Net Blotch and Necrotic Warts Caused by Streptomyces scabies on Pods of Peanut (Arachis hypogaea).

Peanut (Arachis hypogaea L.) grown in rotation with potato (Solanum tuberosum L.) in the Northern Cape Province and at one locality in the Free State Province, South Africa, have developed unusual pod symptoms since the 1994 to 1995 season. Symptoms ranged from a net blotch with scattered lesions to dark brown, necrotic, wartlike lesions on the cv. Sellie. Streptomyces scabies (2) was consistently isolated from both types of lesions. Pathogenicity was confirmed in greenhouse tests. Inoculum was prepared by growing colonies on yeast malt extract agar for 21 days. Ten milliliters of sterile, distilled water was poured over the colonies of two different isolates and lightly scraped with a sterile needle. Separate sets of sterile soil were amended with spore suspensions of different isolates at a rate of 10 ml/kg and thoroughly mixed. Seeds of cv. Sellie and minitubers of potato cv. BP1 were planted in infested soil in 3-kg plastic bags. Uninfested soil served as controls. Each set of pots for both peanut and potato had three replications. Pots were kept in a glasshouse at 27°C for 12 weeks. Plants were lifted, disease development recorded, and infected plant parts prepared for reisolation on antibiotic-amended water/ agar according to the protocols of Loria and Davis (3). Both lesion types recorded under field conditions developed on peanut pods in the glasshouse upon reinoculation. Incidence ranged between 2 and 3 pods out of 8 to 10 pods per plant while minitubers were nearly 100% infected. Streptomyces scabies was isolated from lesions on both peanut and potato. Net blotch caused by an unknown Streptomyces sp. is a significant problem on peanut pods in Israel (1). However, the species found in Israel was not identified and could be different from the one reported here, according to Y. Barash (Tel Aviv University, personal communication). This is a first report of S. scabies being pathogenic on peanut in South Africa. References: (1) Y. Barash et al. (In Hebrew.) Hassadeh 72:688, 1992. (2) D. H. Lambert and R. Loria. Int. J. Syst. Bacteriol. 39:387, 1989. (3) R. Loria and J. R. Davis. Streptomyces scabies. Pages 114-119 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. 2nd ed. N. W. Schaad, ed. American Phytopathological Society, St. Paul. MN, 1988.