A Practical Guide To: Fighting Charcoal Rot and Southern Blight

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CHARCOAL ROT

Southern blight and charcoal rot are two soilborne diseases that pose persistent threats to crop health and farm profitability during the summer heat. While the two diseases are caused by fungi (Macrophomina phaseolina and Sclerotium rolfsii, respectively), fungicides are not effective at controlling them once they’ve taken hold. This article will explore how management practices and seed selection can help reduce the incidence of both diseases and prevent yield loss. The fungus that causes charcoal rot can subsist in soil and plant residue in small fungal structures called sclerotia that can live for up to two years. More than 500 species of plants can play host to the disease, making it tough to eradicate. Infection begins at the roots, then spreads through the stem tissue, where the fungal growth inhibits water intake. The fungus struggles to survive in moist soils, favoring hot, dry conditions.

Signs charcoal rot is in your fields

Soybeans infected with charcoal rot may develop prematurely yellow leaves that later turn brown and die. The lower woody portion of the stem may also display a grayish coloring. “Peppering” is another symptom, when the microsclerotia become visible as small, black specks on the lower portion of the plant.

Signs of charcoal rot in your fields:

Avoid drought stress – Charcoal rot thrives in hot, dry conditions, so irrigation can help lower the ability of the fungus to colonize roots.

Crop rotation – Microsclerotia will die after too much time without a host, so rotating to nonhost crops can help rid fields of charcoal rot.

Seeding rates – Avoid overseeding to lower the incidence of plant stress and lessen fungal growth opportunities.

Tolerant varieties – Look for varieties with good charcoal rot tolerance scores. Corteva Agriscience has many high-scoring options and is always working on improving tolerance to charcoal rot through our robust genetic screening process.

SOUTHERN BLIGHT

Southern blight sclerotia are tough. These fungal bodies can survive three or four years near the soil surface. All types of soybeans (as well as hundreds of other plants) can host this fungus.¹ Favoring hot, humid weather, southern blight often arises in the southern United States — hence its name — but can also appear in temperate locations when the weather conditions are right.¹ Infection often starts in spring, when small, thread-like structures called hyphae infect the soybean at or below the soil line. Within the next few days, soft rot develops, continuing until it encircles the stem, which eventually leads to wilting, dieback and even plant death.²

Southern blight symptoms:¹ Pre- or post emergence damping off is common with seedlings. Older plants will grow a dark brown stem blemish at the soil line. A white, fan-shaped mat of fungus often appears at the base of the stem, growing out to cover nearby soil or plant debris. The disease then produces sclerotia on these fungus mats and lower plant stems. These sclerotia start with a yellow-tan color that changes to brown over time. The plant’s green leaves turn yellow, wilt and finally turn brown, but do not drop off.

Tips for controlling southern blight:

Crop rotation – Rotating to a nonhost crop like wheat or sorghum for two years can help kill off sclerotia living in the soil.1

Cultivation – As southern blight sclerotia survive best near the surface of the soil, deep tillage can bury them, reducing their survival ability.²

Improve drainage – Southern blight thrives in warm, moist conditions, so ensuring proper drainage and avoiding excessive irrigation can help limit its spread.³

Plant spacing – A dense canopy of plants can trap moisture, contributing to favorable conditions for the disease. Decreasing plant density can foster improved air circulation and help plant tissues dry more fully, reducing the chances of infection.³

Tolerant varieties – Planting varieties with good tolerance scores for southern blight can be one of the most critical disease control steps. Corteva Agriscience offers a range of varieties and different tolerances.

While no chemical or biological treatments are effective against these diseases, selecting tolerant varieties and following recommended cultural practices can make a difference in the odds of infection. Add diligent and frequent scouting, and you can help minimize the effect these pathogens can have in your fields to help ensure productive and profitable results.

¹ Travis Faske et al., “Soybean Diseases,” University of Arkansas Division of Agriculture Research and Extension, April 2014, https://www.uaex.uada.edu/publications/pdf/mp197/chapter11.pdf.

² Jackie Mullen, “Southern Blight, Southern Stem Blight, White Mold,” The American Phytopathological Society, January 1, 2001, https://www.apsnet.org/edcenter/pdlessons/Pages/SouthernBlight.aspx.

³ Emily Pfeufer et al., “Southern Blight,” University of Kentucky Extension Plant Pathology, August 2018, https://plantpathology.ca.uky.edu/files/ppfs-gen-16.pdf.