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 wetne ss duration in controlled-environment experiments on the development of lig ht leaf spot on oilseed rape (cv. Bristol) leaves inoculated with Pyrenopez iza brassicae conidial suspensions. A Gompertz function was used to describ e the progress with time in percentage leaf area with sporulation, and incl uded the parameters maximum percentage leaf area with sporulation (c); maxi mum rate of increase in percentage leaf area with sporulation (r); and late nt period (l, the time from inoculation until the leaf area with sporulatio n reached 37% of c). The effects of leaf wetness duration on c and r were a lso described with Gompertz functions, which included the parameters minimu m 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 describ ed by quadratic functions, and the effect of temperature on m(r) was descri bed by a linear function. The combined model described the progress with ti me in percentage leaf area with sporulation, including the effects of tempe rature and leaf wetness duration on the parameters c, r and l. It generally fitted well to the observed data. Latent periods in previously published e xperiments were predicted accurately by the model, but percentage leaf area with sporulation was not. Assuming a great number of conidia were disperse d and infection occurred when there was 2 mm h(-1) rain for 0.5 h, the mode l estimates for latent period were used to predict the dates when large inc reases 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.