By Travis Faske, Extension Plant Pathologist
Southern blight is the most common peanut disease that is found in Arkansas peanut fields. This disease typically occurs on peanut during the hot summer months, hence the name southern blight (syn. white mold, southern stem rot, and stem rot). A few weeks ago (late July 2015) the fungus that causes this disease was observed in the lower canopy of some fields, which is an indication the fungus is active and will soon infect nearby plants. So, now is a good time to review fungicide timing and application recommendations to manage southern blight.
Southern blight is caused by the fungus Sclerotium rolfsii. Early symptoms consist of yellow, wilted stem and leaves (sometimes called flagging, Fig. 1);
however, white fungal mycelium can often be seen on lower stems and soil surface prior to symptoms 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 soil-borne disease that has one infection cycle per year so it will not spread like foliar disease in a single season. Given it is soil-borne, it will also reoccur in the same fields that are planted in peanut.
The severity of southern blight is much, much less than that observed in the SE United States. Therefore, our management does not need to be as aggressive as the programs used in Georgia, Alabama, and Florida. For the past few years some Arkansas peanut producers have only needed 3-4 fungicides to complete the season. This is much less than the 6 or 7 fungicides needed in other peanut producing states. Most 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.
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). 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 in 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. Fungicides applied before a ½ in. rain was very effective in improving coverage for effective southern blight control. Additionally, those producing peanut in a pivot irrigated field can irrigate shortly after fungicide applications to improve distribution of the fungicide within the dense peanut canopy.