Stem canker disease, caused by the fungus Diaporthe phaseolorum var. The cankers tend to be most abundant in lower to mid stems, generally at one or more of the first eight nodes. In the early stages of canker development, cankers will be superficial and reddish-brown in color. As the disease progresses, cankers will enlarge, longitudinally, turn dark brown to black in color, become slightly sunken and eventually girdle stems completely. At this point, affected plants will die due to disruption of the free flow of nutrients and water in the plant.
Plant death is also due to activity of a fungus-generated plant toxin.
Invaded stem tissues can become brittle and cause plants to break easily at the canker. Serious stem canker development in a field only occurs when plants are infected in the early stages of vegetative plant development. The most critical time is the V3 stage, the stage at which three nodes are present on the main stems of plants. When young plants are infected, some will die rather quickly. The bulk of infected plants, however, will survive and the infection becomes symptomless.