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Cercospora leaf blight, caused by the fungus Cercospora kikuchii, can survive and overwinter on plant residue and infected seed. Warm temperatures (75 to 80ºF) and humid or wet conditions favor Cercospora spore development. Spores can be spread by wind and rain to new soybean tissue where infection occurs. Other factors that can favor development include poor drainage, high plant densities, and poor air circulation.
Figure 1. Upper leaves of Cercospora-infected plants turn reddish and have a leathery appearance.
During seed set, the upper leaves of infected plants become dark red, orange, or bronze colored and leathery in appearance (Figure 1). Infected leaves have very small, dark lesions on or near major leaf veins and on petioles. This condition can lead to premature defoliation. Other conditions that can commonly be confused with Cercospora leaf blight include sunburn and early senescence.
When the fungus grows into the upper vein on a pod, seed can become infected, resulting in purple seed stain (Figure 2). Purple seed stain may range from tiny purplish marks to blotches covering most of the seed. Planting infected seed the following year can result in reduced germination, emergence, and vigor.
Figure 2. Cercospora-infected plants can produce seed with a purple stain.
The potential for the disease to reduce yields ranges from very low to substantial depending on the timing of disease onset, the speed of development, and environmental conditions.
In-season. Management during the growing season may include the application of a labeled foliar fungicide. Fungicide applications should be based on disease severity and timing. Applications for late-season diseases are generally made between growth stages R3 and R5 (pod development stages). Spraying fungicides after plants reach full maturity, or after R6 growth stage, is generally not recommended.
Next season. When planning for the next growing season, consider management tools such as tillage and crop rotation, which can help reduce disease inoculum, and use certified disease-free seed.
It is important to scout corn fields 3 to 5 days after the water has receded.1,2 Pull up seedlings and look at the growing point. A white or cream-colored growing point that is still firm means the plant is recovering. Growing points that are darkening and soft are beginning to die.2 Stand counts need to be taken to see if a desirable plant stand survived.
Several options are available if you need to replant a field. More geography and timing specific information on stand evaluation and replant decisions can be found from state Extension offices. If replanting with corn, minimum or no tillage is recommended to maintain efficacy of any herbicides and/or soil insecticides already applied to the field.
Switching to alternative crops when replanting corn fields must be carefully considered. Before replanting with soybeans, check your herbicide label and consult local experts to determine if the previously applied corn herbicides could damage the replanted crop. It is important to scout fields entirely before making the decision to replant.
Sources Elmore, R. Daugherty, R.B. and Mueller, N. 2015. Corn and soybean survival in saturated and flooded soils. University of Nebraska—Lincoln. cropwatch.unl.edu. 2 Thomison, P. Ponding effects on corn. May 15, 2006. Corn Newsletter. Ohio State University Extension. 3 2014. Estimating nitrogen fertilizer losses. University of Nebraska—Lincoln. 4 Al-Kaisi, M. and Pedersen, P. 2007. Wet conditions: challenges and opportunities. Iowa State University Extension. Integrated Crop Management. ICM > 2007 > IC-498 (9). Web sources verified 05/14/15