AN ASSESSMENT OF THE SOIL-SAMPLING DENSITY AND SPATIAL DISTRIBUTION REQUIRED TO DETECT VIRULIFEROUS NEMATODES (NEMATODA: LONGIDORIDAE AND TRICHODORIDAE) IN FIELDS

D. J. F. Brown, B. Boag, A. T. Jones, P. B. Topham

Abstract


Twin soil samples were collected from 130 sampling points from each of eight fields, during a survey of 37 fields in eastern Scotland, to determine the spatial distribution of virus vector nematodes. Samples were taken from 10 to 19 cm depth at each site from ten 100 m transects each 11 m apart with distances between samples of 5 cm to 51.2 m. Virus was detected by bait-testing field soil in 10 cm dia., 650 cm³ plastic pots planted with Petunia hybrida and Cucumis sativus seedlings for four weeks. Symptoms in indicator plants, electron microscopy and the alkaline phosphatase version of F (ab¹)[sub2] ELISA were used for virus identification. Viruses were recovered from seven sites with combinations of two viruses occurring at four sites: raspberry ringspot and arabis mosaic (AMV), raspberry ringspot and tomato black ring, and tomato black ting and tobacco rattle (TRV) al two sites, and TRV alone at three sites. AMV was recovered from all 35 samples containing its vector, Xiphinema diversicaudatum, whereas with all other virus and vector combinations virus was not always recovered from samples containing vector nematodes and also the numbers of vector nematodes in samples did not influence the recovery of virus. The distributions of all viruses were highly aggregated and an analysis of all possible pairs of samples, classified by their distances apart, revealed a high probability for the recovery of virus from pairs of samples up to 8 m apart. Sampling for detecting nepoviruses and tobraviruses therefore should be based on a regular grid lattice with 7 m between sampling points.

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