INVOLVEMENT OF SOIL ABIOTIC FACTORS IN THE MECHANISMS OF SOIL SUPPRESSIVENESS TO FUSARIUM WILTS

Two soils from Algerian palm groves were chosen for their abiotic characteristics. The soil from Beni-abbes contains: sand 96% and clay 2.5%; the soil from Tolga: clay 37%, silt 44%, sand 19%. When infested with Fusarium oxysporum f.sp. lini the sandy soil appeared conducive and the clay soil suppressive to fusarium wilt. After addition of glucose the kinetics of CO2 released was faster and greater in the suppressive than in the conducive soil. Study of the population dynamics of an introduced population of F. oxysporum fsp. lini showed that the inoculum density decreased faster and to a lower threshold in the conducive than in the suppressive soil. Suppressiveness of this soil was not related to poor survival of the inoculum. Addition of clay, talc or humus to the sandy soil improved the survival of the pathogen, the mixture clay plus humus enabling the survival of 97% of the introduced population. But addition of montmorillonite or talc to this sandy soil resulted in an opposite effect on the kinetics of CO2 release after glucose amendment. The amount of CO2 released was greater after addition of montmorillonite and lower after addition of talc than in the non-amended control. Similarly, talc and montmorillonite had an opposite effect on the level of soil suppressiveness to fusarium wilt. Montmorillonite made the soil more suppressive and talc more conducive than the non-amended control. These results showed that modifications of the texture of a sandy soil by addition of clay may induce changes of microbial activity resulting in modification of several properties of the soils: the level of fungistasis and the suppressiveness of the soil are not independent of the soil texture.