Modelling the interaction between powdery mildew epidemics and host dynamics of tomato

A model simulating the progress of powdery mildew coupled to the growth dynamics of tomato, with a time step of 1 day, is developed. The model is formulated as a set of differential equations for the rate of change in the amount of healthy, diseased and defoliated leaf area of a diseased plant relative to that of a healthy crop. The main assumption of the model is that the total host area formed is limited and identical in the disease and non-disease situation. Host and disease parameters were estimated through fitting the model to experimental data obtained from glasshouse experiments. Model outputs of powdery mildew severity and tomato leaf area showed a good fit to observed data with R 2 > 0.97. In the two experiments with big pot size, the estimated rates for host growth (r H : 0.110 and 0.123 day−1), defoliation (r D : 0.047 to 0.085 day−1) and disease (r Y : 0.128 to 0.131 day−1) were very similar while the rates in the other experiment with small pots clearly differed. The simulated effect of the disease on the host growth rate was not uniform, predicting in only one experiment that powdery mildew epidemics significantly lowered the growth rate of diseased plants when compared to the healthy plants. The model showed that defoliation of healthy area does not contribute significantly to total defoliated area. Except for slight deviations, there were no significant differences between progress curves of either host or disease dynamics under a constant or variable disease rate influenced by temperature and relative humidity. Since there was a reasonably good fit between model outputs and experimental data, the model can be considered to satisfactorily describe the interaction between powdery mildew epidemics and growth dynamics of tomato.