SIMULATION MODELLlNG OF THE PROGRESS OF LIGHT LEAF SPOT ON WlNTER OILSEED RAPE (BRASSICA NAPUS ) IN RELATION TO CRITERIA FOR INFECTION BY PYRENOPEZIZA BRASSICAE
K PAPASTAMATI1, SJ WELHAM1, BDL FITT1 and P GLADDERS2
1IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK; 2ADAS Boxworth, Cambridge CB3 8NN, UK
Background and objectives
Light leaf spot (Pyrenopeziza brassicae) is a polycyclic disease of winter ouseed rape. Epidemics progress through repeated infections caused by secondary production of inoculum (spores) subject to appropriate weather conditions. Previous research has shown that the criterion for infection of leaves is leaf wetness duration of at least 16 hours at optimum temperature of 150°C. Spores are produced after a latent period of between 150 and 250 degree-days (accumulated temperature) after infection . Furthermore, spores for reinfection of crops can be produced at temperatures below 100°C. Simulation of light leaf spot progress with time over one growing season, incorporating information about infection criteria and the latent period, is intended to provide a better understanding of the mechanisms influencing the epidemic than has been achieved with previous empirical models . This will be the first stage in developing a modelling framework to improve fungicide use and develop more efficient strategies for management of the disease.
Materials and methods
The data used was collected by ADAS Bristol in the 1989/90 season on winter oilseed rape cv. Jet Neuf. Weekly assessments were made on 25 marked plants between the end of August 1989 and January 1990. Leaves were labelled on appearance and so could be identified across assessments. In each assessment, every leaf was recorded as either dead, present with no visual symptoms or diseased (the percentage area with light leaf spot symptoms). Meteorological parameters were also recorded. This data was sufficiently detailed to allow modelling of disease progress by simulation of the process of infection and production of new spores for given weather conditions. The recording of data on identified leaves meant that a realistic estimate of disease progress within the crop could be obtained, as opposed to estimates based on the apparent infection on live leaves alone.
Results and conclusions
Modelling investigated the dynamics of the epidemic in relation to weather factors, with extensions to incorporate stochasticity to allow for natural variation in the pathogen life cycle and the amount of inoculum available and, for the purposes of prediction, to account for uncertainties resulting from the unpredictability of the weather. This project combines modelling work with further biological research into the disease epidemiology, which produces more detailed information on infection and pathogen growth conditions. This benefits the modelling process, where the aim is to produce a simple but accurate description of the epidemic. Conversely, the model-fitting process helps to identify the critical or limiting factors in the epidemic cycle in the crops. Future work will incorporate the effects of fungicide use on disease progress to enable simulation of disease progress for a given fungicide spray programme in combination with different weather patterns. This work will complement the light leaf spot forecasting scheme based on empirical models being developed  and will eventually form a component of a robust decision support system for management of diseases on winter oil seed rape.
1. Figueroa L, Fitt BDL, Welham SJ, Shaw MW, McCartney HA, 1995. Plant Pathology 44, 641-654.
2. Fitt BDL, Gladders P, Turner JA, Welham SJ, Davies JMLI, 1997. Aspects of Applied Biology 48, 135-142.