Modelling the progress of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to leaf wetness and temperature

A compartmental model was developed to describe the progress with time of l ight leaf spot (Pyrenopeziza brassicae) on leaves of winter oilseed rape (B rassica napus) during the autumn in the UK. Differential equations describe d the transition between the four compartments: healthy susceptible leaves. infected symptomless leaves, sporulating symptomless leaves and leaves wit h necrotic light leaf spot lesions, respectively The model was fitted to da ta on the progress of light leaf spot on winter oilseed rape at a single si te during the autumn of the 1990-1991 season. Model parameters were used to describe rates of leaf appearance, leaf death, infection by airborne ascos pores (primary inoculum) and infection by splash-dispersed conidiospores (s econdary inoculum). Infection was dependent on sufficient leaf wetness dura tion. The model also included delay terms for the latent period between inf ection and sporulation and the incubation period between infection and the appearance of necrotic light leaf spot lesions. This modified SEIR model fo rmulation gave a reasonable fit to the experimental data. Sensitivity analy sis showed that varying the parameter accounting for the rate of infection by ascospores affected the magnitude of the curves after the start of the e pidemic, whilst including a parameter for conidiospore infection improved t he fit to the data. Use of ascospore counts from different sites and differ ent years showed variation in spore release patterns sufficient to affect m odel predictions.