INDUCTION OF SYSTEMIC RESISTANCE BY PSEUDOMONAS-FLUORESCENS IN RADISHCULTIVARS DIFFERING IN SUSCEPTIBILITY TO FUSARIUM-WILT, USING A NOVELBIOASSAY

Pseudomonas fluorescens-mediated induction of systemic resistance in r adish against fusarium wilt (Fusarium oxysporum f. sp. raphani) was st udied in a newly developed bioassay using a rockwool system. In this b ioassay the pathogen and bacterium were confirmed to be confined to sp atially separate locations on the plant root, throughout the experimen t. Pathogen inoculum obtained by mixing peat with microconidia and sub sequent incubation for four days at 22 degrees C, yielded a better per centage of diseased plants than a microconidial suspension drench, an injection of a microconidial suspension into the hypocotyl, or a talcu m inoculum. Pseudomonas fluorescens strain WCS374 applied in talcum or peat, but not as a suspension drench, induced systemic resistance. A minimal initial bacterial inoculum density of greater than or equal to 10(5) CFU WCS374 root-(1) was required to significantly reduce the pe rcentage diseased plants. At least one day was necessary between bacte rization of strain WCS374 in talcum on the root tips and inoculation o f the pathogen in peat on the root base, for an optimal induction of s ystemic resistance. Strain WCS374 induced systemic resistance in six r adish cultivars differing in their susceptibility to F. oxysporum f. s p. raphani. Significant suppression of disease by bacterial treatments was generally observed when disease incidence in the control treatmen t, depending on pathogen inoculum density, ranged between approximatel y 40 to 80%. Strains WCS374 and WCS417 of Pseudomonas fluorescens indu ced systemic resistance against fusarium wilt, whereas P. putida WCS35 8 did not. This suggests that the induction of systemic resistance by Pseudomonas spp. is dependent on strain-specific traits.