Loren J. Giesler, Extension Plant Pathologist
Pod and stem blight is caused by the fungus Diaporthe phaseolorumsojae and Phomopsis spp. The fungus overwinters in infested crop debris and in infected seed. In early summer perithecia are produced which give rise to ascospores for initial infections. Another source of the disease will be conidia produced on debris and infection sites on the plants. Both spore types are splashed onto the plant or distributed with wind. Initial infections are latent until the soybean plant begins to senesce. These diseases were found in the United States in 1920, and are now endemic throughout most soybean growing areas. While pod and stem blight had little effect on yield, seed quality may be negatively impacted when seed infections occur and Phomosis seed decay develops.
The most obvious sign of pod and stem blight is the presence of pycnidia on infected material. Pycnidia are small, black fruiting structures which are first seen on the petioles of shed leaves. One of the most definitive diagnostic features of pod and stem blight is the appearance of lines of pycnidia which may cover large sections of the stem or they may occur in clusters near the nodes. Pycnidia can also occur in a scattered arrangement on the pods. Infected seeds may appear healthy or they may be shriveled and cracked. Infected seed are often covered with white mycelium and appear chalky. Severely infected seed may not germinate.
|Stem Pycnidia||Pod Pycnidia|
Favorable Environmental ConditionsProlonged periods of warm, wet weather during pod development and maturation favor the spread of the fungus from the pod to the seeds. Very wet conditions will favor stem infections during pod filling stages.
Rotation with corn and other non-legume crops reduces the amount of inoculum available the following season. Incorporating any residue into the soil also will reduce inoculum as the fungus overwinters in plant residue. Use a certified seed source at planting as this disease will affect the germination rate.
Foliar fungicides may be applied to reduce the amount of Phomopsis seed decay. This practice is only recommended for fields grown for seed production. Applications applied from pod set (R3) through late pod stages (R6) will lower seed infection incidence. Seed known to be infected which will be used for planting should be treated with a seed treatment fungicide.
Foliar Fungicide Use In Soybean (G1862)