A novel source of resistance to the two-spotted spider mite in Lycopersicon pimpinellifolium (Jusl.) Mill.:: its genetics as affected by interplot interference

We have previously found an accession of Lycopersicon pimpinellifolium (Jusl.) Mill. ('TO-937') that appeared to resist attack by the two-spotted spider mite ( Tetranychus urticae Koch). L. pimpinellifolium is a very close relative of the cultivated tomato (Lycopersicon esculentum Mill.) and thereby a potential source of desirable traits that could be introgressed to the crop species. The objective of this study was to investigate the genetics of the resistance present in 'TO-937'. Resistance to infestation by the spider mite was quantified in 24-plant plots of L. pimpinellifolium accessions 'TO-937' and 'PE-10', L. pennellii accession 'PE-45', L. esculentum cultivars 'Moneymaker', 'Roma' and 'Kalohi' (reported to be partially resistant: Stoner & Stringfellow, 1967), and the interspecific F-1 cross, L. esculentum 'Moneymaker' x L. pimpinellifolium 'TO-937'. Only 'TO-937', the F-1, and 'PE-45' were found to be resistant. Resistance of 'TO-937' was complete when evaluated in two small greenhouses completely planted with 'TO-937' so as to simulate the genotypic homogeneity usual in commercial crops. Generations (P-1, P-2, F-1, F-2, BC1P1, and BC1P2) of a P-1 (susceptible) x P-2 (resistant) cross ('Moneymaker' x 'TO-937') were studied for resistance in a single-plant per plot design. Resistance of 'TO-937' was inherited with complete dominance and appeared to be controlled by either two or four genes according to whether segregation in the F-2 or the BC1P1, respectively, were considered. However, calculation of the number of genes involved in the resistance was complicated by negative interplot interference due to the high frequency of resistant genotypes within most of the generations.