Red crown rot (RCR) is caused by the fungus Calonectria ilicicola. Red crown rot causes deterioration of soybean roots and stems. Fields affected with RCR typically show symptoms after R3. At the field level, patches of symptomatic plants often occur in low lying or poorly drained areas of the field.
Foliar symptoms begin with small light green to yellow blotches in between the leaf vein, which expand to cause interveinal necrosis. Leaves may die prematurely. Stem symptoms start as a reddish discoloration of the lower stem. Following periods of high moisture late in the growing season, discolored areas become flush with white hyphae and red/rusty brown spore-bearing structures. The central pith of the lower crown may have a grey discoloration. Roots are rotted and plants are easily removed from the soil. Root rot can occur without foliar symptoms and infected plants may be shorter than nearby healthy plants.
Foliar symptoms of red crown rot. Image: N. Kleczewski
Reddish spore-bearing structures and white hyphae on lower stems are late-season signs or red crown rot. Image: S. Geisler
Foliar symptoms can be confused with soybean sudden death syndrome, brown stem rot, or southern stem canker. Stem symptoms can look like other stem diseases including charcoal rot, Rhizoctonia root rot, and Phytophthora stem rot.
C. ilicicola overwinters in soils as microsclerotia, which can survive several years in the absence of a suitable host. Microsclerotia can be dispersed locally in wind-blown plant debris as well as field equipment. Infection occurs under warm, wet conditions. If infested plants are exposed to warm, wet weather after R3, the fungus may produce a toxin that accumulates in the foliage.
Delaying planting may result in unfavorable soil temperatures for root infection. Crop rotation out of soybean for at least two years can reduce disease-causing inoculum. Improving draining in areas where pooling is problematic, avoiding excessive organic matter in soils, and managing pathogenic nematodes and root feeding insects can help reduce red crown rot issues.