Suppression of Verticillium wilt in olive planting stocks by root-associated fluorescent Pseudomonas spp.

Protection of pathogen-free olive planting material from infection by Verticillium dahliae during plant propagation and/or at planting would help in the management of Verticillium wilt of olive. In this study, 8 isolates of Pseudomonas fluorescens (6) and Pseudomonas putida (2) obtained from roots of olive plants were tested for suppression of Verticillium wilt in nursery-produced olive planting stocks under controlled conditions. All tested bacteria produced the green fluorescent siderophore pseudobactin in vitro but only some P. fluorescens isolates produced either salicylic acid in succinate medium or HCN in vitro assays. The antagonistic activity of P. fluorescens and P. putida isolates from olive against defoliating (D) and nondefoliating (ND) V. dahliae pathotypes varied with culture media. On PDA, isolates of P. putida were more inhibitory to the pathogen than those of P. fluorescens. In planta bioassays were conducted either under growth chamber or greenhouse conditions, by inoculating bacterial-treated and -nontreated 3- to 4-month-old, own-rooted or micropropagated plants of susceptible olive cv. Picual with the highly virulent D V. dahliae. Results from three experiments indicated that root treatment with some of P. fluorescens isolates significantly delayed the onset of symptoms, and reduced the final disease incidence and severity by 31–82% and 73–96%, respectively, compared with the nontreated controls, under conditions of severe Verticillium wilt. In addition, those bacteria counteracted the deleterious effects caused by the pathogen infection through enhancement of plant growth. Our results indicate that root treatment of olive plants with selected P. fluorescens isolates during nursery propagation can help in the biocontrol of D V. dahliae in olive. No correlation was found between efficacy of tested bacterial isolates for in vitro antagonism of the pathogen and in planta suppression of Verticillium wilt.