The first 10 rootstocks from the USDA breeding program were evaluated for sting nematode tolerance during the first week of March, approximately 20 weeks after infesting the seedlings with sting nematodes. Compared to unchallenged trees, the fibrous root damage was not readily evident and the stubby root symptoms were markedly less pronounced than in the previous trial. The morphology of all plants were documented photographically and the shoots, structural and fibrous roots were dried and weighed. The average fibrous root mass of the infested as a proportion of the average of the uninfested lines was marginally correlated (r=0.6; P=0.07) with the subjective visual damage rating assigned each seedling and was used to rank the genotypes for nematode tolerance. All USDA hybrid rootstocks comprised trifoliate orange crossed with either pumelo or mandarin parents. The tolerance ratio of the average fibrous root mass (infested/uninfested) for the seven hybrid rootstocks as well as that of Swingle citrumelo (also a trifoliate hybrid) was 0.99, indicating no root reduction. That of the remaining two non-hybrid rootstocks (sweet and sour orange) averaged 0.71 indicating a 29% reduction of fibrous roots in plants exposed to the nematode. It is unclear why the extent of stubby root symptoms caused by sting nematode differed between the two trials done to date. The most likely explanation is that the infestation in the first trial was done by transplanting infested Bermuda grass into the tanks prior to introducing the seedlings. Due to competition between the grass and the citrus in that trial, we changed the methodology in the second trial and infested the seedlings directly following planting. Nematode populations monitored in each trial after three months and at termination were consistently higher in the second trial; nevertheless, the initial population to which plants were exposed was highest in trial 1. This discrepancy should be overcome in future trials because all tanks are now heavily infested. Regardless, the relationships between the tolerance ratio (infested/uninfested fibrous roots) and variables other than nematode damage which might have affected the ratio were the same in both trials. For example, the ratio might have been affected by the inherent size of the different lines. In both trials the smallest lines tended to have greater tolerance ratios (correlations were significant at P<0.05 and p<0.10, respectively). However, when the tolerance ratio was regressed on two variables (inherent size (average weight of uninfested fibrous roots) and actual reduction in fibrous roots (infested - noninfested fibrous roots), the inherent plant size was not significant (P=0.35 and P=0.55), whereas the net reduction of fibrous roots due to nematodes was highly significant in both trials (P<0.001). The two variables explained 78% of the variability of the tolerance ratio in the first trial and 88% in the second. Validation of the results of both experiments to date will be attempted by comparing the most tolerant lines in both trials to widely used conventional rootstocks including sweet and sour oranges, Kuharske citrange, Cleopatra mandarin, Volkamer lemon and C-35. We have also begun trials to evaluate several of the most and least tolerant lines in trial one for sting nematode resistance (inability to reproduce) in addition to tolerance (ability to thrive despite nematode ability to reproduce). These trials require individually potted plants grown in nematode infested and non-infested soil. We will expand these experiments to evaluate promising lines in trial two and conventional rootstock varieties for comparison. The average number of sting nematodes in the surface soil (0-12 in depth) of the field trial remained low, just 6 nematodes per 250 cm3 soil (0-25 nematodes range). Unlike the summertime measurement, there were no treatment effects on the nematode numbers. Similarly, the fibrous root density was 3 times higher in January than the previous July, but there were no effects of treatment on the root weight or the change in root weight.