Southern Blight of Soybean Observed in a Few Isolated Fields

Southern blight of soybean. Note the thick, white, cottony mycelium growing on the steam below the soil line.

Over the past week I’ve received several calls regarding soybean seedlings that reached a good stand and scattered plants started to die in small groups (typically 2-4 plants, sometimes more).  Two fields in particular, where soybean was planted on heavy soil, had symptoms of southern blight scattered throughout the majority of the field.  Southern blight is a soilborne disease caused by the fungus Sclerotium rolfsii.  Typically, the occurrence of southern blight is rare in MS.  In the 5 years I’ve been here I can count the number of plants that I’ve seen on my 10 fingers and can only recall 3 situations where I remember hearing about the disease (2 in 2011, and 1 in 2010).  But, given the environmental conditions we’ve encountered recently, more specifically the warmer than normal temperatures and lack of rainfall, I’m not surprised that some “odd” diseases are starting to be recognized.

Symptoms of southern blight on foliage can appear as interveinal chlorosis but the more common symptom is a circular, tan to brown lesion with darker margins.  Plants removed from soil will generally have a brown to maroon lesion that can be on a portion of the stem or girdle the entire stem (see image).  A sudden wilting of the infected plants generally occurs as the next phase of the disease.  Careful inspection of plants removed from the soil allows the observation of the key characteristic of the disease: a heavy white mass of fungal growth below the soil surface.  In some cases, white to brown sclerotia are present on the mycelia growing on the stem.  However, I was only able to observe sclerotia on dead plants.  Dead plants will typically have leaves stuck to the dead stem (see photos).  In the two fields I observed Thursday afternoon there was an instance of a 5 foot portion of the planted twin row that was killed by the southern blight fungus.  Southern blight can be observed on soybean plants at any growth stage.

Southern blight can be detected in random areas of a field. Flags were placed in clumps of infected plants in only a small area of the field in question.

Infection by the fungus can quickly move down the planted row if conducive environmental conditions continue.  Typically the disease occurs during hot, humid, wet weather.  However, we’ve had a range of environmental conditions this season that have been warmer than normal but certainly not wet.  But, the two fields in question had received some rain showers over the past 3 weeks and were beginning to dry out.  The shift in environmental conditions and what appears to be bordering on droughty conditions is likely contributing to the presence of the disease since the seedlings in both fields appear to be stressed.

Appearance of plants infected with the southern blight fungus.

Varieties with resistance to S. rolfsii are not available.  The fungus can overwinter in soil, generally closer to the soil surface rather than buried deeply, as sclerotia, a hardened mass of fungal mycelia.  In fields with a history of the disease, rotation to a nonhost crop that includes corn, sorghum, and wheat could prevent the increase of inoculum in the soil profile.  In addition, since the fungus does not like to be buried deeply, tillage to bury sclerotia will reduce the likelihood of the disease occurring in subsequent crops.  Seed treatments are not effective, since in most cases the organism will infect after the seedling emerges through the soil surface.  Even though strobilurin fungicides (e.g. azoxystrobin, fluoxastrobin, pyraclostrobin) are labeled for disease management the fungicide label in most cases suggests the product would only suppress some disease from occurring.  In addition, fungicides would need to be applied by a ground rig and there is no data available from any states in our region to provide guidance on the efficacy of one product over another.  In addition, in situations where the disease has already been observed a fungicide application would be considered a “rescue treatment” and not likely beneficial.

Sclerotia, a hardened resting structure of the fungus can appear either on the soil surface or immediately below the soil line. The sclerotia are the white to brown, round structures.