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Figure 1. Anthracnose on watermelon fruit

Figure 2. Anthracnose on cucumber 

Anthracnose, caused by the fungus Colletotrichum obiculare, is a major foliage disease of watermelon, cucumber and muskmelon, but is not a serious problem on squash, pumpkin, or gourd. Anthracnose is most severe during relatively warm periods with frequent rainfall and a high relative humidity.

SYMPTOMS

Symptoms of the disease vary somewhat on the host plant. On watermelon, leaf spots first appear light yellow or tan but soon turn black. The roughly circular lesions expand rapidly during favorable weather conditions. Elongated lesions also may form on the petioles and stems, causing death of plant tissue beyond the lesion. Severely blighted plants appear scorched and unthrifty. The fungus also affects the fruit. Young fruits are either killed or become malformed. Larger fruits develop sunken, roughly circular lesions that are water-soaked. During humid weather, masses of pink to salmon-colored spores are extruded from fruiting structures in the center of the lesions. This spore matrix is visible during wet weather.

Symptoms on cucumber and muskmelon leaves are somewhat similar to those on watermelon except that the color of the spots tends to be lighter. In most cases the lesions are light tan to red. The necrotic tissue tends to crack and fall out, giving the leaf a tattered or shot-hole appearance.

DISEASE CYCLE

The fungus overwinters in infected plant debris or on seed. In the spring, spores of the fungus are dispersed by splashing water and infect the foliage and vines.  The disease usually becomes established after the canopy has become fully developed.

CONTROL

Many cultivars of cucumbers have resistance to anthracnose and should be used whenever possible. Most watermelon and muskmelon varieties are susceptible to one or more races of the anthracnose fungus. Hence genetic control of anthracnose on these crops is usually not sufficient.

Anthracnose can be reduced through the use of clean seed and crop rotation to non-cucurbit crops. Rotation periods should not be shorter than 3 years since the fungus can survive in crop debris. In locations where crop rotation is not possible (e.g. home gardens) deep plow or till debris into the soil after fall harvest to hasten decomposition. In small plots consider removing dead vines.

Avoid overhead irrigation. Apply water through drip lines or tapes or soaker hoses. Plastic or straw mulches may also help suppress disease.

Fungicide applications may be necessary to control anthracnose. Make applications at 7- to 21-day intervals with a labeled fungicide (see table) during the growing season. A weather-based computer software program called MELCAST is available to help time fungicide applications.

Homeowner: Daconil, Fungi-Gard, Liquid Fungicide, Encore, Monterey Bravo

• Copper products may cause plant injury and reduce yields of watermelon and muskmelon

Homeowner: Copper Fungicide, Bordeaux

Homeowner: Mancozeb, maneb

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Web updated 9/01/06
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