Identification of the East African Strain of Sweet Potato Chlorotic Stunt Virus as a Major Component of Sweet Potato Virus Disease in Southern Africa.

Sweet potato virus disease (SPVD) is the most damaging disease of sweet potato Ipomoea batatas (L.) Lam. in Africa. It is caused by sweet potato feathery mottle potyvirus (SPFMV) plus either the West African strain of sweet potato chlorotic stunt crinivirus (Closteroviridae) (SPCSV-WA) (2) or the serologically distinct and apparently more severe East African strain (SPCSV-EA) (1). Typical symptoms of SPVD include severe plant stunting, leaf distortion, chlorosis, mosaic, or vein clearing (1). During a survey done in February 1998 of 48 farmers' fields in Lusaka Province and North Western Province of Zambia, sweet potato plants with typical SPVD symptoms were observed. Incidence was generally 1 to 5% but occasionally >20%. To determine which viruses (SPFMV, SPCSV-EA, SPCSV-WA) were present in symptomatic plants, enzyme-linked immunosorbent assays (ELISAs) were done on leaf sap extracts. Twenty-two SPVD-affected plants from Lusaka Province and 15 from North Western Province were tested and SPFMV and SPCSV-EA (but not SPCSV-WA) were detected in all samples. SPCSV-EA by itself may cause purpling or yellowing of lower or middle leaves (1). Eight plants showing these symptoms were collected from North Western Province, and SPCSV-EA only was detected in six of the samples. SPVD was also observed in a 1997 survey of crops near Antsirable, Madagascar; incidence was generally <1% but occasionally >20%; SPFMV and SPCSV-EA, but not SPCSV-WA, were detected in two SPVD samples tested. Our results are the first report of SPCSV in southern Africa. SPVD in the regions surveyed appears to be due to SPFMV and SPCSV-EA; SPCSV-WA was not detected. References: (1) R. W. Gibson et al. Plant Pathol. 47:95, 1998. (2) G. A. Schaefers and E. R. Terry. Phytopathology 66:642, 1976.