A GENE FOR RESISTANCE TO THE SOUTHERN ROOT-KNOT NEMATODE IN SOYBEAN

The inheritance of resistance in soybean [Glycine max (L.) Merrill] to the southern root-knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood] was determined for a cross of Bossier (susceptible) X Forrest (resistant). The F-1, F-2, and F-3 generations were screened in a greenhouse for resistance to gall formation by counting individual galls on the roots of plants inoculated with nematode eggs. The mean number of galls on F-1 plants derived from reciprocal crosses of Bossier and Forrest were similar to each other and to the midparent mean, indicating that inheritance was not maternal and that neither resistance nor susceptibility was dominant. F-2 plants which had fewer galls than the upper limit of the 95% confidence interval (CI) about the mean of Forrest or more galls than the lower limit of the 95% CI about the mean of Bossier were classified as resistant or susceptible, respectively. If an F-2 plant had a gall number between the confidence intervals of Forrest and Bossier, it was classified as intermediate. These classifications were verified by family gall means and within-family variances of F-3 progeny harvested from each F-2 plant. Both the F-2 and F-3 populations segregated into distinct resistant, intermediate, and susceptible classes in a ratio of 1:2:1. In addition, low within-family variances indicated that F-3 families in the resistant and susceptible classes were derived from homozygous F-2 plants. High within-family variances for F-3 families in the intermediate class indicated they were derived from heterozygous F-2 plants. These data indicate that Forrest contains a single additive gene for resistance to galling, designated Rmi1.