- Primary disease gradients of wheat stripe rust, caused by the aerially dispersed fungal pathogen Puccinia striformis, were measured in Madras and Hermiston, OR in the springs of 2002 and 2003. Disease foci were created by inoculating a 1.52m x 1.52m area in each of three replicate field plots (6.1m wide x 131 to 171m long) in each of the four experiments. Primary disease gradients were measured both upwind and downwind of the focus. Four gradient models the power law, the modified power law, the exponential model, and the general model proposed by Lambert et al.were fit to the data. Five of the eight gradients were better fit by the power law, modified power law,
and Lambert model than by the exponential, revealing the non-exponentially bound nature of the gradient tails. The other three datasets, which were comprised of the fewest data points, were equally well fit by all the models. By truncating the largest datasets it was shown how the relative suitability of dispersal models can be obscured when data is available only at a short distance from the focus. The truncated datasets were also used to examine the risks of extrapolating gradients to distances beyond available data. The power law and modified power law predicted dispersal at large distances well, even given limited data, while the Lambert and exponential consistently and sometimes severely underestimated dispersal at large distances. Field data on disease gradients are useful for helping to confirm or refute the validity of gradient models derived from the physical mechanisms of dispersal, as well as to provide accurate information for models designed to predict the behavior of expanding epidemics. The velocity of expansion of focal epidemics was studied using an updated version of the simulation model EPIMUL. Dispersal data were derived from the Hermiston 2002 field experiment described above. Lesion growth rate, latent period, infectious period, multiplication rate, and dispersal gradient steepness were varied within ranges reasonable for P. striformis. All but the infectious period had a strong influence on velocity. Three different equations were employed in turn to describe dispersal: the modified power law, the exponential, and Lambert's general model. The exponential,
which fit the gradient data from the field epidemic poorly, yielded an epidemic that expanded at a constant velocity, after an initial buildup period. Both the modified power law and the Lambert model fit the field data well, and produced gradient curves of similar shape. Simulations run with the modified power law and the Lambert model resulted in velocities that increased over time for the entire course of the epidemic, supporting the existence of dispersive epidemic waves.