Bean Plant Diseases Caused by Fungi

Green beans, or string beans, are a common garden staple in the U.S. String beans favor warmer temperatures, typically after the last ground frost, and moist soil. Bean plants are relatively easy to grow and usually are problem free; however, fungal diseases can infect bean plants under ideal environmental circumstances. Early identification and prevention are important for controlling bean plant diseases caused by fungi.

Fusarium Root Rot

Root rot in bean plants, caused by the fungal pathogen Fusarium solani, is one of the most common diseases of the bean plant. The disease thrives on moist soil and weak plants. According to the University of Minnesota, symptoms of root rot consist of reddish to brown discolorations on the taproot, yellowing of leaves, branch dieback and plant death. Because the disease primarily takes place below the soil surface, it is important to avoid overwatering and using well drained soil. Control for root rot consists of avoiding overwatering, crop rotation and avoidance of fertilizer during dry. Fungicides are not a primary control as they cannot penetrate below the soil in a way that kills the fungus.

Anthracnose

Caused by the fungus Colletotrichum lindemuthianum, anthracnose is a disease that affects all the above ground tissue on the bean plant. Symptoms of anthracnose consist of dark water soaked legions on leaves and stems and change in legion color from red to purple to a dark brown or black. According to the University of Minnesota, the disease favors temperatures between 55 and 70 degrees F with available moisture on the plants surface. Anthracnose, like most other fungal related diseases, thrives when there is free moisture on the plants surface. Avoiding overwatering and watering at night, where water takes longer to evaporate are preventative measures. Control for the disease consists of the use of copper based fungicide and the use of resistant bean plants.

Pythium Root Rot

According to the North Carolina State University, phythium root rot is caused by a variety of different fungus species including ultimum, debaryanum, aphanidermatum and myriotylum. The disease affects the seeds and roots of the bean plant. Symptoms include dark water soaked legions on the plants roots, soft and mushy seeds and visible white fungal growth. The disease favors both cool and warm temperatures around 60 to 90 degrees F, states the University of Alabama Extension. Like other fungal infections, phythium root rot needs free moisture in the soil for the disease to develop. Preventative measures are the most important method of control as fungicides do not work well underground after the plant takes root. Prevention includes using well drained soil (reduces moisture build up), soil based fungicide before planting and using soil free of past phythium root rot infections.

Sources:

http://www.extension.umn.edu/distribution/horticulture/dg6144.html
http://www.aces.edu/pubs/docs/A/ANR-1005/
http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/vg7.htm