Cause Pythium debaryanum and P. ultimum are fungus-like microorganisms that persist indefinitely in soil or plant debris. Favored by wet soils, they increase when vegetable and legume crops are planted too frequently in the rotation.
Symptoms In the intermountain west, the disease appears when bean plants are in the pre-bloom stage and continues to affect plants for 2 to 3 weeks thereafter. The stem and lateral branches develop a rot that extends from the soil line to several inches above it. The stem cortex initially becomes soft and slimy and turns dry, shrunken, and tan after a few days. Infected plants usually wilt and die quickly. However, some infected plants may survive into the pod-filling stage. Those plants have a white fungal growth near the soil line extending an inch or two up the stem. The rotted portion of the stem and lateral branches is light brown and shrunken in dead plants. The lack of sclerotia and the occurrence of the disease well before row closure helps distinguish Pythium wilt from white mold.
Control No controls are recommended because the disease rarely affects more than a small percentage of plants in the field and adjacent healthy plants compensate for their dead neighbor.
Reference Hall, R. 1991. Compendium of Bean Diseases. St. Paul, MN: APS Press.