Science into practice better risk assessment of onion stem nematode

Stem nematode is a very destructive pest of bulb onions that would, if left untreated, cause an estimated annual crop loss of around £1m. Unless growers are confident that the risk of infestation of fields is low, they are loathe to leave crops untreated, but supermarkets are requesting nematicide-free production.

The pest spends most of its life in the plant so numbers in soil are usually small. Soil sampling protocols are the only way to assess risk, therefore they need to give the maximum chance of detection. HDC project FV 327 aimed to develop optimum methods of soil sampling and analysis.

For mapping the in-field distribution of stem nematode the project took advantage of soil sampling equipment linked to differential GPS. Intensive soil sampling was concentrated on three fields known to pose low, medium and high risk to crops. Stem nematode distribution was mapped and a sampling method drawn up to give the best chance of detecting it. As not all soil extraction methods are equally effective at recovering stem nematode, four methods were compared in the laboratory.

More nematodes were recovered from the pneumatically-sampled than hand-sampled soil, irrespective of extraction method. Stem nematodes occurred in distinct patches in infected fields, so a systematic rather than random sampling pattern is needed for accurate population counts.

A sampling grid of 100 points covering 4ha is the best compromise between accuracy and the time required, replacing the method of sampling along a W pattern. Of the extraction methods tested, the Seinhorst two-flask technique extracted two or three times as many nematodes as others.

Horticultural Development Company

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