Modelling crop management and environmental effects on the development of Leptosphaeria maculans pseudothecia

The timing of ascospore release is critical in the prediction of blackleg infection, particularly if the timing of spore release coincides with early development of canola seedlings. Historically, prediction models have used average daily temperature and an environment calibration to estimate a minimum rainfall amount to trigger development. This paper describes a different approach based on hydro-thermal time, where soil evaporation and rainfall are used as a surrogate to estimate when the stubble is wet, and temperature is accumulated on an hourly basis. Furthermore, the stubble orientation due to differing harvest management practices is considered, as stubble knocked down has greater contact with the soil compared to stubble which remains in a standing position. Pseudothecia and ascospore development in the standing and lying treatments was monitored weekly to measure the rate of development in diverse environments. The new modelling approach was used to describe the pseudothecial maturation rate and predict the timing of ascospore release. Subsequently, a range of Australian and international datasets were used to assess the robustness and accuracy of this new model's predictions. When tested across multiple locations and seasons globally, the hydro-thermal time approach had similar performance (R-2 = 0.94, RMSD = 16 days) to the existing Sporacle Ezy model (R-2 = 0.90, RMSD = 22 days), but without the need to calibrate in different environments and account for the delayed rate of development in standing stubble. Integrating this new approach into early warning systems for canola growers will help in the management of control measures.