Fungi have been very active this season in some soybean fields. Right now at the University of Illinois Plant Clinic we are seeing samples that are the result of a very wet season. All that rain couldn’t have made fungi more active! High on our recent disease diagnosis list has been Pod and Stem Blight caused by the Diaporthe/Phomopsis complex. Did you know that you can look for infection by this disease well before harvest? Late July and early August are ideal for looking for the fruiting structure of Pod and Stem blight on the petioles of fallen soybean leaves. The pycnidia will look like small pieces of soil on the petiole, only they won’t rub off. So take a look at fallen leaves for this early warning sign. Certainly we have recently received numerous samples with this early sign of infection. We have also seen samples in which the disease progressed beyond early infection and is now working on the stems. Look for the small black pycnidia of the fungus arranged in straight rows up and down the stem of diseased plants. In a few weeks you will also see pycnidia 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.
Soybean plants affected by pod and stem blight
Sudden death syndrome Fusarium virguliforme– infected plants have also been a frequent sample this past week. Interveinal chlorosis and necrosis of the leaves (veins remain green while the tissues between the veins turn yellow and then brown) should be a red flag to you that the stem needs investigation. This leaf symptom is common to several stem and root diseases. Split the stem. The pith will be a normal color and the exterior vascular tissue will look a little grayish. Cool and wet weather after planting and the recent rainfall received in parts of the state were favorable for infection and disease development, and are the reasons why SDS incidence is high in some areas this year. The primary method of managing SDS is to choose the most resistant soybean varieties available.
Sudden Death Syndrome
Plant Clinic Director and Illinois NIFA-Extension IPM Coordinator
University of Illinois Extension