The early symptoms appear as small, slightly sunken circular spots. The spots increase in size, sometimes to 1 cm in diameter, and often coalesce to cover a large area of the fruit. Spots are usually most numerous on the side of the fruit adjacent to the soil; they are usually depressed and have concentric ring markings. The centres become tan and have numerous dark specks, which are the bodies in which the fungus spores are produced.
The spores are produced in large numbers and are spread by rain to green fruit, where infection occurs. The fungus can penetrate the cuticle of uninjured fruit. Fruit infected when it is green and small shows no spotting until it begins to ripen. The fungus persists from season to season on infected plant refuse in the soil. The anthracnose fungus, which is sometimes found as a weak parasite on the roots of greenhouse tomatoes affected by corky root rot, where it is the cause of black dot root rot. Occasionally, the disease may be severe in hydroponic production.
