting body. The fruiting body is slightly saucer-shaped and lined with microscopic stalks. The microscopic stalks produce the conidia, which amass in sticky, flesh-colored tendrils. As long as the tendrils remain dry, the conidia cannot escape, but a heavy dew may dissolve the mucilaginous covering and then rain drops splash them several feet.
Anthracnose usually becomes established in mid-season after the plant canopy has developed.
Cultural control methods include:
- The use of disease-free seed.
- Practicing a three-year rotation.
- Employing good sanitation by plowing under fruits and vines at the end of
- Choosing anthracnose-resistant varieties, if at all possible. Resistant cucumber slicers include ‘Cobra’ and ‘Thunder’ (Stokes Seeds), ‘Dasher II’ and ‘Speedway’ (Harris Seeds), ‘Daytona’ (Jordan Seeds Inc.), ‘Indy’ (Seminis), and others.
Many pickling cucumbers are tolerant or resistant, as are many watermelon varieties. No anthracnose-resistant muskmelon varieties are currently available for the Southeast.
Apply approved fungicides to the crop at regular intervals, more often if frequent rains occur.
If angular leaf spot is a problem, tank mix with a copper compound.