Abstract
Some isolates of nonpathogenic Fusarium oxysporum were isolated from the crowns of healthy strawberry plants. When strawberry plants were inoculated with the nonpathogenic F. oxysporum by dipping the roots into the spore suspension and/or adding the branculture to soils, prior to the inoculation with the pathogen (F. oxysporum f. sp. fragariae), the number of diseased strawberry plants was decreased, indicating that preinucolation of the nonpathogenic F. oxysporum suppressed the disease development of Fusarium wilt of strawberry. An isolate of nonpathogenic F. oxysporum C-8 was selected from the tested 11 isolates, all of which have some suppressive effect against the disease. The isolate C-8 was nonpathogenic to 18 plant species out of 10 families tested. The disease suppressive effect of the isolate C-8 was lost after the heat treatment (100°C, 5min) of the bran-culture. The effect was decreased remarkably, when the bran-culture was kept at room temperature for 2 years. F. oxysporum was frequently isolated from the strawberry plants 2 months after inoculation with the isolate C-8, indicating that the nonpathogenic F. oxysporum can grow in the strawberry plants. The isolate C-8 and the pathogen did not compete each other on PDA medium. It is suggested that the suppression of the disease is due to the growth of C-8 in strawberry plants reducing the infection and/or the growth of the pathogen. An isolate of nonpathogenic F. oxysporum obtained from a healthy tomato plant also had the suppressive effect against Fusarium wilt of strawberry, indicating that nonpathogenic F. oxysporum which is effective against the disease is distributing not only in strawberry, but also in other plant species. The protective effect of the isolate C-8 was demonstrated in the field when it was preinoculated, and the effect was increased when the isolate C-8 was inoculated to strawberry plants in combination with soil fumigation with methyl isothiocyanate (MITC). It is clear that the protective effect of the isolate is very high, when the density of the pathogen is low after soil sterilization with MITC.
