Biological control of Verticillium wilt of olive by Paenibacillus alvei, strain K165

In the present study, the efficiency of the biocontrol agent Paenibacillus alvei (strain K165) to suppress Verticillium wilt of olive tree was evaluated in greenhouse and field experiments. In planta bioassays were conducted under greenhouse conditions and revealed that K165 significantly decreased symptoms on the susceptible cultivar ‘Amfissis’ by 44.5 and 51.6 % of the final disease severity index and relative area under disease progress curve (AUDPC), respectively. Thereafter, the suppressive effect of K165 against Verticillium dahliae was studied for two consecutive years (2007 and 2008) in a newly established olive orchard of the susceptible cv Amfissis and the resistant cv Kalamon, naturally infested with V. dahliae. The evaluation of K165 was carried out by recording symptoms, isolations and qPCR quantification of the pathogen in olive tissues. In both years, ‘Amfissis’ trees treated with K165 showed significantly lower final disease severity and relative AUDPC values compared to the non treated controls, whereas, in 2008 decreased symptom severity was associated with significantly lower V. dahliae DNA levels in plant tissues, indicating the suppressive effect of the biocontrol agent. However, no significant suppression was observed in ‘Kalamon’. Pathogen isolations along with qPCR quantification revealed a seasonal fluctuation of V. dahliae biomass in olive tissues with higher amounts occurring in May, and lower amounts in February, August and November. This is the first report of biological control of Verticillium wilt of olive tree under field conditions, associated with reduced pathogen levels inside the xylem tissues.