Abstract
Puccifeia Menthae PERS. (a causal fungus of mint rust) which is distributed in Okayama district, infects M. arvelcsis var. piperascens (Japanese mint), and M. arvensis var. canade, vsis which is indige-nous to North America and phylogenetically near to Japanese mint, and brings about a large damage to them. Besides, the fungus also infects M. japoleica which is native of Japan, and gives it a slight injury. The mint rust, however, did not infect European arvensis and dther species of mint, not only by natural inoculation in the field but also by artificial inoculatian in the laboratory. Both tetra-ploid and triploid clones of Japanese mint were as susceptible to rust as the respective original clones of diploid Japanese mint.So it seemed to be difficult to expect a rust-resistant Japanese mint by the doubling of chromosome number, because response to rust did not change by this method. However, one tetraploid mutant was found to be extraordinarily resistant to rust and this characteristic was domi-nantly inherited. The F1 between Japanese mint and M. japonica was as susceptible as Japanese mint, and the F1’s between Japanese mint and other mint species which ;tre themselves immune to mint rust, suffered a slight injury or none from mint rust. That is, in the latter case, resistance of mint to rust is of complete or approximately complete dominance in heredity.
