Southern Blight of Peanut
The first readily apparent symptom of southern blight is rapid yellowing and wilting of limbs or entire plants. Affected limbs and plants then turn brown and die as a result of decay of the lower stem. Advanced symptoms of southern blight are very similar to those of Sclerotinia blight. The base of diseased plants must be closely examined for signs of the fungus to distinguish between the two diseases. Early detection of the disease is also possible if lower stems are examined prior to development of advanced symptoms. Southern blight infection is characterized by a white or cream colored moldy growth (mycelium) covering lower stems and imparting a white-washed appearance to the base of affected plants. The mycelium of the southern blight fungus is coarse, rope-like, closely appressed to stems, and may radiate out over the soil surface. This is best observed when the soil surface is moist. Mycelium may also be seen growing from decaying plant matter on the soil surface. Lesions (dead areas) on infected stems and pegs are at first light brown, then become dark brown. Advanced symptoms develop when lesions coalesce to girdle the lower stem. Small, round, mustard seed-like structures soon develop on the mycelium. These structures are called sclerotia and serve as reproductive and survival structures of the fungus. The sclerotia first appear as white tufts that later become light brown and finally dark brown at maturity. Sclerotia are uniformly round, about 1/16 inch in diameter, and are produced in large quantities on dead peanut tissue.
Cultural practices to manage southern blight can be very effective, because actions taken to reduce the populations of sclerotia at the soil surface can directly reduce disease incidence. Integration of cultural practices with chemical applications as warranted will generally result in the most satisfactory and economical control. Moldboard plowing in the spring turns under crop residue and buries sclerotia where they cannot cause disease. Most aspects of cultivation during the growing season encourage southern blight development. Increased soil aeration from cultivation allows deeper penetration of the fungus where pegs and pods can be readily attacked. Cultivation is a useful means of weed control; however, cultivation, known as dirting, that moves the soil onto the limbs or crown of the plant should be avoided. Crop rotation is an effective practice for management of southern blight, but rotation crops must be carefully selected because the southern blight fungus has a wide host range. Fungicide applications are often required in fields where southern blight is a main factor limiting production. Please contact your local county extension office for current information.