MECHANISMS INVOLVED IN BIOLOGICAL-CONTROL OF FUSARIUM-WILT OF CUCUMBER WITH STRAINS OF NONPATHOGENIC FUSARIUM-OXYSPORUM

Two nonpathogenic strains of Fusarium oxysporum (C5 and C14) introduced into soil at 1.08 X 10(4) and 5 X 10(4) colony-forming units (cfu) per gram of soil significantly decreased the slope values of inoculum density-disease incidence curves generated for Fusarium wilt of cucumber induced by F. oxysporum f. sp. cucumerinum. Strain C14 reduced disease significantly more than C5. A three-phase mechanism was associated with biological control of Fusarium wilt by these agent. There was a significant reduction in germination of chlamydospores of F. o. cucumerinum in the rhizopheres of cucumbers infested with C5 or C14. Competition for infection sites with F.o. cucumerinum was demonstrated in soil infested with C14 but not C5. Enhanced systemic resistance of the host to inoculation with F. o. cucumerinum or Colletotrichum lagenarium occurred when wounded or nonwounded roots wer exposed to C14. Strain C5 significantly reduced disease induced by F. o. cucumerinum in similar experiments only if roots were wounded when the agents were introduced. When roots were exposed to either C5 or C14 and microconidia of F.o. cucumerinum applied to transversely severed stems of cucumber, significant reduction in germination of the pathogen occurred 72-96 hr after introduction of the biocontrol agents; however, wounding of roots was necessary for C5 to induce the response. Therefore, mechanisms of suppression of Fusarium wilt of cucumber by these nonpathogenic agents involved competition in the rhizosphere and infection sites, as well as induction of enhanced resistance in the host. Apparently, the ability of C14 to compete for infection sites, penetrate through intact root tissue, colonize the host, and induce resistance reactions in the host were attributes that contributed to its superior performance in biological control in comparison with C5.