QUANTIFICATION OF PYTHIUM-ULTIMUM VAR SPORANGIIFERUM ZOOSPORE ENCYSTMENT PATTERNS USING GEOSTATISTICS

Geostatistical analysis was used to quantify spatial patterns of encysted Pythium ultimum var. sporangiiferum zoospores on pea roots. In one experiment, peas were grown in sand at 20 C for 5 days, then roots were exposed to zoospore suspensions of P. u. sporangiiferum (150, 1,500, or 15,000 zoospores/g sand). After 3 h, roots were removed from the sand, stained with 0.05% trypan blue, and zoospore counts were made at 100X, using each square (83 X 83 mu m) of a 10 X 10 reticle as a sample unit. Coordinates and number of encysted zoospores were recorded for each sample unit. Spatial statistics (geostatistics) were used to create covariograms and fit a spherical model for each inoculum density level. Fitted models provided estimates of the size of the spatial influence (range) in each treatment, random (nonspatial variation), or measurement error (nugget), and the value (sill) around which the variogram became stable. The spatial organization of cysts changed with inoculum density. At low and intermediate densities cysts were either randomly or uniformly distributed over the root surface. At the highest inoculum density, cysts had an aggregated spatial arrangement (nugget approximate to 0.41, sill approximate to 1.0, range approximate to: 355 mu m). When peas were exposed to zoospores after 2, 3, or 5 days after planting, spatial patterns of encysted zoospores again showed spatial structure only at the high inoculum density. Root system age did not affect spatial patterns.