Cercospora Leaf Blight and Purple Seed Stain of Soybeans
Field Facts by Steve Butzen
Cercospora leaf blight and purple seed stain is caused by the fungal pathogen Cercospora kikuchii, which attacks both the leaves and the seeds of soybeans. As a leaf blight, Cercospora is one of many diseases that can infect soybeans in late summer, usually with only minor effects on yield. As a seed disease, Cercospora diminishes seed appearance and quality, but usually does not decrease yields significantly.
The causal organism of Cercospora leaf blight and purple seed stain, Cercospora kikuchii , overwinters in soybean residue and seed coats. The disease is spread as spores are blown or splashed onto soybean plants from infected residue, weeds or other soybean plants. Infection is more likely during periods of warm, wet weather.
Symptoms begin primarily after flowering as angular to somewhat circular lesions on soybean leaves. These red-brown to red-purple spots may merge to form large lesions, and leaves may become leathery in texture. Often, symptoms are seen only in upper canopy, for example, in the uppermost three or four trifoliate leaves. Leaves will have a general bronzing to purpling discoloration. Symptoms worsen as the crop matures, and premature defoliation of affected leaves may occur during pod-fill.
|Leaf "bronzing" at top of soybean plant due to Cercospora.|
Lesions may also develop on stems, leaf petioles and pods. Seeds are infected through their attachment to the pod, the hilum. Infected seeds may show a pink to pale or dark purple discoloration, which varies in size from specks to blotches to the entire seed coat.
|Soybean seeds showing purple seed stain caused by infection with Cercospora kikuchii.|
Yield losses from Cercospora are typically considered to be minimal in the U.S. and Canada. The leaf phase is not expected to reduce yield unless significant leaf area is lost before soybeans near maturity. Yield loss from the purple seed stain phase of the disease is negligible unless infection is severe, but quality may be reduced. In many cases, the seed discoloration is only cosmetic, affecting the appearance of the grain but little else.
When Cercospora is found late in the growing season, managing the disease is usually not recommended. In such cases, it may be too late to apply a preventative fungicide, and risk of yield loss is low. However, if Cercospora is known to be a recurring problem in a field, the following practices can help reduce the incidence and severity of the disease.
- Rotation of soybeans with crops other than legumes can help reduce Cercospora in future soybean crops.
- Tillage can reduce soybean residue on which Cercospora pathogens survive, but this option must be weighed against the cost of tillage and the loss of the soil conservation benefits of the residue.
- Foliar fungicides can control both the foliar and seed phases of this disease, but such treatments are rarely recommended by extension plant pathologists. If a fungicide is needed, the University of Nebraska provides application tips and fungicide efficacy ratings for Cercospora and other soybean leaf diseases.