Influence of plant population in incidence , progress and severity of cassava mosaic disease in central Uganda

Cassava (Manihot esculenta Crantz) continues to be important food crop in Uganda, being only second to banana in total production and per capita consumption. However, cassava mosaic geminiviruses (CMGs), family Germiniviridae: Genus Begomovirus remains the major problem limiting cassava production in Sub-Saharan Africa, India and Srillanka. The viruses are maintained by man through planting infected cuttings and are spread in the field by their whitefly vector (Bemisia tabaci). Losses have been especially sever in the Lake Victoria region, and the Democratic Republic of Congo. In Uganda, the disease devastated cassava in the 1980s/1990s leading to widespread famine in the country. Intervention using resistant varieties has to some extent, controlled the disease and restored cassava production in the country. However, adjusting dates and other cultural practices such as varietal mixtures and intercropping are also known to decrease the efficiency of the vectors in spreading the viruses. Nonetheless, the effects of host –plant population on spread of CMGs are not clearly understood but in general, farmers plant cassava in irregular populations which may be lower or higher than the standard recommended populations of 10,000 plants /ha. Experiments were therefore conducted at Makerere University Agricultural Research Institute Kabanyolo (MUARIK) to determine the effects of plant population on the whitely vector and on the progress and severity of CMD. Three cassava varieties, Bao (CMD-susceptible), Nase 2 (moderately resistant) and SS4 (highly resistant) were each planted at four densities (5,000. 6,666, 10,000 and 20,000 plants/ha) using symptom less planting materials in a split-plot arrangement of a randomized complete block design with three replications. Adult whitefly populations and CMD incidence and severity were monitored monthly beginning 6 weeks after planting until 7.5 and 6.5 months after planting (MAP), RESPECTIVELY. Plant canopy spread and height were also measured monthly AND 6.5 MAP, plant leaf area was measured. The effects of CMD on growth and yield of cassava were quantified by measuring leaf area and stem girth, number of tuberous roots per plant, individual tuberous root weight per plate. There was a cassava varietal preference by B. tabaci. Significantly (P<0.001) more vectors were recorded on cultivators Bao and SS4 which had larger and more sufficient looking leaves than Nase 2. These features probably provided a better feeding and oviposition environment in Bao and SS4 than Nase 2. Planting population statistically (P=0.009) influenced vector population per shoot which increased as plant population per unit area decreased. This was because plants at lower density received more of the vectors per plant compared to their closely spaced counterparts. Widely spaced plants may have also been preferred by the vectors for their vigor, which provided a better feeding ground. Correspondingly, CMD incidence differed between cassava varieties and increased significantly (P=0.029) with a decrease progress curves (AUDPCs) values that were significantly (P<0.05) greater in Nase 2 followed by in Bao and least in SS4 and decreased with increase in plant population per unit area. Reduced CMD incidence in the highly resistant SS4 compared to Nase and Bao, irrespective of the high number of whitefly populations it bore is an indication that the ability of cassava varieties to tolerate CMG depended more on inherent resistance to CMGs rather than to vector repellence per se. Higher incidence at lower plant population resulted from the large dose of the CMG inoculum transmitted by the higher vector numbers, which infested the plants. Cassava varietal difference or plant population did not affect severity of the disease significantly. Disease severity only increased with time and leveled off beyond 4 months after planting. However, the disease index (the measure of damage the disease caused to the plants) was significantly (P<0.01) higher in Nase 2 followed by Bao and lowest in SS4. Significantly, more damage also occurred in the lower plant population and reduced as plant population was increased. There were significant (P<0.05) effects of stage CMG infection of cassava on the growth and yield parameters with the earlier affected plants especially of Bao, being more damaged and yielding less compared to the lately infected plants. However, SS4 was not infected lather than 4 MAP. This indicated that earlier infected plants suffered more loss than the lately infected plants. Ultimately, the earlier infected plants yielded significantly (P<0.05) lower than the lately infected and the healthy plants, which did not differ significantly in most of. It is concluded that cassava resistance to CMD can be augmented by limiting B.tabaci infestation of the through plant population manipulation. Ultimately, it is recommended that farmers who wish to grow cassava should use CMG-resistance cultivators and supplement their inherent resistance by planting at optimally higher populations with other control measures. However, it is also recommended that the study should be repeated in different agro-ecologies of different infection pressures to extend the findings of the present study and assess their ability.