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Peanut producers: Spotting leaf spot diseases and fungicide timing for southern blight
Author: Travis Faske, Extension Plant Pathologist

Though it is very common to find spots on peanut leaves in Arkansas, the disease called early leaf spot (ELS) of peanut (and late leaf spot of peanut) has yet to be confirmed in the state. Herbicide injury is most commonly mistaken for ELS (Figure 1). Early leaf spot is caused by the fungus Cercosproa arachidicola, which produces silvery, fuzzy tufts of spores on the TOP side of the leaf. Symptoms consist of circular, brown to dark brown lesions surrounded by a yellow border or halo. An incubation test can be helpful determine if spots are from herbicide or ELS. Incubate symptomatic leaves in a sealed plastic bag with a moist paper towel (squeeze water out of a saturated towel) and after 24 hours inspect leaves for spores on the TOP side of the leaf with the aid of a 20x hand lens. If fuzzy tufts (spores) are not present then it likely herbicide injury. Because ELS can quickly defoliate a field causing significant yield loss, a broad spectrum fungicide like chlorothalonil is commonly recommended in a peanut fungicide program. Additionally, many of the fungicides used to manage southern blight, the most widespread diseases of peanut in the state, will also control ELS of peanut.

Figure 1.  Leaf spots caused by herbicide injury on peanut.

Figure 1. Leaf spots caused by herbicide injury on peanut.

Southern blight is caused by the fungus Sclerotium rolfsii, which typically occurs on peanut during the hot summer months, hence the name southern blight (syn. white mold, southern stem rot, and stem rot). Early symptoms consist of yellow, wilted stem and leaves; however white fungal mycelium can be seen on lower stems and soil surface prior to symptoms development (Fig. 2). Though other fungi produce white hyphae, S. rolfsii produces course strands of hyphae with orange to brown sclerotia (shape and size of a mustard seed). Early season symptoms are often observed in mid to late-June, but given the delayed planting of the 2014 crop symptoms may develop later than 2013.

Southern blight is managed with fungicides due to lack of host plant resistance in commercial peanut cultivars. See peanut section of MP 154 for fungicide efficacy information for southern blight and other peanut diseases.   Fungicide timing and coverage are key factors for southern blight management. Consider a fungicide when white hyphae with sclerotia are observed or when symptomatic plants are noticed, less than 1% of the field. 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), applying fungicides at night (leaves close to allow fungicide penetration into the peanut canopy), or before a light rain. In 2013, fungicide application before a ½ in. rain was very effective in improving coverage for effective southern blight control.

Figure 2.  White mycelium of Sclerotium rolfsii colonizing a peanut plant

Figure 2. White mycelium of Sclerotium rolfsii colonizing a peanut plant

Finally, other diseases that look like southern blight include Sclerotinia blight, which was reported last year; however, it typically does not occur until late September or early October when the high temperature is near 80°F. Only two fields were confirmed to have Sclerotinia blight in 2013 and extension agents and plant pathologist will be scouting fields later this season to estimate the incidence of this disease in Arkansas peanut fields.

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