Soil suppressiveness by organic amendment to Fusarium disease in cucumber: effect on pathogen and host

Soil suppressiveness to Fusarium wilt and crown rot disease, caused by Fusarium oxysporum f. sp. radicis-cucumerinum in cucumber, develops following the incorporation of wild rocket (Diplotaxis tenuifolia (L.) DC.) crop residues, manifested as reduction in disease incidence. We studied mechanisms that might be involved in disease suppression. Exposure of nonamended-nonsuppressive soil to volatile organic compounds generated from wild rocket decomposing in the soil induced soil suppressiveness to Fusarium disease in nonamended soil. Similarly, a crude aqueous soil extract from wild rocket-amended (suppressive) soil induced suppressiveness to Fusarium disease when cucumber seedlings were transplanted into nonsuppressive soil amended with this soil extract. Suppressive soils did not show any effect on F. oxysporum f. sp. radicis-cucumerinum macroconidial germination, mycelium growth or new chlamydospore production. Root colonization with Fusarium spp. was suppressed in inoculated plants after 6 days, but not after 3 days. No evidence for induced resistance was found when plants were first grown in suppressive soil followed by inoculation and transplanting in nonsuppressive soil, or when plants were inoculated with Botrytis cinerea. Findings suggest that the microorganisms perpetuate the mechanisms of suppressiveness following the introduction of organic amendments, and that this occurs in the root zone.