SPATIAL PATTERNS OF MICROSCLEROTIA OF VERTICILLIUM-DAHLIAE IN SOIL AND VERTICILLIUM WILT OF CAULIFLOWER

The spatial patterns of microsclerotia of Verticillium dahliae in soil and wilt symptoms on cauliflower were determined at three sites in ea ch of two fields in 1994 and 1995. Each site was an 8 x 8 grid divided into 64 contiguous quadrats (2 by 2 m each). Soil samples were collec ted to a depth of 15 cm with a probe (2.5 cm in diameter), and samples from four sites in each quadrat were bulked. Plants in each quadrat w ere cut transversely, and the number of plants with vascular discolora tion and the number without discoloration were recorded. The soil was assayed for microsclerotia by the modified Anderson sampler technique. Lloyd's index of patchiness (LIP) was used as an indicator to evaluat e the aggregation of microsclerotia in the field. Spatial autocorrelat ion and geostatistical analyses were also used to assess the autocorre lation of microsclerotia among quadrats. The LIP for microsclerotia wa s greater than 1, indicating aggregation of propagules; however, the d egree of aggregation at most sites was not high. Significant autocorre lation within or across rows was detected in some spatial autocorrelog rams of propagules, and anisotropic patterns were also detected in som e oriented semivariograms from geostatistical analyses for microsclero tia, indicating the influence of bed preparation in the fields on path ogen distribution. The parameter estimates p and theta in the beta-bin omial distribution and the index of dispersion (D) associated with the distribution were used to assess the aggregation of diseased plants a t each site. A random pattern of wilt incidence was detected at 7 of 1 2 sites, and an aggregated pattern was detected at 5 of 12 sites. The degree of aggregation was not high. A regular pattern of wilt severity was detected at all sites. The high disease incidence (77 to 98%) obs erved at 11 of the 12 sites could be explained by high inoculum densit y.