Soybean farmers may see more purple stain in soybeans this year.

In 2021, the University of Missouri Plant Diagnostic Clinic received seven soybean samples diagnosed with Cercospora leaf blight. The samples came from Vernon, Cooper, Montgomery, Chickasaw, Clark and Johnson counties.

And according to MU Plant Diagnostic Clinic director Peng Tian, most of the samples were submitted toward the end of the growing season.

Disease basics

The disease is a caused by a fungus called Cercospora kikuchii. Symptoms appear later in the R5 stage as plants start to set seed. This disease is often seen when there are dry conditions at pod fill followed by wet weather.

In the field, farmers will first see the disease develop as purple, brownish spots on leaves. As it progresses, these leaf spots deepen in color and combine to form large necrotic areas that eventually lead to premature defoliation, Peng explained in a recent MU-IPM Newsletter. Farmers will experience yield loss from the leaf loss. However, seed quality will likely suffer.

The purple seed stain occurs when the fungus infects the seed. The seed coat turns purple and reduces overall soybean seed quality.

Cercospora leaf blight can survive in plant debris left over from the previous season and unclean seeds, Peng added. “When the weather becomes humid and warm in the spring, the fungus begins to produce spores [conidia] that can be spread to the newly planted soybeans with the help of wind or rain splash and infect whatever tissue they land on,” he said.

The disease can also be transmitted by planting a seed infected with the pathogen.

“Based on the communications with the clients this year,” Peng noted, “this disease progressed very fast and normally caused severe defoliation within two to three weeks.”

Management options

Cercospora leaf blight is a disease farmers need to anticipate early. Application of foliar fungicides during early pod stages may help prevent foliar blight and pod infections.

Unfortunately, once purple seed stain sets in, there is little a farmer can do to bring the bean back from the fungal infection. However, farmers should make a note of the problem and prepare for the next growing season.

To help mitigate the disease, Peng offers the following advice:

Crop rotation. Reduce the number of pathogens present in plant debris by rotating the field with other crops.

Seed selection. Plant disease-free seeds.

Early planting. This cultural method avoids the hot, dry weather of late summer that can exacerbate disease symptoms.

Later-maturing cultivars. Soybeans vary in their response to Cercospora, and later-maturing cultivars are typically more resistant to the disease than early-maturing cultivars.

Chemical control. A foliar fungicide application may be applied in the early pod development stage (R2-R4). However, this has only shown limited results. Seed treatments may also be used if seed lots have higher percentages of infected seeds

MU-Integrated Pest Management contributed to this article.