INDUCTION OF SYSTEMIC RESISTANCE IN CUCUMBER AGAINST FUSARIUM-WILT BY PLANT GROWTH-PROMOTING RHIZOBACTERIA

Plant growth-promoting rhizobacteria (PGPR) strains 89B-27 (Pseudomonas putida) and 90-166 (Serratia marcescens) were tested for their ability to induce systemic resistance against Fusarium wilt, a vascular disease of cucumber, using a split-root assay. PGPR strains and Fasarium oxysporum f. sp. cucumerinum were inoculated on separate halves of roots of cucumber seedlings at the same time and then planted in separate pots. Both PGPR strains induced systemic resistance against F. oxysporum f. sp. cucumerinum as expressed by delayed disease symptom development and reduced number of dead plants after PGPR treatments compared to the nonbacterized, F. oxysporum f. sp. cucumerinum-inoculated controls 5 weeks after inoculation. F. oxysporum f. sp. cucumerinum was recovered from lower stems 2 weeks after root inoculation and from the first, second, and third petioles 5 weeks after inoculation in the nonbacterized control. In contrast, F. oxysporum f. sp. cucumerinum was isolated from stems of plants treated with PGPR only 4 weeks after inoculation and from the first petiole 5 weeks after inoculation, indicating that PGPR treatment reduced spread of the pathogen. Movement of PGPR in cucumber split-root systems was monitored with a bioluminescent derivative of 89B-27, strain L211, that was detected with a charge-coupled device camera. Strain L211 provided protection against F. oxysporum f. sp. cucumerinum at levels similar to the wild-type PGPR strain. L211 colonized cucumber roots up to 5 weeks after root inoculation and was not detected inside stems or petioles. The bacterium showed only limited movement within inoculated pots and did not move to the pots in which the pathogen was inoculated, demonstrating that the PGPR and pathogen remained spatially separated. We conclude that the two PGPR strains induced resistance systemically in cucumber against Fusarium wilt.