Effects of quadrat size and time of year for sampling of Verticillium dahliae and lesion nematodes in potato fields

Six potato fields were randomly sampled using quadrat areas ranging from 10 m(2) to 4,000 m(2). A composite soil sample consisting of 20 soil cores wa s taken from each of 20 quadrats of each unit area. All samples were assaye d for Verticillium dahliae and nematodes. The area in which a composite sam ple was taken had little effect on estimated mean and variance of CI dahlia e until quadrat size was greater than or equal to 1,000 m(2). Then variance s dramatically increased with size. With Pratylenchus spp., there was no sy stematic relationship between quadrat size and variance. Seven additional f ields were divided into 2,000 or 4,000 m2 grids, and each grid was sampled in both spring and fall for V. dahliae and plant-parasitic nematodes (Praty lenchus spp. and Meloidogyne hapla). Density of V. dahliae was lower, and i n some cases, degree of aggregation was higher, in the spring than in the f all. Sampling for V. dahliae in the fall required 2 to 7 composite samples to estimate mean density within 25% of the true mean, while in the spring, 10 to 19 samples were required. Assays of Pratylenchus spp. required a simi lar number of samples in spring and fall surveys, while M. hapla required f ewer samples with spring than with fall sampling when estimating densities >20 second-stage juveniles per 100 cm(3) of soil.