Tomato anthracnose, caused by several members of the genus Colletotrichum, produces black, sunken lesions on the ripening fruit. Although symptoms do not appear until the fruit is ripening, the infection actually occurs when fruits are small and green. After infection, the fungus "rests" between the cuticle and the epidermis of the fruit. The fungus is activated by exposure of the fruit to low temperatures, fruit maturation or plant stress. As fruit ripens, symptoms begin to appear and susceptibilities increase.
Symptoms begin as small, depressed lesions that are circular in shape. Lesions enlarge and become more sunken (figure 1.). If you were observant and caught these symptoms early, you could slice the lesion off and observe that the flesh underneath was pale and the texture granular. As the lesion develops, target-like rings appear. As the lesion matures, the center turns tan and small black fruiting bodies appear (figure 2.). If the weather is wet, salmon-colored spores can be observed on the lesion surface. Lesions can coalesce if infection becomes severe. It is not uncommon for secondary pathogens, particularly bacteria, to invade and quickly render the fruit more inedible than it already appears to be. For this reason, infected fruit does not have an appreciable storage life and should be discarded.
Despite all this apparent destruction, Colletotrichum is considered a weak pathogen and can often be controlled through good cultural practices. The pathogen overwinters on infected plant debris, so it is very important to dispose of rotten fruit and infected plants. Because of the ability of this fungus to persist in the soil, tomatoes and other solanaceous crops, like peppers and eggplants, should be rotated on an every-other-year basis. Weed control is extremely important because of the broad host range of this fungus. Staking plants and mulching around them will help reduce losses due to this disease. Water plants at their base, as overhead watering splashes and spreads the fungus that causes this disease. Pick fruit promptly to prevent over-ripening.
Minimizing overhead watering, staking and mulching plants, and removing infected fruit will lessen or even prevent this problem next year. However, if this is a repeated problem in your garden, you may wish to consider the use of copper-based fungicides when fruit first develops.