Beans

[toc]

Insect Damage

Stems Cut Off Near Soil Surface


Cause

Cutworms--the larva of several species of moths. Cutworms appear in a variety of colors including gray, brown, and black and may be stripped or spotted. Cutworms like other caterpillars are soft bodied and reach sizes up to 1 1/4 inch in length. When disturbed, cutworms curl up.

Treatment

Remove cutworms by hand in small plantings. In large plantings, use approved formulations of Bt, or Sevin. Apply these pesticides when cutworms are young and small if possible.

Round Holes in Leaves


Cause

The bean leaf beetle (Cerotoma trifurcata), a 1/4 inch long green-yellow, or red beetle with a black triangle-shaped spot behind the head. Some individuals have multiple spots similar to a lady beetle. Several generations hatch in Minnesota in a single season. Larva live in the soil and feed on bean roots, although this damage is not considered to be a problem. Adult feeding to leaves can create wounds that potentially expose plants to disease. Feeding on seedlings is especially damaging and can kill plants.

Treatment

Treat with insecticide if more than 10% of a field has damage during the cotyledon stage of crop development. Approved Orthene and Sevin formulations are available. Also avoid planting beans near alternative host crops such as soybeans and alfalfa. Rotate crops annually and remove crop residue to disrupt overwintering.

Hollow Stems or Tunneling within the Plant


Cause

European corn borer (Ostrinia nubilalis), the larval stage of a moth. In most of Minnesota, only one generation occurs per year.

Treatment

Infected sites are sometimes difficult to spot until infestation is underway. Larva enter stems through small holes, then begin tunneling throughout the plant. Wilting and broken stems often follow this tunneling activity.

Pesticide treatments are difficult since the larva is only active outside the plant for a few days between egg hatch and tunneling activity--although labeled formulations of Orthene, Sevin, Bt, and permethrin are commercially available. Watch for flat white egg masses on leaf surfaces, or use black-light insect traps to scout for adult females in mid-June. Pesticides should be applied immediately after egg hatch, or 7 to 10 days after females begin to appear in June.

For additional control remove overwintering sites by eliminating crop residue from corn, pepper, potato, tomato, bean, and beet fields.

Disease

Brown, sunken lesions


Cause

Anthracnose (Colletotrichum lindemuthianum), a fungal disease.

Treatment

Plant disease-free seed, and use resistant varieties whenever possible. Remove crop residue each season and rotate crops annually. Approved fungicides include various chlorothalonil and thiophanate-methyl formulations.

Orange pustules


Cause

Bean rust (Uromyces appendiculatus), a fungal disease.

Treatment

Plant disease-free seed, and use resistant varieties whenever possible. Remove crop residue each season and rotate crops annually. Approved fungicides include various chlorothalonil formulations.

Cottony fungal threads


Cause

White mold (Sclerotinia sclerotiorum), a fungal disease.

Treatment

White mold problems are most severe under wet conditions. Adjust plant spacing to allow air circulation and rapid drying of plants. Also avoid overwatering. Approved fungicides may be available. Contact your local Extension agent for current information.

Brown or Red, swollen and brittle stems accompanied by yellowing leaves


Cause

Fusarium wilt (Fusarium oxysporum), a fungal disease. Other symptoms include wilting and yellowing leaves, starting from the bottom of the plant and moving up.

Treatment

Resistant cultivars are available, contact your local Extension agent or seed dealer for currently available varieties.

Seedlings that shrivel and die


Cause

Damping off, a disease condition caused by two fungi (Pythium spp. and Rizoctonia solani).

Treatment

Avoid planting in wet soils, use seed treated with fungicides if possible.

Shiny Dark, Wet Lesions


Cause

Bacterial blight (Pseudomonas syringae). Lesions become dark and necrotic and are worse in rainy and windy conditions.

Treatment

Avoid overhead irrigation. Plant resistant varieties.