Cercospora Leaf Blight

By Cory Cowan, Crop Sales Specialist, AGRIS Co-operative - Chatham.

This past growing season during the late summer and early fall we came across quite a few cases of Cercospora Leaf Blight in soybean fields throughout Chatham-Kent. The impact of this disease can be loss of yield and bean quality especially for IP soybeans.

Image credit: Daren Mueller, Iowa State University, Bugwood.org

The disease Cercospora Leaf Blight is caused by a fungus called Cercospora kikuchii. While observing your soybean crop you may start to see purple to brownish spots on leaves when the disease is in its early stage. As Cercospora progresses leaf spots become darker in colour and eventually results in premature leaf drop and reduced yields.

Beyond a yield reduction this disease lesions on the pods can cause purple staining on the seed which result in loss of quality for IP soybeans, rejection and loss of premiums.

Purple stained soybean seeds.
Image credit: Adam Sisson, Iowa State University, Bugwood.org

Image credit: Daren Mueller, Iowa State University, Bugwood.org

Cercospora pathogen survives in plant residue, which can be a source of spores that spread the infection to the next crop. Infected seed could also be an issue.

Initial infection can occur with wind or rain delivering spores onto new leaves and warmer weather favours infection.

The disease may stay dormant until more favourable conditions develop later in the season. Later in the growing season, blight symptoms appear on the leaves, stems, and seed pods. This year I saw symptoms causing severe defoliation within one to two weeks of the beginning of seed fill at the R4 to R5 growth stages.

Some of the best ways to manage Cercospora Leaf Blight is through crop rotation which reduces pathogens present in the field and incorporating soybean crop residue.

Focus on planting clean, good-quality seed free of disease. Plant early in the spring and try and avoid warmer and dry weather that can increase disease pressure during reproductive stages at seed fill.

Seed treatments can be effective; selecting for tolerant varieties and foliar fungicide applications between R2 and R4 timing may also be used but may have mixed results.

At the end of the day, the best practice is crop rotation and planting a certified, disease-free seed and tolerant varieties.

Contact an AGRIS Crop Sales Specialist or AGRIS location near you.

References:
https://ipm.missouri.edu/IPCM/2021/11/cercosporaLeafBlightSoybean-PT/
Crop Protection Network images
OMAFRA Diseases of Field Crops