Anthracnose

Anthracnose, caused by the fungus Clletotrichum orbiculare, is an important disease of cucumber, muskmelon, and watermelon in the Midwest. Pumpkin and squash are less susceptible. This disease causes significant yield losses when conditions are favorable to disease development.

Symptoms. All parts of foliage, stems, and fruit can become infected (Fig. 1). Symptoms of anthracnose are roughly circular, light brown to reddish, and can reach 0.5-inch in diameter (Figs. 2 & 3). The leaves may be distorted, and the center of the lesions may crack or drop out, creating a shot-hole appearance (Fig. 3). On petioles and stems, lesions are shallow, elongated tan areas. On fruit, lesions are circular, sunken, water-soaked areas which first develop as the fruit approaches maturity (Fig. 4). In the moist weather, these lesions turn black and are covered with pink spore masses.

Disease Cycle. The pathogen survives between crops on infected plant residue or infected volunteer plants and can be carried on seed harvested from infected fruit. Conidia (spores) are produced and disseminated primarily by splashing water and to a lesser extent by wind and on people and machinery. Spore germination and growth are optimum at 72- 81°F (22-27°C) and 100% relative humidity for 24 hours. Visible symptoms appear about 96 hours after infection. Several races of anthracnose pathogen vary in virulence on cucurbit hosts are known.

Disease Management. Anthracnose is difficult to manage once it becomes established in the field, thus practices should be employed to prevent contamination of clean field. Cucumber cultivars resistant to anthracnose are available. Also, watermelon cultivars resistant to some races of the pathogen have been developed. Since the pathogen is a seed-borne fungus, planting should be initiated with pathogen-free seed. Deep plowing of crop residue immediately upon completion of the harvest effectively reduces the inoculum level and should be combined with a crop rotation schedule in which no cucurbits of any kind are grown for at lease one year. Overhead irrigation should be avoided or minimized. Also, working in wet field should be avoided. Anthracnose is controlled with foliar fungicide applications. Many protectant fungicides (e.g., chlorothalonil and strobilurins) control anthracnose.

Fig. 1. A cucumber field with anthracnose.

Fig. 2. Anthracnose symptoms on cucumber leaves.

Fig. 3. Anthracnose lesions on a cucumber leaf.

Fig. 4. Anthracnose symptoms on watermelon fruit (left, courtesy T. A. Zitter) and muskmelon fruit.