Anthracnose
Anthracnose is a seed-transmitted disease caused by the fungus Colletotrichum lindemuthianum. Although once a severe problem for Michigan bean growers, development of resistant, improved seed production and certification programs has reduced the treatment of Anthracnose. However, the disease is still found in a few fields every year and occasionally causes severe losses. Black beans are especially susceptible. In 1981and 1994 the use of contaminated seed caused severe disease problems. There are numerous strains or races of the bean anthracnose fungus which are distinguished by their ability to attack certain bean varieties. The alpha, beta and gamma strains are found in Michigan. Disease symptoms produced by all strains are identical. The initial symptom of an infection is a dark brown to black lesion along the leaf veins on the underside of the leaves. Leaf petioles and even stems may also show this symptom. The plants may appear fairly normal until the disease is well advanced because infections are initially restricted to the lower leaf surface. Symptoms of pod infection appear as circular, reddish-brown to near-black lesions on the pods. The spots develop a sunken center as the lesions enlarge. The center first shows brown, dead tissues which are quickly replaced by pink fruiting structures which contain the spores of the fungus. The fungus may then penetrate seeds. Such infected seeds, when planted, serve as the source of infection for succeeding crops. Because anthracnose infrequently overwinters in Michigan, infected seeds serve as the primary source of inoculum. Anthracnose development is favored by cool to moderate temperatures and prolonged periods of high humidity or free water on foliage and young pods. The pathogen can be easily transmitted to healthy plants by storms, people, and machinery moving through the field when plants are wet. Anthracnose is usually not a problem in semi-arid regions. It is important to plant treated, disease-free seed to minimize the risk of infection. Some Michigan growers use western grown seeds to control possible outbreaks of anthracnose.

A one year rotation out of beans is recommended because studies indicate the fungus may overwinter. It is recommended that fields remain clear of walking and scouting when plants are wet. If anthracnose is found in a field, spread of the disease can be limited by the use of fungicides. A single application of benlate when disease first appears is important. However, chemical control is dependent on identifying the disease early and will be ineffective once the disease is well established. Chemical control is often erratic and satisfactory control may not be achieved.