We have recently received several cucurbit samples in the Plant Disease Clinic with anthracnose. Anthracnose is a common fungal disease of cucurbits, including watermelons, muskmelons, and cucumbers. Squash and pumpkins are rarely affected by anthracnose.
Anthracnose causes small, yellow, water soaked lesions to appear on the leaves near the veins. Over time these lesions enlarge and turn brown and the dead tissue may fall out, creating holes in the leaves. Spots on watermelon leaves are irregular in shape and turn dark brown or black. Lesions on cucumber and muskmelon leaves tend to be round, brown, and quite large. When lesions occur on the stems, they may cause wilting of the foliage when the stems are girdled. Round, sunken, water soaked lesions also may appear on the fruits. In moist weather, the center of these spots may be covered by slimy, salmon-colored spore masses.
Anthracnose is caused by a fungus, Colletotrichum lagenarium. It is favored by warm, wet conditions. Although initial infection usually occurs in the spring, the disease typically does not become established until mid-season when the canopy is fully developed.
The fungus that causes anthracnose can survive the winter on cucurbit residue; therefore, all diseased debris should be removed or plowed under at the end of the season. Growing cucurbits in a three-year rotation with other types of crops can help prevent disease. Anthracnose can enter the garden on diseased seed, so only disease-free seed should be used. Resistant varieties of watermelons and cucumbers are available. Fungicide sprays also may be used.
More information on cucurbit diseases can be found in PM 1049, Curcurbit Diseases An Aid to Identification and Control (PDF).
This article originally appeared in the 8/13/2004 issue.