SUMMARYWe reformulate a model for botanical epidemics into an SIR form for susceptible (S), infected (I) and removed (R) plant organs, in order to examine the effects of different models for the effect of host responses to the load of infection on the production of susceptible tissue. The new formulation also allows for a decline in host susceptibility with age. The model is analysed and tested for the stem canker disease of potatoes, caused by the soil-borne fungus, Rhi octonia solani. Using a combination of model fitting to field data and analysis of model behaviour, we show that a function for host response to the amount (load) of parasite infection is critical in the description of the temporal dynamics of susceptible and infected stems in epidemics of R. solani. Several different types of host response to infection are compared including two that allow for stimulation of the plant to produce more susceptible tissue at low levels of disease and inhibition at higher levels. We show that when the force of infection decays with time, due to increasing resistance of the host, the equilibrium density of susceptible stems depends on the parameters and initial conditions. The models differ in sensitivity to small changes in disease transmission with some showing marked qualitative changes leading to a flush of susceptible stems at low levels of disease transmission. We conclude that there is no evidence to reject an SIR model with a simpler linear term for the effect of infection load on the production of healthy tissue, even though biological considerations suggest greater complexity in the relationship between disease and growth. We show that reduction in initial inoculum density, and hence in the force of infection, is effective in controlling disease when the simple model applies.

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Screenhouse experiments conducted in Kenya showed that inoculation of cabbage seedlings with Turnip mosaic virus (TuMV), either alone, or in combination with Cauliflower mosaic virus (CaMV), reduced the number and weight of marketable harvested heads. When viruses were inoculated simultaneously, 25% of cabbage heads were non-marketable, representing 20-fold loss compared with control. By contrast, inoculation with CaMV alone had insignificant effects on cabbage yield. This suggests that TuMV is the more detrimental of these pathogens, and its management should be a priority. Early exposure to TuMV produced cabbages that were 50% lighter than non-infected plants, but later infection was less damaging suggesting that controlling virus infection at the seedling stage is more important. TuMV was far less damaging to kale than it was to cabbage; although high proportions of TuMV-inoculated kale plants showed symptoms (>90%), the marketability and quality of leaves were not significantly reduced, and no clear relationship existed between timing of infection and subsequent crop losses. Early inoculation of Swiss chard with Beet mosaic virus (BtMV) significantly impaired leaf quality ( ∼ 50% reduction in marketable leaf production), but the impact of disease was greatest in plants that had been inoculated at maturity, where average leaf losses were two and a half times those recorded in virusfree plants. Disease-management of BtMV in Swiss chard is important, therefore, not only at the seedling stage, but particularly when plants are transplanted from nursery to field.

Bait plants, comprising micropropagated and commercial seed tubers, were used to estimate the effects of rotation on the density and spatial pattern of inoculum of Rhizoctonia solani in large field plots of potatoes. Soilborne inoculum of R. solani produced economically significant levels of stem canker and its incidence and severity varied with rotation, with most disease in 2-year and less in 4-and 6-year rotations. The rates of loss of inoculum during intercrop periods differed amongst rotations with a rapid fall to low levels occurring after 1 year in a 6-year rotation and after 2 years in a 4-year rotation. Replenishment of inoculum to soil was rapid following the growth of a susceptible crop, with comparatively high levels of infection and disease, even in long rotations. Disease occurred in patches and the size of patches and the density of R. solani within patches differed with cropping frequency. The degree of spatial autocorrelation also differed amongst rotations but there was no evidence for any significant differences in the rate of change of spatial autocorrelation during intercrop periods in the three rotations.

A storage rot of yams (Dioscorea alata), commonly referred to as ‘dead skin′, has recently become a major constraint to yam production in the Caribbean. The fungal pathogen Colletotrichum gloeosporioides was consistently isolated from affected tubers and re‐inoculation of healthy yam tubers resulted in the development of typical symptoms of dead skin.