Mechanism of Biological Control in a Fusarium-Suppressive Soil

Metz fine sandy loam soil from the Salinas Valley in California was suppressive to the Fusarium spp. which induce wilts of flax and carnation. Suppressiveness to Fusarium oxysporum f. sp. dianthi was transferred to conducive soil when the Metz fine sandy loam was added in small amounts to steamed greenhouse soil. Aerated steam treatment of the suppressive soil at 54 C for 30 min eliminated the suppressive effect. Lowering of pH values of the Metz fine sandy loam from 8.0 to 6.0 in unit increments eliminated the suppressive effect. Bacteria were isolated from mycelial mats of F. oxysporum f. sp. lini buried in the suppressive soil and conducive soils. Two isolates from suppressive soil introduced into conducive soil at 10⁵ cells per gram of soil significantly reduced disease incidence of Fusarium wilt of flax. The more effective of these isolates inducing suppressiveness was a Pseudomonas sp. Viability of this organism was drastically reduced when soil was treated with aerated steam at 54 C. These results suggest that suppressiveness in the Metz fine sandy loam is biological in origin and that control of Fusarium wilt diseases may be accomplished through introduction of appropriate species of bacteria into conducive soil.