Symptoms, aetiology and serological analysis of sweet potato virus disease in Uganda R. W. Gibson a , I. Mpembe b , T. Alicai b , E. E. Carey c , R. O. M. Mwanga d , S. E. Seal a and H. J. Vetten e a NRI, University of Greenwich, Central Ave, Chatham Maritime, Kent ME4 4TB, UK; b Namulonge Agricultural and Animal Production Research Institute, PO Box 7084, Kampala, Uganda; c CIP, Regional Office for SubSaharan Africa, PO Box 25171, Nairobi, Kenya; d Department of Horticultural Science, Box 7609, North Carolina State University, Raleigh, NC 27695–7609, USA; and e Institut fu ¨ r Biochemie und Pflanzenvirologie, Biologische Bundesanstalt fu ¨ r Land und Forstwirtschaft, Messeweg 11/12, 38104 Braunschweig, Germany Sweet potato virus disease (SPVD) is the name used to describe a range of severe symptoms in different cultivars of sweet potato, comprising overall plant stunting combined with leaf narrowing and distortion, and chlorosis, mosaic or vein-clearing. Affected plants of various cultivars were collected from several regions of Uganda. All samples contained the aphid-borne sweet potato feathery mottle potyvirus (SPFMV) and almost all contained the whitefly- borne sweet potato chlorotic stunt closterovirus (SPCSV). SPCSV was detected by a mix of monoclonal antibodies (MAb) previously shown to react only to a Kenyan isolate of SPCSV, but not by a mixture of MAb that detected SPCSV isolates from Nigeria and other countries. Sweet potato chlorotic fleck virus (SPCFV) and sweet potato mild mottle ipomovirus (SPMMV) were seldom detected in SPVD-affected plants, while sweet potato latent virus (SPLV) was never detected. Isolates of SPFMV and SPCSV obtained by insect transmissions together induced typical symptoms of SPVD when graft-inoculated to virus-free sweet potato. SPCSValone caused stunting and either purpling or yellowing of middle and lower leaves when graft-inoculated to virus-free plants of two cultivars. Similarly diseased naturally inoculated field plants were shown consistently to contain SPCSV. Both this disease and SPVD spread rapidly in a sweet potato crop. Introduction Sweet potato virus disease (SPVD) is the name com- monly used in Africa to describe a range of severe symptoms on sweet potato generally attributed to virus infection. Symptoms vary with plant genotype but typically include stunted plants (Fig. 1a) with small leaves, the latter often also being distorted, narrow (strap-like) and crinkled, with a chlorotic mosaic and/or vein-clearing, giving affected plants an overall pale appearance (Fig. 1c). SPVD is widespread and regarded as a serious problem in Africa, affected plants commonly yielding less than half that of symptomless ones (Mukiibi, 1977; Hahn, 1979). In Nigeria, the disease has been associated with the presence of an aphid-borne potyvirus now known to be sweet potato feathery mottle virus (SPFMV) and with a closterovirus transmitted by the whitefly Bemisia tabaci (Schaefers & Terry, 1976; Clark & Moyer, 1988). This Nigerian closterovirus was first called sweet potato chlorotic stunt (SPCSV) (Schaefers & Terry, 1976) and later SPVD-associated closterovirus (Winter et al., 1992). A closterovirus isolated in Israel from sweet potato was named sweet potato sunken vein virus (SPSVV) (Cohen et al., 1992) but has since been shown to be serologically similar to the West African closterovirus (Hoyer et al., 1996; Vetten et al., 1996). Consequently, it has been proposed to the Closterovirus Study Group of the International Committee for the Taxonomy of Viruses that the name SPCSV should be retained for this closterovirus (H. J. Vetten, Institut fu ¨r Biochemie und Pflanzenvirologie, 1996, personal communication) and this name is there- fore used throughout this paper. Results of serological tests with polyclonal and monoclonal antibodies (MAb) have recently shown (Vetten et al., 1996; H. J. Vetten, 1997, unpublished data) that an isolate of SPCSV from Uganda and others from Kenya and Tanzania are serologi- cally distinct from most other non-East African isolates (Nigeria, Gabon, Israel, Brazil, Argentina, USA and Taiwan). African sweet potato production is concentrated in East Africa, especially around Lake Victoria; Uganda has the largest sweet potato production in Africa and the fourth largest in the world. SPVD is the most important disease of sweet potato in the Lake Victoria region (Geddes, 1990). Sheffield (1957) identified two viruses Plant Pathology (1998) 47, 95–102  1998 BSPP 95 Accepted 17 August 1997.