Start looking for early signs of pod and stem blight in soybean fields now. Full-blown symptoms of pod and stem blight usually don't show up until late August and into September. That's a little late for seed producers to do anything to curb disease development.|
Now as we move into August, small black fruiting structures, called pycnidia, of the fungus will be evident on the petioles of fallen leaves.
Black pycnidia on fallen petioles.
The pycnidia will look like small pieces of soil on the petiole, only they won't rub off. So take a look at abscised leaves for this early warning sign of the disease.
Pod and stem blight is caused by the fungus Diaporthe phaseolorum var. sojae or by Phompsis longicolla. The fungus is typically seedborne, and most infection takes place after R7. The disease can also survive and be spread by infected soybean residue.
The disease occurs primarily on plants nearing maturity and will be most severe in wet seasons with delayed harvest. Late in the season, the disease is not hard to diagnose; signs of the pathogen are obvious. Look for the small black pycnidia of the fungus arranged in straight rows up and down the stem of diseased plants. Pycnidia may also be found scattered on the pods. The fungus also can rot the seed as the plants mature. Pod and stem blight is an important factor in reduction of seed quality in seed production fields. Infected seeds produce low-quality oil and flour.
Disease management options include planting high-quality certified seed that is disease free and has a warm germination of 80 to 85%, or greater than 75% in a cold germination test. A fungicide seed treatment is recommended at planting because of the seedborne nature of the fungus. To manage disease in-season, foliar applications of fungicides are recommended in seed production fields. The fungicides Benlate, Bravo, and Topsin-M are labeled for control. Harvest in a timely manner because if weather conditions have been wet, all seeds on diseased plants can quickly be infected. Bury infected crop residue and implement a rotation with nonhost crops, such as corn, sorghum, small grains, alfalfa, or small legumes.--Suzanne Bissonnette