CBN-V Video Archives - S8-21
The Effect of Symptom Severity, Virus Type and Content on the Epidemiology of Cassava Mosaic Virus Disease in Different Cassava Varieties in Uganda

Sserubombwe W.S.^1,3, J.M. Thresh², J.P. Legg³, G.W. Otim-Nape⁴ and A. Bua¹

1. Namulonge Agricultural and Animal Production Research Institute, P.O. Box 7084, Kampala, Uganda wsserubombwe@hotmail.com
2. Natural Resources Institute, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK
3. International Institute of tropical Agriculture, East and Southern Africa Branch, P.O. box 7878, Kampala, Uganda
4. National Agricultural Research Organization Secretariat, P.O. Box 295, Entebbe

        An experiment was conducted at Namulonge Research Institute to assess the influence of symptom severity, virus type and content on the progress of cassava mosaic virus disease and on the growth and yield of 20 local cassava varieties. These varieties emerged following the cassava mosaic epidemic that occurred in the country a decade ago and were being grown regularly by farmers. The response of the varieties to the disease was unclear and it was necessary for determination in line with the dynamics of the disease. The sprouting %, irrespective of the initial symptom category, was high and not significantly different among the varieties in the first season. Significant differences, however, occurred in the second and third propagation cycles. In the first cycle, % reversion averaged 50% compared to 90% for resistant standards and 0% for susceptible. More reversion was recorded on “mild” than “severe” symptom categories. Reversion declined significantly during the second and third cycles but such decline was minimal in resistant standards. Although almost total infection was recorded in the initially symptomless categories for all the varieties after a year, there were differences in the onset of infection and symptom severity and virus content. In the first season, the “mild”, “severe” and seldom “symptomless” categories were associated with a complex of viruses; either ACMV or EACMV-Ug alone and dual infections of the two viruses. EACMV-Ug and the dual infections occurred more frequently but EACMV-Ug infection occurred most often in subsequent propagation cycles on “mild” and “severe” plants. Some diseased samples consistently tested negatives to the existing primers specific to ACMV, EACMV and EACMV-Ug, although they were positive to universal primers. The varieties produced acceptable storage root yield at harvest. The implications of cultivating these varieties on the long-term dynamics of cassava mosaic virus disease is discussed.

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