Temporal spread of cassava mosaic virus disease in a range of cassava cultivars in different agro‐ecological regions of Uganda

Abstract

Summary.The spread of cassava mosaic disease (CMD) in a range of cassava cultivars was studied in experiments and on‐farm trials in different agro‐ecological regions of Uganda in 1989–1990 and 1990–1991. No spread occurred in either experiment at the southernmost site near Kampala, but there was considerable spread at the four sites elsewhere and also in the on‐farm trials in Luwero district. There were significant differences in the final incidence of disease between locations and between cultivars at each location. Where spread occurred it was more rapid in the Ugandan cvs Ebwanateraka, Senyonjo and Bao than in four of the five improved TMS cultivars introduced from Nigeria. These usually showed an apparent decline in incidence of CMD after reaching maxima 4 to 8 months after planting (MAP). The areas under the disease progress curves (AUDPCs) differed significantly between locations and cultivars and were less for cvs TMS 30572, TMS 30395, TMS 30337 and TMS 60142 than for cvs Ebwanateraka, Senyonjo, Bao and TMS 30786. Overall, the mean AUDPCs were greatest at Migyera in Luwero district in 1989–1990 and at Kagando in Kasese district in 1990–1991. They were significantly less at Mubuku in Kasese district in 1989–1990 than at the other two experimental sites where spread occurred. Adult whitefly vector populations were highest at Migyera and Kagando in the 1989–1990 and 1990–1991 trials, respectively, and they were higher on cvs Bao, Ebwanateraka and TMS 30786 than on other varieties. Mean numbers of adults increased until 3–5 MAP and then declined, but CMD incidence increased progressively to reach maxima at or near crop maturity. Locations with the largest numbers of adults also had a relatively high incidence of CMD. Symptoms of CMD were usually more severe on cvs Ebwanateraka, Bao and Bukalasa 11 than on the TMS cultivars, on which symptoms remained slight throughout growth and usually decreased from 5 MAP. The differences between sites, the resistance of the cultivars and the relationship between CMD incidence and whitefly populations are discussed.