1Natural Resources Institute, University of Greenwich, Central Avenue, Chatham Maritime, Kent ME4 4TB, UK; 2Namulonge Agricultural and Animal Production Research Institute, PO Box 7084, Kampala, Uganda; 3CIP, Regional Office for Sub-Saharan Africa, PO Box 25171, Nairobi, Kenya

Background and objectives
Sweet potato is an important staple food crop in the smallholder, subsistence-orientated farming system in much of Uganda and in eastern and central Africa. Uganda has the greatest production of sweet potato in Africa and ranks third globally. Sweet potato virus disease (SPVD) is the most important disease of the crop in sub-Saharan Africa. The cause of SPVD in Nigeria had been reported to be infection with the aphid-borne sweet potato feathery mottle potyvirus (SPFMV) and with the whitefly-borne sweet potato chlorotic stunt closterovirus (SPCSV) [1], but this has not been confirmed elsewhere in Africa. A 3-year study was therefore undertaken to determine the cause and incidence of the disease in Uganda. The overall conclusions will be reported.

Materials and methods
Aphid and whitefly transmission studies, coupled with serological analyses using monocional antibodies kindly provided by Dr HJ Vetten (Institut fur Biochemie und Pflanzenvirologie), were done using field-grown sweet potato plants affected with SPVD and the indicator plant species Ipomoea setosa. Sweet potato crops were examined for incidence of SPVD from the Ugandan border with Kenya in the east around the shore of Lake Victoria to the Ugandan border with Tanzania in the south.

Results and conclusions
The presence in SPVD-affected plants of SPFMV and SPCSV were confirmed by aphid and whitefly transmissions respectively to I. setosa seedlings and by serology [2]. SPCSV isolates from Uganda had more severe symptoms in both sweet potato and I. setosa, and were serologically distinct from Nigerian isolates. The incidence of SPVD varied widely throughout the region surveyed, ranging from a negligible level in the east of Uganda to 25-30% mean incidences in the south (Masaka and Rakai Districts) near the Tanzanian border. In the latter districts, both moderately resistant and moderately susceptible cultivars are grown, and there was a negative correlation between yield and SPVD resistance. Consequently, yield losses in the latter districts derive from poor yields of those plants which become diseased. but also, when somewhat resistant cultivars are grown, from their only moderate ability to yield.

This document is an output from projects funded by the UK Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of DFID.

1. Schaefers GA, Terry ER, 1976. Phytopathology 66, 642-645.
2. Gibson RW, Mpembe I, Alicai T et al., 1998. Plant Pathology 47 (in press).