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Posted 14 April 2010. PMN Crop News.

What Discolors Beans?

Source: University of Illinois Press Release.

Urbana-Champaign, Illinois (April 5, 2010)--Question: What causes brown spots on beans? Many different diseases can attack beans resulting in a discoloration of the seed coat. Those diseases, including soybean mosaic virus, bean pod mottle virus and purple seed stain, can result in blotching or streaking.


Soybean mosaic virus is transported from plant to plant by aphids and possibly other vectors that briefly stop in bean fields on their way elsewhere. Bean pod mottle virus can be transported by bean leaf beetles. Purple seed stain resides in infected seed or crop residue. Of the diseases, soybean mosaic virus and bean pod mottle are often two good explanations for bean discoloration.

Another potential culprit could be a common fungal disease also related to infestations of bean leaf beetles. Bean leaf beetles don't actually transport this disease-causing organism on their own, but their feeding predisposes plants to infection by something called Alternaria tenuis.

Alternaria tenuis and other Alternaria fungi are possibly the most common disease causing organisms in the environment. Many different species of plants may be infected by this fungus, which occurs throughout the state and throughout other soybean producing regions. Favored by prolonged periods of warm, moist weather, the disease can infect any above ground tissue on the plant. On leaves the disease will eventually form several dark brown, irregular shaped lesions. The lesions are easily distinguished from other lesions by concentric rings of dead tissue. Interestingly, these concentric rings will appear on other plants as well, including vegetables in the garden.

Spread by wind born spores or by splashing water containing spores, Alternaria tenuis is a weak organism and has a very difficult time penetrating leaf tissue. In order to infect a plant, a wound or some other form of injury is required to provide entry. It is at this point that the bean leaf beetle begins to play a significant role. Bean leaf beetles feed on all portions of the plant, including soybean pods. Once a pod is injured by the feeding of the bean leaf beetle, the pod and the seed contained within the pod become susceptible to the Alternaria fungus. In seeds, that infection appears as a brown or "dark" discoloration of the soybean seed.

It is worth noting that bean leaf beetle populations are influenced by the severity of the winter in an area. Bean leaf beetles over winter in Illinois beneath debris, and a mild winter means high surviving populations going into the growing season. In the early 80's, high populations of bean leaf beetle coincided with a high incidence of Alternaria seed decay. While the population capable of causing extensive pod injury may appear this year, the conditions necessary for Alternaria to appear may not. No one knows if warm, moist weather will sweep into the area providing the right environment for the disease to appear in August and September.

Matt Montgomery
County Extension Director