by Linda Geist

Farmers struggled during the drought of 2023 to identify what was happening in their soybean fields as plants showed similar symptoms with different diseases.

University of Missouri Extension plant pathologist Mandy Bish says doppelgangers, look-alikes that mimic other diseases with confusing similarities and perplexing differences, created problems for soybean growers last year. To make matters worse, old soybean foes and emerging threats combined to create challenges.

Bish looked back on soybean diseases from last year and what farmers should know heading into this growing season:

Sudden death syndrome. As its name implies, SDS strikes quickly as plants begin to flower. However, initial infection is in the roots. Symptoms include yellowing and browning of the leaves between leaf veins, followed by tissue death. Bluish fungal growth may appear on the roots. Roots show signs of rot, and severely infected leaves eventually fall off.

The fungus can survive in the soil and on soybean residue for several years. It favors cool, wet soils at planting time, so wait to plant when soil is warm. Aim to plant fields with SDS or SDS-like symptoms last. Plant an SDS-resistant variety into well-drained soil, and always rotate crops. Seed treatments can help to reduce early-season infections.

Charcoal rot. This disease favors dry, warm soils and is problematic during drought conditions. Charcoal rot’s fungus grows inside the roots and stem. Symptoms most often occur late in the season, but in 2023, it came in early.

Bish received reports of charcoal rot the week of June 5, which is early for Missouri. Most samples submitted to the MU Plant Diagnostic Clinic in June came from central and west-central Missouri, and many samples throughout the season tested positive for both charcoal rot and SDS.

The fungus disrupts water and nutrient uptake, which can result in yellowing leaves, wilting and premature leaf death. Early-season symptoms may be mistaken for Phytophthora or Pythium rots, but drought conditions favor charcoal rot. Black fungal structures called microsclerotia fall from mature plants at harvest and return to the soil, where they can infect future crops.

Disease symptoms typically appear first in the driest regions of the field, such as edges, hillsides and areas with compacted soils.

Phytophthora root and stem rot. This was substantial in areas along U.S. Highway 36 and north in 2023. Phytophthora generally favors warm and wet conditions, so its appearance during a droughty season initially seemed unusual, Bish says. But it appeared in irrigated fields where moisture created favorable conditions.

This disease can infect soybeans at any stage and cause 100% yield losses. The fungus survives on crop residue and in the soil. The pathogen is becoming more complex, Bish says.

The best management tool for this disease has been planting resistant varieties. However, the same soybean genetics have been in use since 1985, and the pathogen continues to adapt. The most commonly used resistance genes, Rps1c and Rps1k, are rapidly losing their resistance in the U.S. and some other countries.

Other strategies to help with disease management include good drainage and seed treatments with active ingredients mefenoxam, metalaxyl and ethaboxam for early-season protection. To minimize spread, work suspect fields last and clean soil from equipment before moving to other fields or storage.

Red crown rot. This disease presents symptoms that mimic sudden death syndrome, but it has not been confirmed in Missouri. Still, it is one to watch, as in neighboring Illinois it causes losses up to 30 bushels per acre.

The fungus that causes red crown rot can survive winter and infect soybean roots shortly after planting. Symptoms include reddish discoloration in the root area near the soil line. Tiny, red ball-like structures called perithecia may grow on the crown and root.

Wilting and death may not appear until late in the season. Leaves usually remain on the plant, unlike cases of sudden death syndrome. Co-infection can occur with plants having symptoms of both red crown rot and sudden death syndrome.

Later planting dates, rotation and avoiding poor drainage areas can help with managing the disease. Seed treatments may be helpful. Bish says that the disease can be spread through equipment.

For more information on soybean diseases, visit the MU Integrated Pest Management website at ipm.missouri.edu/croppest.

If you suspect disease in your soybean field, send samples to the MU Plant Diagnostic Clinic. Visit plantclinic.missouri.edu or call 573-882-3019.

Geist is a senior strategic communications associate for MU Extension. She writes from Columbia, Mo.