Red Crown Rot

White mold in soybean field

By Dylan Mangel | Extension Plant Pathologist

Pathogen

Red crown rot (RCR) is caused by the fungus Calonectria ilicicola. Historically, reports of this disease were limited to the eastern regions of Louisiana, but in recent years it has expanded north into Missouri, Kentucky, Illinois, Indiana, Ohio, and the latest expansion includes Wisconsin and Minnesota. While it is not yet found in Nebraska, the nearest known infected field is less than 100 miles from our border. The Crop Protection Network features a map of current RCR distribution.

Disease Symptoms

RCR is often first noticed through foliar symptoms that resemble those of sudden death syndrome (SDS), brown stem rot (BSR), or triazole injury. Early symptoms appear as light green or yellow interveinal spots that expand and turn necrotic (brown and dead tissue). In some cases, foliar symptoms may be absent altogether.

To distinguish RCR from other look-alike diseases, examine the lower stem near the soil surface. Infected plants typically show a reddish discoloration at the stem base. Under warm, moist conditions late in the growing season, the discolored area may develop white fungal growth and small, red to rusty-brown spherical spore-producing structures slightly larger than this period. Infected roots are often rotted and can be easily pulled from the soil.

Close-up of a soybean stem showing clusters of small red fungal structures characteristic of red crown rot infection.
Figure 1. Red perithecia of Calonectria ilicicola, the causal agent of red crown rot, forming on the surface of an infected soybean stem. These spore-producing structures develop under humid conditions later in the season and are a diagnostic feature of the disease. (Photo courtesy Boris Camiletti)
Soybean plant showing yellowing and browning leaves with red crown rot symptoms near the stem base.
Figure 2. Foliar symptoms of red crown rot. (Photo courtesy N. Kleczewski)
Soybean field with widespread yellowing and browning plants showing symptoms of red crown rot infection.
Figure 3. Red crown rot impacted plants in the field. (Photo courtesy N. Klexzewski, S. Geisler, and D. Telenko)

Favorable Environmental Conditions

This fungus survives in the soil as microsclerotia, which can persist for several years in the absence of a host. Infection occurs when conditions are warm and wet, particularly during the spring and early summer. Although soybean planting in Nebraska has generally trended earlier, RCR tends to favor warmer soil temperatures, meaning the disease may become more common in later-planted soybeans or years with extended warm, wet periods.

Management

Genetic Resistance

Commercial soybean varieties are not currently graded for resistance. There is potential for varietal differences or resistance but identification research will take time.

Cultural Practices

In In fields known to be infested with RCR:

  • Avoid planting soybeans late into wet soils.
  • Consult with local agronomists or seed advisors about varieties that have shown better performance under RCR pressure.
  • Rotate soybeans with non-host crops to reduce inoculum levels in the soil.

Chemical/Biological Control

Several seed treatments have demonstrated effectiveness against RCR in research trials, and some products are now being labeled for its control. More treatment options are expected as additional labels are approved. University pathologists across affected states are continuing to collect efficacy data, which will be incorporated into the Crop Protection Network publication Fungicide Efficacy for Control of Soybean Seedling Diseases as new data become available: https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-soybean-seedling-diseases

Additional Information