Cercospora in Soybeans
Cercospora in Soybeans
In soybeans, Cercospora kukuchii, is the culprit. This later season disease shows up after pollination in soybeans and during pod fill. Its signature symptomology is a give away by virtue of the purpling and bronzing of leaves located in the uppermost portion of the canopy. The leaves become almost leathery as the season progresses and the coloration deepens within the leaf. The seeds of the soybean plant can also be infected, and when this occurs, the common name of the disease is purple seed stain. The disease is most commonly found in Southern locations and in the North Central states.
Scouting and Identification When scouting in late season, Cercospora can be mistaken for other conditions and diseases such as sunburn or sudden death syndrome. Other fungi like Diaporthe and Phomopsis which cause pod and stem blight can also be confused with Cercospora, as can early onset of senescence.
However, if you closely examine individual leaves, you’ll see the unique symptoms described above. At maturity, check the seed for purple discoloration, which is the seed phase of the disease.
Significant yield loss is rare across areas where Cercospora leaf blight is found in Soybeans; however, where seed infection is significant, the value of the crop can be reduced and dockage if harvested for grain; or outright denial of certification if grown for seed. When severe, the impact from kukuchii or sojina on germination and/or seedling vigor can be significant.
Management options for Cercospora in Soybeans integrate cultural practices and host resistance. Rotation of crops to minimize buildup of the disease, clean tillage, destruction of residue, using disease free seed, locating fields to maintain good air flow, and avoiding poorly drained low lying fields all contribute to lessening the disease. Later-maturing seeds tend not to be as susceptible to Cercospora, and some seeds claim fairly good resistance in their genetics.
Chemical control of infected seed with seed treatment fungicides can be effective. Foliar fungicides which are applied during early pod stages R3-R5 may offer some control, but the decision to use them should be based mostly on fields which have a history of the disease and were planted without recommended residue controls, combined with favorable overwintering conditions. When these situations exist with current weather conditions favorable to the disease, a foliar treatment may be effective. Applying foliar fungicide outside of this recommended window is not likely to provide an economic return.