CONSEQUENCES OF LONG-TERM CROPPING WITH RESISTANT CULTIVARS ON THE POPULATION-DYNAMICS OF THE ENDOPARASITIC NEMATODES HETERODERA-AVENAE ANDPRATYLENCHUS-NEGLECTUS IN A CEREAL PRODUCTION ECOSYSTEM

The population dynamics of the cereal cyst nematode, Heterodera avenae , and the root lesion nematode, Pratylenchus neglectus, were studied i n three la-year experimental cropping sequences. Rotation treatments c ompared a high frequency of crops of the H, avenae resistant oat cv. P anema with susceptible cereal monoculture. H. avenae densities fluctua ted according to 1) susceptibility of the crop, 2) nematode density-de pendant factors, and 3) degree of fungal parasitism of cysts. H. avena e densities were reduced to almost undetectable levels by frequent cro pping with resistant oats which consequently favoured the root lesion nematode, P, neglectus. Heterodera avenae numbers increased rapidly fo llowing the reintroduction of susceptible cereals into the rotation. T he rate of this increase was proportional to the number of preceding c rops of resistant oats. This increase was associated with a very low d egree of fungal parasitism of the cysts and their contents. The popula tion of H. avenae re-established in this way apparently included some nematodes able to break the resistance of cv. Panema. The observations made in these long term investigations contribute to the development of a strategy for using resistant cultivars to maintain optimally bala nced biological equilibria within cereal agroecosystems.