One of the most common fruit rots of tomato, especially in vegetable gardens, is caused by several species of the fungus Colletotrichum. This pathogen can infect green fruit, but symptoms do not develop until fruit begin to ripen. Spots on fruit initially are small, circular, and depressed. They can enlarge considerably over time, and may develop concentric rings. The center of anthracnose spots become dark as the fungus produces spore-containing structures (microsclerotia and acervuli).
Masses of pink to orange colored spores are released from these structures when weather is wet or humid. These spores are dispersed to other fruit by splashing water. Eventually the entire fruit will rot, especially when there are several anthracnose spots or decay organisms enter the diseased tissue. Fruit nearest to the ground are most likely to be affected. The fungus can also infect roots.
Manage anthracnose by controlling sources of the pathogen, minimizing the opportunity for dispersal of the pathogen, reducing favorability of environmental conditions for disease development, and applying fungicides. There are no resistant varieties.
The spore-containing structures provide a means for the causal fungus to survive between crops. Consequently, important practices for managing anthracnose include rotating where tomatoes are grown. Not growing tomatoes or other solanaceous plants (especially potato) in the same area for 3 to 4 years is ideal. A practice that can be implemented in a small garden is removing affected fruit rather than letting them drop to the ground.
The pathogen can also be seed-borne, Therefore seed should not be saved from diseased fruit.
Covering the ground with black plastic mulch, straw, or other material provides a barrier between the pathogen in the soil and fruit. Trellising plants increases the distance between the fungus in the soil and fruit, plus air circulation will be improved enabling plant tissue to dry more quickly.
Many fungal pathogens need plant tissue to be wet in order to infect. Practices to minimize the length of time that fruit will be wet from rain or dew include trellising, locating tomatoes where there is good air movement and no shade, and orienting rows parallel to the predominant wind direction. Providing water to the base of plants rather than using a sprinkler not only avoids wetting fruit, but avoids the opportunity for splash dispersal of the pathogen.
Pick fruit as soon as it is ripe to minimize the time for anthracnose to develop, but note that development of symptoms is not completely prevented by taking fruit from plants to drier, protected, indoor conditions.