Red crown rot, also called Cylindrocladium root rot, black root rot, and Calonectria root rot, occurs worldwide in warm-temperate and tropical regions. Red crown rot is uncommon in North Carolina, but can be found causing local losses sporadically when conditions are conducive.

Red crown rot is caused by the fungus Calonectria ilicicola (anamorph: Cylindrocladium parasiticum). The fungus produces microsclerotia that serve as overwintering structures that can survive several years in the soil. On the surface of stems above the soil line, bright, reddish-orange reproductive structures (perithecia) may form.

This pathogen survives as microsclerotia (small, fungal overwintering structures) in soil or crop residue and can survive several years without a suitable host. Roots and Rhizobium nodules are penetrated within 24 hours of microsclerotia germination and hyphae (fungal strands) begin colonizing plant roots. As the fungus depletes resources in the roots, it produces more microsclerotia. Microsclerotia can be dispersed by wind-blown plant debris as well as mechanical transmission. Perithecia will develop abundantly along stem if adequate moisture is available. Secondary disease spread in the growing season can be attributed to the development of mature ascospores which can be discharged by ejection, rain splash, and water runoff.

Moderate soil temperatures (approx. 77-86°F) favor development of disease and disease frequency decreases as soil temperatures increases above 86°F. Poorly drained soils and soils with high clay content may increase disease severity.

Symptoms can appear anywhere between R3 and R7 growth stages. Single plants or small patches of infected plants appear randomly throughout the field. Infected plants are not commonly found evenly distributed through entire field and incidence rarely exceeds 30 percent. The first symptom typically observed in the field is interveinal chlorosis which later can lead to necrosis. Plants may begin defoliating prematurely if disease severity is high.

Right above the soil line, the base of the stem may display reddish discoloration. The appearance of bright, reddish-orange reproductive structures called perithecia in the same region right above the soil line provide further evidence of a red crown infection. Perithecia develop more frequently after periods of high moisture when the plant is near maturity. Another symptom observed in the stem can be found by splitting the stem, greyish-brown discoloration of the vascular tissue can be observed in infected plants. Below the soil line, roots become deteriorated and rotted in the later stages of disease development.

The symptoms found with red crown rot, including interveinal chlorosis, stem discoloration, and root rot, are found in many other stem and root diseases of soybean. Field diagnosis is more evident with the presence of bright perithecia above the soil line. In the absence of these reproductive structures, identifying multiple symptoms and common environmental conditions or sending samples to NCSU Plant Disease and Insect Clinic can help confirm diagnosis.

Fungicides not typically used to manage soybean root diseases; adjusting cultural practices is more economically and practical for this crop and disease. Crop rotation can be used to decrease inoculum levels between years. Rotating with peanut should be avoided as it shares a common pathogen and causes Cylindrocladium black root rot of peanut. Rotating for more than one year decreases chances of disease occurrence. Avoid mechanical transmission to surrounding fields by identifying areas where the disease is present and sanitizing equipment before leaving infected field. Infection of pathogens who penetrate through the roots will be exacerbated by the presence of root damaging nematodes. Damage caused by nematodes provides an easy access point for other pathogens. Controlling the nematode population can reduce severity of infection.

The Crop Protection Network Fact Sheet for Red Crown Rot has some excellent images and other useful identification notes.

The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations.

The NC State Extension Plant Pathology portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.

Acknowledgements

This factsheet was prepared by the NC State University Field Crops and Tobacco Pathology Lab in 2020.

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