Modelling the effects of temperature and wetness duration on development of light leaf spot on oilseed rape leaves inoculated with Pyrenopeziza brassicae conidia

A model was developed to describe the effects of temperature and leaf wetness duration in controlled-environment experiments on the development of light leaf spot on oilseed rape (cv. Bristol) leaves inoculated with Pyrenopeziza brassicae conidial suspensions. A Gompertz function was used to describe the progress with time in percentage leaf area with sporulation, and included the parameters maximum percentage leaf area with sporulation (c); maximum rate of increase in percentage leaf area with sporulation (r); and latent period (l, the time from inoculation until the leaf area with sporulation reached 37% of c). The effects of leaf wetness duration on c and r were also described with Gompertz functions, which included the parameters minimum leaf wetness duration (v(c) or v(r)); and maximum of c (m(c)) or maximum of r (m(r)). The effects of temperature on m(c), v(c) and v(r) were described by quadratic functions, and the effect of temperature on m(r) was described by a linear function. The combined model described the progress with time in percentage leaf area with sporulation, including the effects of temperature and leaf wetness duration on the parameters c, r and l. It generally fitted well to the observed data. Latent periods in previously published experiments were predicted accurately by the model, but percentage leaf area with sporulation was not. Assuming a great number of conidia were dispersed and infection occurred when there was 2 mm h(-1) rain for 0.5 h, the model estimates for latent period were used to predict the dates when large increases in light leaf spot severity occurred in experiments at Rothamsted on winter oilseed rape (cv. Bristol) under natural conditions in 1998/99 and 1999/2000. The predictions agreed with the observations.