INFLUENCE OF A NEMATODE-RESISTANT TRAP CROP RADISH ON SOIL POPULATION DYNAMICS OF HETERODERA SCHACHTII IN SUGAR BEET FIELDS IN THE UNITED STATES FA GRAY and DW KOCH Department of Plant Sciences, University of Wyoming, Laramie, WY, 82071-3354, USA Background and objectives Cultivars of sugar beet nematode-resistant radish, Raphanus sativus L., and mustard, Sinapis alba L., have been developed in Germany in response to the removal of nematicides, due to ground water contamination. These German-bred trap crops are being used to control the sugar beet nematode (SBN), Heterodera schachtii in several European countries. Trap crops are planted following harvest of a cereal crop and grown until frost-down. Root exudates from these densely-planted crops stimulate juveniles within cysts to hatch and emerge. Although juveniles migrate to and infect roots, reproduction does not occur [1]. Trapped juveniles, as well as those still migrating in search of roots, die of starvation and exposure over the winter months, reducing soil populations of the SBN [2]. The objective of this study was to determine the effect of growing degree days (GDDs), base 4.4 C on growth of SBN-resistant radish, and its effect on soil population dynamics of the SBN in Wyoming. Materials and methods SBN-resistant radish (cvs. Adagio and Pegletta) were either seeded in replicated plots or left unseeded within fields naturally infested with the SBN following harvest of either malt barley (Hordeum vulgare L.), silage corn (Zea mays L.), or dry beans (Phaseolus vulgaris L.). Plots were seeded to sugar beet the following year. Soil nematode samples were collected at radish planting and at plow down, as well as at sugar beet planting, mid-season and at harvest and number of cysts and eggs determined. Radish growth, accumulated GDDs and reproductive indices of the SBN were determined. Results and conclusions Radish growth varied from 101 to 2518 kg ha-1 and was closely related to planting date and GDDS, which was influenced by harvest date of the main crop. GDDs following radish planting varied from 496 to 1758. Radish growth was reduced by uncontrolled volunteer barley at one site and by lack of irrigation water at another site following dry beans. Of the three rotation crops preceding radish, maximum radish growth was obtained following malting barley. Initial soil population varied from 1.5 to 24.1 SBN eggs/cm3, while reproductive indices ranged from 0.25 following barley to 0.96 following silage corn. In the barley-sugar beet rotation, soil populations of the SBN were reduced in both trap crop radish and fallow treatments. Greater reduction in SBN numbers occurred when initial populations were high. Following sugar beet planting, soil populations showed a steady increase until harvest. Soil populations in trap crop plots increased similarly or at a slower rate than in unseeded plots. Two of the test sites showed a significant reduction in SBN numbers due to radish at sugar beet harvest. Populations of the SBN were relatively low (<5 eggs/cm3) at both sites following silage corn and populations appeared to be unaffected by trap crop planting. Populations at both sites had increased by sugar beet harvest, however, differences were not significant. Following dry beans, soil populations were relatively high (>12 eggs/cm3). At one site, populations declined following radish planting and increased slightly the following year when sugar beets were harvested. At the other site, where irrigation water after radish seeding was unavailable, soil populations remained static during the radish crop and showed only a slight increase at beet harvest the following year. Results indicate that planting SBN-resistant radish after malting barley harvest provides sufficient GDDs for good radish growth resulting in a trend toward greater reduction of soil populations of the SBN than with the irrigated fallow check. Soil populations of the SBN during the sugar beet crop increased at a much slower rate in radish plots, as compared to fallow plots. References 1. Gardner J, Caswell-Chen EP. 1993. J. of Nematology 25:695-702. 2. Steudel W, Schlang J, MO]Ier J, 1989. Nachrichtenblatt fur den Deutschen pfianzenschtzdienst 41:199-203.