Evaluation of Pseudomonas fluorescens PICF7 as potential biocontrol agent against Plasmopara halstedii in sunflower

One of the most important biotic constraints for sunflower production is the soil-borne, obligate-parasitic oomycete Plasmopara halstedii Farl. Berl. & de Toni, which causes sunflower downy mildew (SDM). Control of SDM by means of biocontrol agents (BCAs) has been poorly investigated so far. Our previous studies have demonstrated that Pseudomonas fluorescens PICF7 is an effective BCA against Verticillium wilt of olive, a disease caused by the soil-borne pathogenic fungus Verticillium dahliae Kleb. Moreover, strain PICF7 was confirmed as endophyte in olive roots. Recently, colonization and persistence in Arabidopsis thaliana roots of strain PICF7 and one spontaneous rifampicin-resistant mutant derivative (PICF7Rf) were assessed, thus confirming the ability of this strain to colonize different plant species. Considering these antecedents, we aimed in this study to determine whether P. fluorescens PICF7: (1) colonize and persist in sunflower roots; (2) promote growth of sunflower at early stages; and (3) control SDM. Hybrid seeds genetically susceptible to SDM were dipped in bacterial suspensions of PICF7 (aprox. 1 x 109 cfu/mL) or PICF7Rf, (aprox. 1 x 109 cfu/mL) or in 10 mM MgSO4·7H2O (control treatment) at 23ºC, 150 rpm for 5h. Seeds of each treatment were germinated and subsequently immersed in