Effects of prevailing wind direction on spatial statistics of plant disease epidemics

A stochastic simulation model was used to study the effects of the strength of prevailing wind (W), the size/ shape (Q) of sampling quadrats and their orientation in relation to the prevailing wind direction (D) on spatial st atistics describing plant diseases. Spore dispersal followed a half-Cauchy distribution with median distance mu, which depended on simulated wind spee d. The relationship of spatial autocorrelation at distance k (rho (k)) to d isease incidence (p) and distance was well described by a four-parameter (a lpha, beta (1), beta (2), beta (3)) power-law model at a given p, rhok decl ined exponentially with distance. A total of 35 different quadrat sizes, ra nging from 4 to 432 plants, were used to sample the simulated epidemics for estimating intraclass correlation (kappa). The kappa -values decreased exp onentially with increasing quadrat size, a binary power law model with thre e parameters (alpha, beta (4), beta (5)) successfully related kappa to p. I n general, the effect of W and D was greatest on the parameters; alpha, bet a (1), beta (2) and beta (3). The effect of W on alpha, beta (1), beta (2) and beta (3) depended critically on the spatial pattern of initial infected plants (Y) W had greatest effect for the random pattern. In contrast, the main effect of D and its interaction with W on the parameters alpha, beta ( 1), beta (2) and beta (3) were large and consistent over different initial conditions. Variations in alpha (1), beta (4) and beta (5) were predominant ly due to Y and Q. Only for beta (5) under the clumped pattern was the effe ct of W very large. For the parameters alpha (1), beta (4) and beta (5) the re was a large interaction among W, Q and D for the clumped and regular pat terns. As expected, in general, the effect of D increased with increasing p revailing wind strength, quadrat size and quadrat length width ratio. Using square quadrats reduced significantly the effect of W on the parameters al pha (1), beta (4) and beta (5); however, the effect of W on beta (5) was st ill very large for the clumped pattern. Sampling perpendicular to the preva iling wind direction generally resulted in larger differences in the nine e stimated parameters.