Chromosome 5B is the most important chromosome differentiating the root rot resistance of the hollow-stemmed cultivars, Apex and Cadet, from the root rot susceptibility of the solid-stemmed cultivars, S-615 and Rescue. A pair of chromosomes 5B of Apex substituted for their homologues in S-615 usually made the latter as resistant to root rot as Apex. Conversely, the substitution of 5B from Rescue into Cadet made Cadet as susceptible as Rescue. The root rot reactions of the 5B monosomics of the cultivars and substitution lines, and of the F1 between S-615 and the S-615-Apex 5B substitution, showed that the genes for resistance and susceptibility are antimorphic alleles, with susceptibility dominant.The substitution of chromosome 2B or 2D from Apex for their homologues in S-615 increased the root rot resistance of S-615, but not to the same extent as did the substitution of chromosome 5B. Their effect, although usually hypostatic to that of 5B, was evident under some test conditions. Dosage studies showed that genes on chromosomes 2B and 2D promote the disease, those from Apex less than those from S-615. Substitutions of these chromosomes from Rescue into Cadet did not affect root rot reaction.