White mold is the most serious yield limiting disease of edible beans under wet growing conditions. Recent weather conditions have been very conducive to white mold disease development. Scout your fields!

White mold is caused by a fungus and develops as a white cottony growth on the stem, stem branches and pods of bean plants. The fungus also produces black hard mats, which appear near the cottony growths. These black structures are the survival structures, which allow the organism to survive adverse (winter) conditions.

The disease cycle starts when the leaf canopy covers the row spaces, within and between the rows. Germination of the survival structures occurs when soil conditions of near field water holding capacity for 10 to 14 days and temperatures between 59-65 degrees F. Upon germination, small mushroom-like bodies appear on the soil surface. Spores are produced by these mushroom-like structures, which infect wilted flowers or other dead plant tissue, later spreading to living plant tissue.

Infection kills some plants and severely reduces the yield of plants with pod infections. High humidity and plant canopy temperatures between 68 and 78 degrees F favor the spread of white mold.

Fungicides are effective in controlling this disease, and timing is important for the most efficient control. Research data has revealed a high correlation between the amount of rainfall 10 days before bloom to 10 days after bloom to final disease. This gives us information on whether or not we should spray. Sprays need to be made before the disease develops for best control. It is important to follow label directions for the best control. University research shows that a single banded application or 2 broadcast applications are effective.

Crop rotation using the right crops will help prevent the build up of inoculum. A rotation of three to four years between susceptible crops is necessary to accomplish this. Sunflower, potato, canola, mustard, and soybeans should not be grown in close rotation with edible beans because they are susceptible. Small grain and corn are recommended in a rotation with edible beans because they are not susceptible. Sclerotia from a previous bean crop will germinate in a small grain canopy and can serve as a source of spores for nearby bean fields. Nevertheless small grain is a good rotation crop as it rids the soil of many sclerotia through germination. The small grain crop is not affected by these fungus spores. Small grain fields should be planted downwind (the prevailing wind) from bean fields. This could help prevent some spores from reaching the bean fields. The use of bean varieties with an upright growth habit in wide rows and the use of recommended fertility and seeding rates will also help alleviate the disease pressure.

Will R. Yliniemi
Hubbard/Becker Extension Educator