DISTRIBUTION AND YIELD-LOSS RELATIONS OF VERTICILLIUM-DAHLIAE, PRATYLENCHUS PENETRANS, P-SCRIBNERI, P-CRENATUS, AND MELOIDOGYNE-HAPLA IN COMMERCIAL POTATO FIELDS

Potato yields and population densities of organisms that cause or pote ntially influence the early dying syndrome were measured by sampling a long linear transects in commercial potato fields. The distributions o f the five organisms were fitted with a negative binomial distribution (P = 0.05) in six of 10 fields for Verticillium dahliae, six of seven fields for Meloidogyne hapla, one of seven fields for Pratylenchus pe netrans, four of six fields for P. scribneri, and six of seven fields for P. crenatus. Hill's two-term local quadrat variance method for V. dahliae indicated that aggregation generally increased or did not chan ge with plot size, except in two fields where aggregation was highest at or near the smallest plot size (2 m), i.e., the lowest tested spati al scale. With the three species of Pratylenchus, aggregation generall y increased with plot size; and with M. hapla, the peak of aggregation was highly variable from field to field. Taylor's power law was used to estimate a minimum sampling number for each organism, With a coeffi cient of variation of the mean (C) of 0.50, six to eight samples were necessary for all five species; the required sample number increased d ramatically if precision was increased to C = 0.20. Significant spatia l autocorrelations of low order were observed most frequently for V. d ahliae and M. hapla. Autocorrelation patterns were not clearly evident in most of the fields for P. penetrans. No significant covariation wa s seen between V. dahliae and any nematode species density. There was a low degree of positive covariation observed between M. hapla and the three species of Pratylenchus and a high degree of positive covariati on among the three species of Pratylenchus. Yields were negatively cor related with preplant densities of V. dahliae and P. penetrans or thei r interaction in three of seven fields and with M. hapla in three of 1 0 fields. Yields were also correlated negatively with V. dahliae and P . penetrans individually, positively with interactions between M. hapl a and V. dahliae, and negatively with V. dahliae and Pratylenchus spp. (species not identified) and V. dahliae and P. crenatus in one or two fields each.