Abstract
Three species belonging to the genus Fusarium, F. fujikuroi, F. oxysporum and F. solani, were isolated from healthy tissues of rakkyo (Allium chinense), and the ability of these isolates to colonize the roots of rakkyo plants was examined. These species were isolated easily from roots, scales, leaf tissues and rhizosphere soil of healthy rakkyo plants, although the distribution of Fusarium species compared with other microorganisms was not abundant in the cultivated soil. The average isolation rates of F. fujikuroi, F. oxysporum and F. solani from these parts were 72 %, 8.7 %, and 0.2 %, respectively. The isolation rates of F. fujikuroi and F. oxysporum from root tissues in the field was higher in the 2 nd year than in the 1 st year after planting. When rakkyo plants were inoculated with nitrate-nonutilizing mutants (nit mutants) of nonpathogenic F. fujikuroi and F. oxysporum before planting by the drenching method in the pot test, a large number of nit mutants were frequently isolated from the rakkyo roots and basal part of scales at 97 days after inoculation. These results indicate that the nonpathogenic F. fujikuroi and F. oxysporum can survive for a long period of time in the rakkyo plants.
