LINKAGE OF RHM, A RECESSIVE GENE FOR RESISTANCE TO SOUTHERN CORN LEAF-BLIGHT, TO RFLP MARKER LOCI IN MAIZE (ZEA-MAYS) SEEDLINGS

The recessive gene rhm confers chlorotic-lesion resistance to Bipolari s maydis race O, the southern corn leaf blight pathogen, in otherwise susceptible maize plants. Because of inconsistencies encountered in sc oring the disease on mature plants in the field, an assay was develope d to monitor the expression of this gene in maize seedlings under cont rolled conditions. One hundred and two related F3 families from the cr oss RH95rhm X B73 were inoculated with conidia of B. maydis race O, an d the genotype at the rhm locus of each F2 parent was deduced from the reactions observed in the progeny seedlings. The F2 genomes were reco nstituted by extracting DNA from leaf tissue pooled from 30-36 F3 prog eny plants per family. The seedling disease ratings were analyzed toge ther with the segregation scores for 14 single-copy DNA probes. Our re sults indicate that rhm is tightly linked to two restriction fragment length polymorphism (RFLP) marker loci (UMC85 and p144) that map to th e short arm of chromosome 6. In addition, a rapid assay based on the p olymerase chain reaction was used to confirm the linkage between rhm a nd the p144 RFLP marker locus in a second unrelated F2 population.