Temporal dynamics of wheat blast epidemics and disease measurements using multispectral imagery

Wheat blast is a devastating disease caused by the Triticum pathotype of Magnaporthe oryzae (MoT). MoT is capable of infecting leaves and spikes of wheat. Although symptoms of spike blast (WSB) are quite distinct in the field, symptoms on leaves (WLB) are rarely reported since they are usually less noticeable. Two field experiments were conducted in Bolivia to characterize the change in WLB and WSB intensity over time and determine if multispectral imagery can be used to accurately assess WSB. Disease progress curves (DPCs) were plotted from WLB and WSB data, and regression models were fitted to describe the nature of WsB epidemics. Although WLB severity was low during the vegetative stages, there was a bimodal shape when WLB incidence DPCs were plotted. The Gompertz model best described WSB intensity change over time in both inoculated and non-inoculated plots from both locations (R2=0.94-0.98; RMSE=0.16-0.58). Lin’s concordance correlation coefficients were estimated to measure agreement between visual estimates and digital measurements of WSB intensity and to estimate accuracy, and reliability. Our findings suggest that the change of wheat blast intensity in a susceptible host population over time does not follow a pattern of a monocyclic epidemic. We have also demonstrated that WSB severity can be quantified using non-green vegetation pixels reliably (0.91-0.960.68-0.83) and accurately (0.86-0.920.56-0.71) at moderately-low-to-high visual WSB severity levels. Additional sensor-based methods must be explored to determine their potential for detection of WLB and WSB at earlier stages.