A late-season bronzing of the leaves is a symptom of infection by the Cercospora leaf blight pathogen. Photo: University of Wisconsin
CERCOSPORA LEAF BLIGHT
Cercospora leaf blight of soybean is caused by the fungus Cercospora kukuchii, a close relative of the frogeye leaf spot pathogen, Cercospora sojina.
This late-season leaf and seed disease shows up in during pod-filling stages. It is relatively easy to identify by a light purple to bronze discoloration that resembles a sunburn on the uppermost leaves. The coloration deepens and takes on a leathery appearance as the plants approach maturity.
Seeds can also be infected, resulting in a seed disease called purple seed stain.
Cercospora leaf spot is becoming more common in the North Central region.
Cercospora kikuchii overwinters in infected seeds and plant residue. The fungus forms spores on the surface of the residue during periods of warm (75-80°F), humid weather. The spores are wind-blown or rain-splashed to new soybean tissue where infection occurs.
Cycles of infection typically begin at flowering. Seeds become infected when the fungus invades the pod and grows through the upper vein. The hilum and eventually the seed coat become infected.
If infected seeds are planted, the fungus grows through the seed coat to the cotyledons and into the stem. In cases of mild seed infection, the coat may be shed before infection of the seedling can occur. In other cases, the young plant may be killed at an early age, or the fungus may become established in the plant, but produce no symptoms until later in the season.
Look for bronzed leaves at the beginning of seed set and into maturity - Photo: North Carolina State University
Scout for Cercospora leaf blight during late-season field scouting. Look for a purplish-bronze discoloration of the canopy. Only the top leaves are affected. As the plants mature, the leaves begin to look leathery. In susceptible cultivars, the entire leaf surface may have this appearance.
Cercospora can be mistaken for sunburn, sudden death syndrome (Fusarium verguliforme), pod and stem blight (a disease caused by Diaporthe and Phomopsis fungi), or even early senescence.
If you look closely at individual leaves, you'll see reddish-purple spots that vary in size. Large brown patches develop as the spots merge. The fungus itself can't be seen without a microscrope.
At maturity, check the seed for purple discoloration, which is the seed phase of the disease.
This seed shows symptoms of purple seed stain caused by the fungus Cercospora kikuchi - Photo: X. B. Yang. Iowa State University
The first sign of Cercospora leaf blight is usually a purplish discoloration of the young uppermost leaves of the canopy during pod-filling stages and as the plant approaches maturity. Severely-affected upper leaves are often shed while lower leaves remain attached.
If the fungus infects the seed, symptoms can range from no symptoms at all to pink or purple discoloration that cover part or all of the seed. This phase of the disease is called purple seed stain.
If infected seed is planted, seedlings will show symptoms of the disease. The cotyledons shrivel, turn purple and drop. Dark stem lesions form which can girdle and kill the seedling. Mildly- infected seedlings survive but produce stunted plants.
Cercospora leaf blight and purple seed stain generally do not affect yields in the North Central region - Photo: L. Giesler, University of Nebraska
Cercospora leaf blight, caused by Cercospora kikuchii, is generally not yield limiting in the North Central region.
Seed infection by Cercospora and the resulting purple discoloration does not reduce yield directly. However, a high percentage of stained seed may reduce the value of the crop, result in grain dockage, or in denial of seed certification.
Planting seeds infected by Cercospora kikuchii, or it's close relative C. sojina (frogeye leaf spot) may result in delayed or reduced germination and poor seedling vigor.
Conditions that favor development of Cercospora are:
Crop rotation and residue management are important control measures for Cercospora - Photo: SS Navi, Iowa State University
A combination of cultural controls and host resistance can provide good control of Cercospora:
Possibilities for chemical control
Seed lots with a high percentage of infected seed can be treated with a seed treatment fungicide.
Foliar fungicides may be applied during early pod stages R3-R5 in fields with a history of the disease, or when weather conditions are especially conducive.
Cercospora leaf blight. - Photo: University of Nebraska
Cercospora leaf blight. - Photo: X. B. Yang, Department of Plant Pathology