Use of Nitrate Nonutilizing Mutants in Ecological Studies of Fusarium Diseases. III. Growth, Benomyl Sensitivity, Pathogenicity, and Stability of nit Mutants of Fusarium oxysporum Compared to Wild-type Strains.

Abstract

Properties of nitrate nonutilizing (nit) mutants of Fusarium oxysporum were compared to those of wild-type strains using F. oxysporum f. sp. raphani, f. sp. spinaciae, and a nonpathogenic strain of F. oxysporum (S-52). Most mutants showed the same growth as the wild-type strain on potato sucrose agar (PSA) medium, though some mutants from F. oxysporum f. sp. spinaciae showed slower growth. No difference was observed in the proliferation in liquid medium or sterilized soil between nit mutants and the wild-type strain for the strains tested. All of the 24 mutants from benomyl-sensitive F. oxysporum f. sp. raphani were also benomyl-sensitive. Most of the 43 mutants from f. sp. raphani and f. sp. spinaciae retained pathogenicity to their respective hosts, although two mutants from f. sp. spinaciae had clearly weakened pathogenicity. Among the total of 195 mutants from 10 formae speciales, most mutants retained their phenotypes at their formation even after three-year preservation by subculture on slanted PSA at room temperature, while 21 mutants (10.8%) changed their phenotypes and recovered the ability to utilize nitrate. When nit mutants of f. sp. raphani were inoculated into nursery soil and reisolated three years after inoculation by soil dilution plating, all of the reisolated F. oxysporum were still nit mutants and no phenotypic change was observed. Population densities of the mutants in the soil were at almost the same level as for the wild-type strain. When mutants of f. sp. spinaciae were inoculated to spinach, reisolated, and tested for their phenotypes, all of the mutants retained their original phenotypes. Thus, although some nit mutants had weakened pathogenicity or were unstable, most nit mutants had the same properties as wild-type strain and were stable over long periods of time. It is thus assumed that nit mutants can be used as markers in ecological studies of F. oxysporum.