Pod and stem blight is caused by various species of the fungi Diaporthe and Phomopsis that are common in Iowa. The fungi survive the winter in infected seed and in crop residue. Most years they do not cause much damage. In other years, build-up of the pathogen in crop residue and favorable weather conditions can lead to high levels of pod and stem blight in some fields.
When pod and stem blight occurs, dark lesions are found on the pod, petioles, and nodes of the stem. The upper portion of an infected plant turns yellow and dies.
Pod and stem blight has little effect on crop yield, but seed quality can be greatly affected. The disease is principally of concern to soybean seed producers.
Seed infection occurs only if pods become infected. Pod infection can occur any time starting at flowering, but extensive seed infections will not take place until plants have pods that are beginning to mature (R7 growth stage). Pod and stem blight often occurs when soybeans mature during wet weather and harvest is delayed.
Damage to pods by insects will favor the development of both pod and seed infections.
Pod and stem blight can be confused with anthracnose. Warm, humid weather favors the development of both diseases, and both diseases frequently occur together late in the season.
Seeds infested with Phomopsis are cracked and shriveled (Figure 1) and usually have a reduced germination rate. If these seeds are planted, emergence would be low due to seed rot or seedling blight. It is recommended to do seed testing if seeds are being saved from a field where pod and stem blight is severe. ISU Seed Science Center provides seed testing for a reasonable charge.
Management of Pod and Stem Blight
Harvest in a timely manner to reduce risk of delayed harvest in wet weather.
Rotate crops to reduce the survival of the fungus in soybean residue.
Till fields with a history of Phomopsis seed rot, pod and stem blight to promote decay of residue.
Seed producers may consider fungicide use. Fungicide treatment does not increase yield, but can preserve seed quality. Some fungicides are effective as seed treatments.