Southern Blight (Sclerotium rolfsii Sacc.) of Cowpea: Genetic Characterization of Two Sources of Resistance

Field studies were conducted to characterize the genetic nature of resistance to southern blight (caused by Sclerotium rolfsii Sacc.) exhibited by the cowpea [Vigna unguiculata (L.) Walp.] cultivars Carolina Cream and Brown Crowder and to determine if a genetic relationship exists for this resistance between the two cultivars. Examination of the comparative frequency distributions of the parental and progeny populations of the “Carolina Cream” x “Magnolia Blackeye” and “Brown Crowder” x “Magnolia Blackeye” crosses and the corresponding segregation data indicates that the southern blight resistances exhibited by “Carolina Cream” and “Brown Crowder” are conditioned by single dominant genes. Examination of the segregation data from the parental and progeny populations of the “Carolina Cream” x “Brown Crowder” cross suggests that the two resistance genes are not allelic. The availability of each of the resistance genes in cultivar-type genetic backgrounds should allow for rapid incorporation of southern blight resistance genes into other cowpea cultivars by the application of conventional plant breeding methodologies. 1. Introduction Southern blight, a stem disease caused by Sclerotium rolfsii Sacc., is a common disease of cowpea [Vigna unguiculata (L.) Walp.] worldwide. Aycock [1] described the general effects of S. rolfsii in causing southern blight on numerous species of host plants. The first observed symptoms are usually a general wilting and yellowing of plants, which is soon followed by drying of foliage and plant death. Plants with advanced disease development characteristically exhibit tan to brown sclerotia and white mycelial growth on the stem epidermis at the soil surface [2]. The disease is best known by its girdling stem lesions near the soil line. Many researchers use the frequency of plant mortality to estimate the impact of southern blight on crop yields. Toler et al. [3], for example, assayed the economic importance of various diseases of cowpea in Georgia, concluding that southern blight, although a widespread problem, was responsible for an economic loss of less than 1%. More recently, Fery and Dukes [4] observed that the impact of southern blight on cowpea yield may be more attributable to reduced plant vigor than to plant mortality per se. They reported that the disease can cause dry-seed yield loss of up to 53.4%. Although southern blight has long been considered an important disease of cowpea, there is only limited information in the scientific literature that deals with resistance in cowpea. Muquit et al. [5] evaluated 20 cowpea