Paramètres influençant la lutte biologique contre la fusariose vasculaire de la tomate et la pourriture noire des racines de concombre

The purpose of this study was to optimise the use of antagonistic microorganisms to control Fusarium wilt of tomato caused by F. oxysporum f. sp. lycopersici and black root rot of cucumber caused by Phomopsis scierotioides. Nonpathogenic F. oxysporum strain Fo47 and Pseudomonasfluorescens strains CHAOO, Pf36, and Pfl53 were used. Additional bacterial strains were isolated from différent soils. About 10% of thèse strains protected cucumber or tomato plants in pot trials. There was no corrélation between in vitro antibiosis and disease protection. Strain Fo47 protected tomato against Fusarium wilt in four différent biotests without being in direct contact with the pathogen and increased chitinase and f3-l,3glucanase activities in the plants, suggesting that it induced host résistance. When added before the pathogen, Fo47 protected tomato plants against Fusarium wilt in natural soil and in rockwool. A ten-fold higher dosage of Fo47, however, was needed for protection in natural soil as compared to rockwool. In rockwool, the iron chelator used in the nutrient solution influenced protection with Fo47; a high level of protection was obtained using Fe-EDDHA, while no protection was observed when Fe-EDTA was used. In natural soil, CHAOO introduced at transplanting with the pathogen enhanced protection by strain Fo47 introduced at seeding but not when the agents were applied simultaneously at seeding time. Protection in rockwool was not enhanced indicating enhancement may be due to suppression of minor pathogens présent in natural soil but absent in rockwool. Strain Fo47 protected the plants and increased tomato yield in an expérimental greenhouse but was less effective in a commercial greenhouse naturally infested with F. oxysporum f. sp. lycopersici and Meloidogyne sp. The combination of P. fluorescens strain Pf36 with Fo47 reduced variation between replications as compared to Fo47 alone, and increased protection and yield in the commercial greenhouse. P. fluorescens Pf153 and four other strains protected cucumber against black root rot in three différent soils. Protection by Pf153 was significantly improved when the inoculant was produced in nutrient-poor médium. There was no effect on root colonization. Increased protection occured only in natural soil with a résident microflora. In continuous cucumber culture, individual soil application of Pfl53 and four other strains at the beginning of the experiment resulted in better protection of the second crop as compared to the first or third. Soil enrichment with Fe-EDDHA increased protection of the first crop by Pf153 more than the second. Effective rhizosphere colonization by Pf153 was passive with irrigation water but not active. In the commercial greenhouse naturally infested with P. sclerotioides, five bacterial strains added at or after transplanting did not significantiy influence cucumber yield. Two strains, CC2c and TMl'b2, however, did significantiy reduce incidence of root disease. Disease control obtained under commercial condition and in pot trials using Fo47 and bacteria strains demonstrated the potential for practical application of biological control of soilborne pathogens.