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Peanut disease update: Scout fields for southern blight and sclerotinia blight
Author: Travis Faske, Extension Plant Pathologist

Southern blight is now beginning to show up in some peanut fields, which is later than normal as southern blight activity typically begins in early July. Unseasonably cooler summer weather contributed to this delay with slower vine growth and less-than-favorable weather conditions for southern blight. Alternately, Sclerotinia blight was observed last week, which is two months earlier than 2013 when it was first detected in the state. Given that both diseases are active, accurate identification is essential to determine the best management option.

Southern blight is caused by the fungus, Sclerotium rolfsii. Early symptoms consist of yellow, wilted stem and leaves; however, white fungal mycelium can be seen on lower stems and on the soil surface or leaf litter prior to symptoms development (Fig. 1). Round, tan to brown sclerotia (shape and size of mustard seed) are commonly observed with the hyphae near the soil line of symptomatic plants. Due to the lack of resistance in peanut, timely fungicide applications are the primary management practice for this disease. Fungicides registered for use on peanut available on the MP 154. Because southern blight activity has been low, many fields with a history of this disease may only need one or two fungicides this year. So, scout fields for disease rather than follow a calendar program, which increases production cost. Direct contact of the fungicide with the fungal pathogen greatly improves disease control. This can be accomplished with using more water per acre (at least 20 gal/ac), apply fungicides at night (leaves close to allow fungicide penetration into the peanut canopy), or before a light rain.

Figure 1. White mycelium of Sclerotium rolfsii colonizing a peanut plant

Figure 1. White mycelium of Sclerotium rolfsii colonizing a peanut plant

Sclerotinia blight is caused by a soilborne fungus, Sclerotinia minor.  This soilborne disease is not as widespread as Southern blight but can cause high yield losses as the disease progressively increases with continued peanut production.  So, from now until harvest it is a good time to determine if this disease is present in your peanut fields. Sclerotinia blight is more active when conditions are cool with a high temperature near 80°F.  The first symptom is the “flagging” or wilting of infected branch tips and petioles (Fig. 2).

Figure 2. Typical blighted plants caused by Sclerotinia blight of peanut

Figure 2. Typical blighted plants caused by Sclerotinia blight of peanut.

White fluffy hyphae often surround infected stems and lesions are initially dark brown and then become straw colored at maturity. Small black sclerotia are common on and inside infected straw-colored peanut stems.  The size, shape, and color of sclerotia produced by each pathogen are key features to disease identification.  Only fields with a history of Sclerotinia blight will need to be managed therefore, detection and accurate identification are important for future disease management considerations.  So, scout peanut fields for these two soilborne diseases and take notes on disease incidence and severity for current and future management decisions.

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