By Travis Faske, Extension Plant Pathologist
Southern blight or southern stem rot is the most common peanut disease in Arkansas. This disease occurs during the hot summer months, hence the name southern blight. Fungal growth and symptomatic plants (i.e. flagging or wilting; Fig. 1) can be observed in fields especially after canopy closure, which creates the ideal conditions for disease development. Recently, there have been a few reports of symptomatic plants in the peanut-growing area of the state, so now is a good time to scout peanut fields for the tell-tale signs and symptoms of southern blight.
Southern blight is caused by the fungus Sclerotium rolfsii. Early symptoms consist of yellow, wilted stem and leaves (Fig. 1); however, white fungal mycelium can often be seen on lower stems and soil surface prior to symptom development (Fig. 2). Though other fungi produce white hyphae, S. rolfsii produces coarse strands of hyphae with orange to brown sclerotia (shape and size of a mustard seed, Fig. 2). It is noteworthy to point out that this is a soilborne disease that has one infection cycle per year, so it will not spread like a foliar disease in a single season. Given that it is soil borne, it will also reoccur in previously infected fields when planted to peanut.
Southern blight is managed with fungicides due to lack of host plant resistance in commercial peanut cultivars. There are several options for fungicide selection (see MP 154); however, fungicide timing and coverage are important key factors in southern blight management. Consider a fungicide when southern blight hyphae are observed or when symptomatic plants are first noticed in the field (Fig. 1). When using a calendar program in fields with a history of disease, consider a fungicide with excellent or good efficacy for the first or second fungicide application. Direct contact of the fungicide with the pathogen greatly improves disease control. This can be accomplished by using more water per acre (at least 20 gal/A), and applying fungicides at night (when leaves are closed, which allows fungicide penetration into the peanut canopy) or before a light rain. Fungicides applied before a ½ in. rain or overhead irrigation can distribute fungicides into the dense peanut canopy.
Overall, fewer fungicides are needed in Arkansas than in states like Georgia where disease severity is often very high in some peanut fields. On average, the University of Arkansas recommends 3 to 4 fungicides (depending in disease history) for effective disease control. In some cases, fields with no history of southern bight and planted in peanut for the first time may only require two fungicide applications to prevent disease development. Typically, fungicides provide 14-21 days of protection depending on the weather thus, when conditions do not favor disease or there is little or no southern blight activity, longer intervals can be used between fungicide applications.