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The root‐lesion nematode (Pratylenchus penetrans Cobb) is a significant pest of many crops in Ontario. For example, more than 25 000 ha of land cropped to flue‐cured tobacco (Nicotiana tabacum L.) is fumigated annually at a mean cost for the fumigant materials alone of $484 ha−1. The objectives of these trials were to develop and evaluate a marigold (Tagetes sp.) rotation cropping system for the biological control of root‐lesion nematodes. In 1995 (site A) and 1996 (site B), field plots of Tagetes patula L. cv. Creole and Tagetes erecta L. cv. CrackerJack were established as rotation crops in comparison with the traditional rye rotation crop plus chemical fumigation prior to transplanting flue‐cured tobacco. Within 75 d of seeding marigold, P. penetrans population densities were reduced to less than 100 kg−1 soil, below the economic threshold of 500 P. penetrans kg−1 soil for tobacco. A marigold plant density of about 20 plants m−2 reduced P. penetrans population densities to levels below the economic threshold for the rotation crop year and the two following years. Tobacco yield was increased by a mean of 197 kg ha−1 by marigold rotation crops compared with rye plus chemical fumigation. The seed cost for a marigold crop at 20 plants m−2 varied from $221 ha−1 for T. patula to $294 ha−1 for T. erecta This new cropping system for the biological control of root‐lesion nematodes is a functional alternative to chemical fumigation for Ontario flue‐cured tobacco growers.

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Re"search lnstitute 6f Ontario, p Scott and Ogle (1952) and Scott and Bouwkamp (1975)

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