1Iwate Prefecture Agricultural Research Center, Kitakami, Iwate O2-OOO3, Japan; 2Iwate Prefecture Government Office, Japan

Background and objectives
In Iwate, Japan, spinach was produced in the same vinyl house over a year, demonstrating injury by continuous cropping. Fusarium wilt of spinach, caused by F. oxysporum f.sp. spinaciae (FOS), is an economically important disease. Because the protective effect of soil sterilization is not continuous, a stable control method is required. In this study, control of fusarium wilt of spinach by transplanting seedlings pretreated with non-pathogenic F. oxysporum was assessed in a field and the cross-protective ability of non-pathogenic F. oxysporum against other soilborne diseases was studied.

Materials and methods
Isolate: non-pathogenic F. oxysporum S3H03 isolate (NPF). NPF was isolated from the root surface of healthy spinach growing in a field infested with fusarium wilt, cross-protected against FOS, when the soil was amended with the isolate before infestation with the pathogen [1].
Combination of NPF and transplanting on spinach: the bud-cell suspension of NPF was amended in nursery soil before sowing. Then inoculum density was 106 cfu dry soil/g. Spinach seedlings were grown 15 days before transplanting to a naturally infested field. The protective effect was compared with transplanting culture with no treated seedlings and direct sowing culture with solar heating sterilization on 13th and 18th days alter transplanting.
Assessment of cross-protective ability against other soilborne diseases: The ability of cross-protection was assessed against diseases as follows: damping-off of spinach, fusarium wilts of cucumber, melon, watermelon, radish and garland chrysanthemum.

Results and condusions
This cross-protective ability of NPF against FOS did not continue through the harvesting. In order to extend the duration of disease suppression, pre-treatment of spinach seedlings with NPF was combined with subsequent transplanting to reduce disease development. Disease suppression then lasted through harvesting when pre-treated seedlings were transplanted in infested soil. In a naturally infested field, the protective effect of this method was as effective as solar heating sterilization in a dosed vinyl house in summer. These results suggest that control of fusarium wilt of spinach by transplanting spinach pre-inoculated with NPF should be practical. The cross-protective effect of NPF was shown against not only fusarium wilt but also damping-off of spinach when the naturally infested soil was drenched with a celI suspension of NPF. Furthermore, this NPF was able to cross-protect against other pathogenic fusaria of cucumber, melon, watermelon, radish and garland chrysanthemum when the soil was amended with NPF before infestation with respective pathogen. Hence the authors hope that this isolate will be an effective biological control agent against diverse soilborne diseases.

1. Katsube K, et al., 1994. Mn. Rept. Plant Prot. North Japan 45, 72-75.