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 Wh ite) Chitwood] was determined for a cross of Bossier (susceptible) X F orrest (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 n umber of galls on F-1 plants derived from reciprocal crosses of Bossie r and Forrest were similar to each other and to the midparent mean, in dicating that inheritance was not maternal and that neither resistance nor susceptibility was dominant. F-2 plants which had fewer galls tha n the upper limit of the 95% confidence interval (CI) about the mean o f Forrest or more galls than the lower limit of the 95% CI about the m ean of Bossier were classified as resistant or susceptible, respective ly. If an F-2 plant had a gall number between the confidence intervals of Forrest and Bossier, it was classified as intermediate. These clas sifications were verified by family gall means and within-family varia nces of F-3 progeny harvested from each F-2 plant. Both the F-2 and F- 3 populations segregated into distinct resistant, intermediate, and su sceptible 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 v ariances for F-3 families in the intermediate class indicated they wer e derived from heterozygous F-2 plants. These data indicate that Forre st contains a single additive gene for resistance to galling, designat ed Rmi1.