Evaluation of improved and local cassava varieties in Uganda for resistance/ tolerance to Cassava Mosaic Virus Disease (CMD)

Cassava Mosaic Disease (CMD) caused by whitefly-transmitted cassava mosaic geminiviruses has been a major constraint to cassava production in Uganda since 1988 when a particularly severe epidemic of the disease was first reported. The introduction and dissemination of new resistant varieties has restored cassava production in many districts. However, there is inadequate information on the susceptibility and resistance of these new varieties to infection, the mechanism of resistance they exhibit, and the yield loss due to CMD. understanding resistance and integrating it into a holistic strategy for management of CMD is of significant importance. Also, in spite of the availability of a number of cassava mosaic disease (CMD) resistant cultivars, several landraces have continued to be grown in many parts of Uganda. This is because the landraces possess certain desirable traits, which may not have been fully met by the resistant varieties. Little information exists on the response of these varieties to CMD and the effect of CMD on the yield of these varieties. The objectives of this study were; to evaluate the resistance/ tolerance of some local cassava varieties to CMD, assess the effects of CMD on yield and growth of the local varieties that have emerged in post-epidemic areas, understand the resistance mechanisms deployed by recently released improved and advanced stage varieties to CMD and assess the effect of CMD on the growth and yield of the recently released improved and advance stage varieties. Field based trials were set up to evaluate the performance of four exotic varieties (i.e. Nase 9, Nase 11, 00036 and 00057) and some Ugandan landraces that became prominent since the 1990s pandemic. Experimental plots for the improved varieties were located in an epidemic hotspot at Namulonge in central Uganda to determine their resistance, while those for the local landraces were locted in areas where the CMD is reportedly to have subsided, i.e.Mukono (Central Uganda) and Bulindi (Western Uganda). The landraces were evaluated in Mukono were Njule, Kabwa and Matooke, and those evaluated in Bulindi were Tongolo, Nyaraboke and Sibampale. These sites were used for the locals because of preference among farmers. Plots for both experiments were established with planting materials obtained from either diseased and healthy plants. Resistant (Nase 4) and susceptible (Bao) standards were included for comparison. The results showed differences in sensitivity to CMD among varieties, with landraces consistently having higher CMD incidences than the resistant check (Nase 4). Like previously found (Otim Nape et al., 2001), low spread of CMD was observed among local landraces in areas where CMD has almost subsided (low pressure areas) such as Bulindi. The spread was even lower in plots grown from healthy cuttings. Surprisingly, the results indicate that Mukono is a high disease pressure zone (epidemic area) with high amount of inoculum in the locality causing rapid and high CMD spread. Thus disease spread among the landraces was very rapid and equally high incidences of CMD were recorded regardless of whether plots were originally containing healthy or diseased cuttings.