The root-lesion nematode,Pratylenchusspp. is proved to be the primary causal agent of the brown root rot disease of tobacco in Ontario. The root lesions resulting from nematode attack are undoubtedly further broken down by the activity of other soil organisms. At least two species ofPratylenchusare involved, differing significantly in pathogenicity and morphology. Nine other parasitic and potentially dangerous nematode genera also were found associated with tobacco roots in Ontario, but they occurred in very low numbers. The effect of the preceding crop upon brown root rot of tobacco was found to be related to nematode populations; and for eight preceding crops, significant effects upon the growth of tobacco occurred only in the presence of high populations of parasitic nematodes. SincePratylenchus minyusSher and Allen, 1953, is the more pathogenic species, the relation between this nematode and the tobacco host was studied in some detail. The optimum soil temperature for this nematode is at least 100deg; F. and it causes little stunting of tobacco below a soil temperature of 70deg; F. In a resistant variety of tobacco studied, an unusual mechanism of resistance upholds a hostndash;parasite equilibrium wherein, despite high populations of parasitic nematodes within the roots, normal plant growth is maintained.
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