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Ninety accessions of non-tuber bearing Solanaceae were screened for (i) resistance to and (ii) stimulatory effect on juvenile hatch of potato cyst nematodes, and (iii) their growth under temperate climatic conditions. All plant species belonging to the genus Solanum tested induced hatching but this effect was most pronounced for plant species of the Solanum nigrum complex. Hatching ofjuveniles was hardly or not stimulated by other plant genera of the Solanaceae. Solanum sisymbrilfolium combined a high hatching effect with complete resistance to both Globodera rostochiensis and G pallida. Two S. nigrum varieties showed full resistance to G rostochiensis and a high level of resistance to G pallida. Moreover, S. sisymbriifolium and the two varieties of S. nigrum performed very well under Dutch field conditions and, therefore, they are suggested as candidate trap crops for the control of potato cyst nematodes.

In 1997 and 1998 the stimulation of hatch of potato cyst nematodes (PCN) by a trap crop was studied at various times during the growing season in a container and a field experiment. Solanurn nigrum '90-4750-188' was used as the trap crop in both experiments and was sown on 1 May, 16 June or 1 August in two successive years on different plots. Neither experiment revealed much seasonal variation in hatchability of PCN juveniles under a trap crop. In the container experiment, the hatch of the Globodera pallida Pa3 population was equally and strongly stimulated (89%) at all sowing dates in both years, except for the 1 August sowing in 1998 (when the hatch was 77% under extremely wet soil conditions). In the control treatment with non-hosts (flax followed by barley) the total spontaneous hatch was 50% over 2 yr. In the field experiment, the hatch of PCN, averaged over the four populations, was also equally stimulated (71%) at all sowing dates in both years. In the control treatment with non-hosts (flax-barley) the total spontaneous hatch was 36% over 2 yr. Total hatch under the trap crop over 2 yr varied between the four PCN populations from 63% to 80%.In 1998 and 1999, control of potato cyst nematodes (PCN) by the potential trap crops Solanum sisymbriifolium and S. nigrum '90-4750-188' was studied in the field. Potato was also included as a trap crop. In the 1998 experiment, potato, S. sisymbriifolium and S. nigmm strongly stimulated the hatch of PCN compared with the non-host white mustard (Sinapis alba). Roots of potato and white mustard were mainly found in the top 10 cm of soil, whereas roots of S. sisymbrigolium and S. nigrum were also abundant at depths of 10-20 cm and 20-30 cm. In the 1999 experiment, soil infestation with PCN decreased markedly with potato and S. sisymbriifolium as trap crops. In plots moderately to severely infested with 2yr old cysts (2-29 juveniles r n -' air dried soil), potato reduced soil infestation by 87% and S. sisymbriifolium by 77%. In plots moderately to severely infested with 1-yr old cysts the reductions were 74% and 60%, respectively. The reduction was least on plots very severely infested with PCN (110-242 juveniles ml-' soil): 69% and 52% for potato and S. sisymbriifolium, respectively. Soil infestations of plots that were initially slightly to severely infested with the root-knot nematode Meloidogyne hapla were greatly reduced under fallow and S. sisymbriifolium but increased under potato. From these and previous experiments it was concluded that, for several reasons, S. sisymbriifolium is a promising trap crop.

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A field experiment in which main-crop potatoes were grown every other year was conducted on a sandy soil from 1994 to 1999. The aim of the experiment was to control soil-borne pathogens of potato with ecologically sound methods. Potato grown as a trap crop from the end of April to the end of June (8 wk) was used to control potato cyst nematodes (PCN) (Globodera pallida), and its effects on other important soil pathogens and on the growth of a subsequent potato crop were also assessed. Additional experimental treatments were a potato crop from which the haulm was removed and a green manure crop. Three potato cultivars with different degrees of resistance to PCN were grown as the main crop. Duplicate sets of the experiment were run concurrently.The PCN were effectively controlled by the potato trap crop. When a highly resistant potato cultivar was grown as a main crop after the trap crop, the post-harvest soil infestation was very low. When a moderately resistant cultivar was grown after the trap crop the soil infestation also remained low. When the trap crop was alternated with a susceptible potato cultivar as a main crop, soil infestation increased slightly, but the degree of control when compared with no trap crop averaged 96%.Soil infestation with root-knot nematodes (mainly Meloidogyne hapla) increased when potato was grown as a trap crop, but soil infestation with the root-lesion nematode Pratylenchus crenatus was not affected. Stem canker caused by Rhizoctonia solani was not affected by the trap crop but black scurf (sclerotia of R. solani) on tubers was reduced. Soil infestation with Verticillium dahliae declined in one of the duplicate sets of the experiment but not in the other. However, stem infections by ? ! dahliae were significantly decreased in both sets, although the effect depended on the PCN-resistance level of the potato cultivar. When a highly resistant potato cultivar was grown Verticillium stem infections were not significantly affected, they were decreased with a moderately resistant cultivar but the decrease was most pronounced with a PCN-susceptible cultivar.Senescence of a following potato crop was not influenced by the trap crop when a highly PCNresistant cultivar was grown, but it was delayed in the case of a moderately resistant or a susceptible cultivar, resulting in higher tuber yields for those cultivars.The experiment proved that a trap crop can be an alternative to chemical soil disinfection but, for several reasons, the potato itself is not an ideal crop for this purpose; a trap crop other than potato must be developed.

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