Measurements of spatial patterns of disease in winter wheat crops and the implications for sampling

Over a period of three crop seasons the spatial patterns of some common diseases of winter wheat were investigated at growth stages (GS) 31/33 and 59/61. A large-scale sampling procedure, using randomly positioned transects and based on the theory of autocorrelation analysis, is described. This novel technique enables valid tests of significance to be made on the autocorrelation coefficients calculated. The most complete data obtained were for Septoria tritici blotch which was found to have a near random pattern on scales between 31 cm and 31 m at the growth stages investigated. However, the severity of S. tritici blotch was found to be autocorrelated at scales below 1 m in some fields. With the exceptions of powdery mildew at GS 31/33 and yellow rust at GS 59/61, the other diseases also exhibited a near random pattern. Therefore, almost any convenient sampling pattern, with reasonable overall coverage, will be adequate to obtain samples for monitoring winter wheat at growth stages 31 and 59.