MEASUREMENTS OF SPATIAL PATTERNS OF DISEASE IN WINTER-WHEAT CROPS ANDTHE IMPLICATIONS FOR SAMPLING

Over a period of three crop seasons the spatial patterns of some commo n diseases of winter wheat were investigated at growth stages (GS) 31/ 33 and 59/61. A large-scale sampling procedure, using randomly positio ned 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 comp lete 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 gr owth stages investigated. However, thr severity of S. tritici blotch w as 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. Therefor e, almost any convenient sampling pattern, with reasonable overall cov erage, will be adequate to obtain samples for monitoring winter wheat at growth stages 31 and 59.