Bean anthracnose

Bean anthracnose (Colletotrichum lindemuthianum) is a seed-borne plant pathogenic fungus. It is a major disease of beans causing serious crop loss in many parts of the world.

Symptoms
Seedlings grown from infected seeds often have dark brown to black sunken lesions on the cotyledons and stems. Severely infected cotyledons senesce prematurely, and growth of the plants is stunted. Diseased areas may girdle the stem and kill the seedling.

Under moist conditions, small, pink masses of spores are produced in the lesions. Spores produced on cotyledon and stem lesions may spread to the leaves. Symptoms generally occur on the underside of the leaves as linear, dark brick-red to black lesions on the leaf veins. As the disease progresses, the discoloration appears on the upper leaf surface. Leaf symptoms often are not obvious and may be overlooked when examining bean fields.

The most striking symptoms develop on the pods. Small, reddish brown to black blemishes and distinct circular, reddish brown lesions are typical symptoms. Mature lesions are surrounded by a circular, reddish brown to black border with a grayish black interior. During moist periods, the interior of the lesion may exude pink masses of spores. Severely infected pods may shrivel, and the seeds they carry are usually infected. Infected seeds have brown to black blemishes and sunken lesions.

Chemical
At this time, no seed treatments for control of anthracnose on dry beans are registered. The efficacy of seed treatments is variable because deep-seated infections frequently survive the treatment.

Foliar applications of protectant fungicides may reduce disease severity in the field, but normally fungicides are not economical. The effectiveness of foliar fungicides varies because the wet weather conducive for anthracnose development also washes and weathers the applied fungicide.

Prevention
Control is best achieved by using anthracnose-free seed. Western-grown, certified seed is highly recommended. Anthracnose development is unlikely on seed produced in semiarid areas, which have little rainfall and high temperatures during the growing season. In contrast, seeds produced areas exposed to summer rains and high humidity during the growing season have an increased risk of developing anthracnose.

The fungus survives best on bean seed and to a lesser extent on dried pods and straw. Cleaning and bagging stations in areas where anthracnose has been a problem may be sources of contaminated dust. Thus these stations should be cleaned of debris between shipments and the shipments isolated.

Since the fungus is disseminated in the presence of water, fields should not be entered for cultivation or pesticide applications when the plants are wet. Avoiding unnecessary movement in infested fields will minimize the spread of the disease.

Because the fungus does not survive well under field conditions, infested bean debris should be incorporated in the soil after harvest to reduce winter survival. A two-year crop rotation is highly recommended as insurance against winter survival, and it provides some control of root-rotting organisms.

Bean varieties resistant to anthracnose are available from seed companies. The use of resistant varieties, however, is complicated by the presence of several forms or races of the fungus, and plants resistant to one race may be susceptible to another Varieties must be tested where they are to be grown to determine their tolerance to the locally prevalent races.