Pod and Stem Blight and Phomopsis Seed Decay
Under some conditions this disease can cause yield loss and a significant reduction in seed quality. Pod and stem blight may be more common when harvesting is delayed during wet weather. Symptoms of pod and stem blight can be confused with anthracnose, and both diseases can occur together on plants late in the season.
Plants are usually infected by pod and stem blight early in the growing season. The pathogen may be present in green tissue without causing symptoms. The pathogen can infect all aerial parts of plant, but does not cause distinct lesions. Signs of infection appear on fallen petioles in mid-season and on pods and stems of plants nearing maturity. A key sign of infection is many small, black, raised dots (pycnidia) arranged in rows on infected stems, pods, and fallen petioles late in the season. In wet seasons, pycnidia may cover the entire plant at maturity. Upper portions of infected plants may turn yellow and die. Infected seed are cracked, shriveled, dull, and may have a gray mold on them. Seed infected by pod and stem blight may decompose after harvest and have low viability. Seedlings grown from infected seeds may often blighted.
Conditions and Timing that Favor Disease:
Infection can occur throughout the season, although symptoms are not seen until later in the season. Wet, warm conditions and continuous planting of soybeans favors pod and stem blight. Insect damage or other injury to pods and seeds favors pod and seed infection.
Pod and stem blight is caused by the fungus Diaporthe phaseolorum var. sojae. The seed decay is caused primarily by the related fungus Phomopsis longicola or other species. These fungi overwinter in soybean tissue residue and in infected seeds. Host range includes green bean, pepper, and tomato.
Rotate soybean with non-hosts such as wheat or corn. Use high quality, pathogen-free seed. Some soybean varieties may differ in resistance. Foliar or seed treatment fungicides may be of value under conditions that favor this disease. Harvest seed quickly after it matures.