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 potentially influence the early dying syndrome were measured by sampling along linear transects in commercial potato fields. The distributions of the five organisms were fitted with a negative binomial distribution (P = 0.05) in six of 10 fields for Verticillium dahhae, six of seven fields for Meloidogyne hapla, one of seven fields for Pratylenchus penetrans, 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 change 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 spatial scale. With the three species of Pratylenchus aggregation generally 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 coefficient of variation of the mean (C) of 0.50, six to eight samples were necessary for all five species; the required sample number increased dramatically if precision was increased to C = 0.20. Significant spatial autocorrelations of low order were observed most frequently for V. dahliae and M. hapla. Autocorrelation patterrs were not clearly evident in most of the fields for P. penetrans. No significant covariation was 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 covariation among the three species of Pratylenchus. Yields were negatively correlated with preplant densities of V. dahliae and P. penetrans or their interaction in three of seven fields and with M. hapla in three of 10 fields. Yields were also correlated negatively with V. dahliae and P. penetrans individually, positively with interactions between M. hapla 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.