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A survey on the tomato late blight situation and current practices for disease management was carried out in Uganda using an informal structured questionnaire approach. Ten districts from different agroclimatic zones were selected for the survey. Phytophthora infestans isolates from tomatoes were obtained from the zones and only the A1 mating type was recovered. Tomato cultivation is practised year-round. The major commercial varieties grown, Moneymaker, Marglobe, Heinz and Roma, were susceptible to late blight. Late blight and bacterial wilt (Pseudomonas solanacearum) were the major diseases, with bollworm (Helicoverpa armigera) and thrips (Thrips spp.) the major insect pests. Fungicides, mostly mancozeb, were the predominant control method used by farmers. Sanitation is practised by some farmers, but only in association with fungicides, to control late blight. Plant nutrition is not perceived as playing a major role in late blight management. Problems associated with pesticides and poor marketability were identified as major constraints to tomato production in Uganda. [ABSTRACT FROM AUTHOR]

1. An epidemiological model of African cassava mosaic virus disease (ACMD) was developed in which the dynamics within a locality, of healthy and infected cassava. and of infective and non-infective whitefly vectors, were specified. 2. Cassava is propagated routinely from stem cuttings. If the exclusive use of virus- free cuttings was assumed then either a high rate of virus transmission by the vector or a Large population of vectors could lead to persistent cycles in disease incidence. Such cycling could serious]v impair cassava production. 3. When frequency-dependent use of infected cuttings was admitted, however, persistent cycles of infection were no longer possible. Instead, three states could occur: disease eliminated (E). both healthy and diseased cassava persisted (P). or infection became total (T). The outcome depended on the particular parameter values, but sonic parameter combinations gave state P or T. depending on the initial conditions. In some instances, low initial incidence of ACMD posed a greater risk of progressing to state T than did higher levels of infection. 4. In general, disease incidence exhibited a discontinuous, threshold response to changes in the proportion of infected cuttings used. An increase in their use could appear to have little impact on incidence, although the system was moving closer to a point of sudden increase in infection. 5. Simulation of the removal (roguing) of diseased plants had little influence on disease incidence, but had an important and largely cryptic effect by helping to move the system away from the threshold of collapse of the healthy cassava population. Intensification of cassava production (represented in the model as an increase in the maximum possible abundance of cassava) had the opposite effect, moving the system closer to this threshold. 6. Assuming a degree of disease `reversion', whereby a proportion of the cuttings derived from infected plants are uninfected, had a stabilizing influence on the model, making even damped cycles of disease very unlikely Total infection did not now occur except at very low reversion rates. The response of disease incidence to the increased use of infected cuttings was no longer discontinuous but remained non- linear and there was a region of rapid change corresponding to the threshold seen in the absence of reversion. [ABSTRACT FROM AUTHOR]