Angular Leaf Spot of Dry Beans
Angular leaf spot is caused by the fungus Isariopsis griseola, which overwinters in crop debris in tropical and subtropical climates. It is not know, however, whether this fungus overwinters on crop debris in Michigan. ALS attacks kidney beans and the dark red kidney bean variety, Montcalm, in particular. It does not attack other bean market classes or varieties. Symptoms are generally evident at late flowering or early pod fill. Leaf lesions, initially gray or brown, become necrotic and well defined with the typical angular shape occurring by 9 days after infection. Plant stems, petioles and branches may also be infected. Pod lesions are oval to circular with reddish-brown centers surrounded by darker colored boarders. Infected pods may contain poorly developed or entirely shriveled seeds, and the fungus can be carried on or in the seed. These symptoms should not be confused with bacterial blight. Angular leaf spot is characterized by fluffy growth of the fungus (synnemata which bear spores) in the lesions on the underside of the leaf.

Although generally rare in Michigan, angular leaf spot was a serious problem on the Montcalm red kidney bean variety in the northern production areas, chiefly Presque Isle County. The Montcalm red kidney bean is the dominant variety in northeast Michigan and, therefore, the most severely infected. It was a concern in the mid to late 1980's and was found on kidney beans in Presque Isle county this year, but the incidence was low and not considered epidemic.

Severely infected fields should be rotated out of susceptible bean crops for at least one year. Of the various commercial classes of dry beans, red kidney and cranberry are most susceptible. Until more is know about this disease, rotation to another crop is recommended.

The fungus has been identified on seed used for planting. Therefore, seed treatment with the standard combination of fungicide, antibiotic, and insecticide is recommended. Application of a fungicide to the foliage of bean plants may be beneficial if applied early. Once infections are well advanced, fungicides will be of little benefit. Fungicide spray trials have not been done in Michigan, but benomyl and thiophanate methyl reportedly control angular leaf spot. Fungicides are used only when the disease is positively identified and detected early.