1986 Volume 52 Issue 1 Pages 15-21
No antibiosis was observed in confronting plate culture between nonpathogenic Fusarium oxysporum, cross-protection agent, and F. oxysporum f. sp. batatas, pathogen of Fusarium wilt of sweet potato. Only pre-inoculation with live bud-cells of the agent brought about cross-protection. Neither paraffin plugging of the cut-ends of sweet potato sprouts nor dipping the cut-ends into heat- or ethanol-killed bud-cells of the agent caused any decrease of the disease incidence. Both natural infection and artificial inoculation with the pathogen at the distant portion on the sprout from the pre-inoculated cut-ends, brought about decrease of the disease incidence, suggesting induction of systemic resistance. Simultaneous inoculation with both the pathogen and agent also caused cross-protection. The agent previously inoculated existed at the cut-ends but little growth upwards through the vessels was observed. It multiplied locally in the cortex and parenchyma tissues at the cut-ends caused local le-sions. Excision of the cut-ends immediately after pre-inoculation with the agent nullified the cross-protective effect but excision at two days after the treatment did not. The filtrate of germinated bud-cell suspension of the agent brought about little decrease of the disease incidence. Thus, induction of systemic resistance in this cross-protection phenomenon is assumed to be caused by a slight and local infection with the agent previously inoculated at the cut-ends.