In 2024, red crown rot of soybean (RCR) was confirmed in Missouri for the first time (Figure 1). This disease can cause significant yield losses, estimated up to 80% in affected areas of a field.

Figure 1 First confirmed red crown rot of soybean in Missouri was documented in 2024.

RCR is caused by the soilborne fungus Calonectria ilicicola, which can be moved when soil is disturbed.

Leaf symptoms usually appear during the reproductive stages (generally R3 or later), regardless of when infection occurred. These symptoms can resemble sudden death syndrome (SDS), with interveinal chlorosis (yellowing that forms between the veins) (Figure 2). As the disease progresses, leaves can become necrotic and die. However, unlike SDS, where leaves tend to fall off the plant, RCR-infected plants usually have leaves that remain attached to the plant's petioles.

Figure 2 Foliar symptoms alone cannot be used to differentiate between red crown rot (RCR) and sudden death syndrome (SDS). Figures 2a and 2b show foliar symptoms of plants with RCR. Figures 2c and 2d shows foliar symptoms of a plant with SDS.

To distinguish RCR from SDS and other look-alike diseases, examine the stem and roots. Splitting the stem open typically reveals an intact pith (or middle of the stem), which helps differentiate both RCR and SDS from other diseases that cause similar foliar symptoms like brown stem rot. Externally, RCR can cause reddening at the stem base (Figure 3), severe root rot, and the presence of tiny red reproductive structures (perithecia) near the top of the root and stem base (Figure 4). Co-infections among the pathogens that cause these diseases can occur.

Figure 3 The base of the soybean stem near the soil line is often a red color.

Figure 4 Tiny red sphere (perithecia) will form near the stem base and top of the root. So far, these perithecia have been easiest to find before the leaves have turned completely brown or necrotic.

The RCR pathogen survives in soil as hardened fungal structures known as microsclerotia, which remain dormant until favorable conditions occur. Soil temperatures between 77°F and 82°F are considered optimal for infection. The pathogen can infect soybean seedlings, and may infect soybean at later stages as well, based on what we know about this pathogen's ability to infect peanuts.

While we still have a lot to learn about the RCR infection process, the follow practices may help reduce RCR severity: seed treatments, planting into cooler soils, cleaning equipment when leaving an infested field, and rotating to a non-host crop, such as corn, for at least two years to reduce inoculum in the soil. Currently, no soybean varieties are known to be resistant to RCR although some may be more tolerant than others.

With support from the Missouri Soybean Merchandising Council, we have begun culturing two Missouri isolates of the pathogen. These will be used to study the infection process and screen soybean varieties and breeding lines for resistance. If you suspect RCR in your fields, please send images or samples to help confirm the disease and contribute to our growing isolate collection.

For the most up-to-date map of red crown rot confirmations, please visit the Crop Protection Network map: https://cropprotectionnetwork.org/maps/red-crown-rot-map.