Application methods and modes of action of Pantoea agglomerans and Paenibacillus sp. to control the grapevine trunk disease-pathogen, Neofusicoccum parvum

Despite an increasing number of studies being carried out on the biocontrol of grapevine trunk diseases (GTDs), no commercial bacterial products have yet been developed to control GTDs. Knowledge of the precise modes of action (MOA) and the different application methods (AM) for biocontrol agents is crucial if they are to be successful in the field. In light of this, the present study aimed at selecting the most appropriate AM for eight bacterial strains with high potential for controlling Neofusicoccum parvum. These strains were applied on one-year-old grapevine in pots (grown from cuttings) using three methods: co-inoculation at stem level, preventive inoculation at stem level and preventive inoculation at the soil surface. The inhibitory activity of the bacterial strain against N. parvum was significantly dependent on the AM. Application of bacterial strains to stems, especially in a preventive way, was much more efficient than inoculation in the soil. When performing preventive inoculation on stems, the inhibition of N. parvumwood necrosis reached 50 and 65 % for Pantoea agglomerans (S1) and Paenibacillus sp. (S19) respectively. To decipher the underlying processes linked to fungal inhibition, the way in which several MOA affected the antagonistic capacity of these two strains was studied via in vitro and in planta assays. While P. agglomerans (S1) inhibited N. parvum by the secretion of antifungal volatile compounds, Paenibacillus sp. (S19) mainly inhibited this pathogen by antibiosis. In addition, both bacterial strains induced systemic defenses in grapevine. However, this affect tended to be higher at 15-dpi after inoculation with P. agglomerans (S1) than after innoculation with Paenibacillus sp. (S19) (three defense genes repressed versus five respectively). Finally, P. agglomerans (S1) and Paenibacillus sp. (S19) were shown to be potential biocontrol candidates for fighting N. parvum in grapevine, due to the combination of direct control via their antifungal activity and indirect control via their ability to activate the grapevine defense system.