REACTION OF WHEAT TO COMMON ROOT ROT: IDENTIFICATION OF A MAJOR GENE, Crr, ON CHROMOSOME 5B

Triticum aestivum L. cv. S-615 has a dominant gene for susceptibility to common root rot and cv. Apex a recessive gene for resistance on chromosome 5B. One hundred and twenty-two lines, each homozygous for a different random crossover between loci on Apex and S-615 chromosomes 5B, but near-isogenic with S-615 for the other 20 chromosomes, were tested for root rot reaction in soil naturally infested with the causal organism, Cochliobolus sativus (Ito and Kurib.) Dachs. ex Dastur, to determine whether the two genes are alleles. The ratio of resistant to susceptible lines was 1:1, and there were no intermediate or transgressive types. It was concluded, therefore, that the genes for resistance and susceptibility are alleles. The recessive gene for resistance has been given the symbol Crr for Cochliobolus root rot. Minor genes appear to exist on chromosome 5B of Apex which, when combined in some of the crossover lines with the dominant gene for susceptibility, crr, increase the percentage of seedlings killed by the disease but not the mean root rot rating of these lines