Type of crop, variety and environmental conditions will influence the severity of symptoms. Gray to tan-colored lesions appear on leaves, stems, pods or fruits. These spots can be circular, oval or irregular in shape and with dark brown, reddish or purplish margins. Under favorable weather conditions, they become more numerous, enlarge and coalesce, turning dark brown or black in the process. Their center gradually becomes grayish and, in the later phases of the infection, it may show tiny dispersed black flecks. A reddish discoloration of the midrib of leaves is also common in some crops. In severe cases, the leaves wilt, dry and fall off, causing premature defoliation of the plant. On stems, lesions are elongated, sunken and brownish, also with darker margins. As they enlarge, the lesions may encircle the base of the stem, causing the plant to wilt and lodge. Top dieback of stems or branches is also common.
The symptoms are caused by several species of fungi of the genus Colletotrichum spp. They survive in the soil, associated to seeds, or on plant debris and alternative hosts for up to four years. There are two ways by which the infection is carried over to new plants. Primary infections happen when soil- or seed-borne spores infect seedling during emergence, growing systemically in the tissues. On other cases, the spores are splashed onto the lower leaves by rain drops and start an infection that spread upward. Secondary infections start when the spores produced within the leaf or fruit lesions are dispersed by rain splashes, dew, sucking insects or field workers to upper plant parts or to other plants. Cool to warm temperatures (optimal 20 to 30 °C), soils with high pH, prolonged leaf wetness, frequent rainfalls and dense canopies favor the disease. A balanced fertilization makes crops less prone to anthracnose.
The spreading of the disease can be prevented by plunging seeds in a warm water bath before sowing (temperature and time depend on the crop). Neem oil can be sprayed. Biological agents might also help to control the infection. Products based on the fungus Trichoderma harzianum and the bacteria Pseudomonas fluorescens, Bacillus subtilis or B. myloliquefaciens can also be used as part of a seed treatment. Organically approved copper formulations can be sprayed against this disease in a variety of crops once the symptoms have been detected.
Always consider an integrated approach with preventive measures together with biological treatments if available. Spray early in the day, and avoid applications during hot weather. Also, treat seeds prior to planting. Seed dressing can be used to kill the fungi before sowing. Fungicides containing azoxystrobin, boscalid, chlorothalonil, maneb, mancozeb or prothioconazole can be sprayed preventively to lower the risk of infection (please check the specific formulation and recommendations for your crop). Some cases of resistance to some of these products have been described. In some crops, no effective treatments is available. Finally, post-harvest treatments together with a food-grade wax can be applied to reduce the incidence on fruits bound to be shipped overseas.