Assessment of the in vitro, in planta and in orchard efficacy of three plant extracts against Erwinia amylovora, responsible agent of fire blight disease in Tunisia

Fire blight, caused by Erwinia amylovora, is a serious disease affecting pear production in Tunisia and worldwide. The present investigation was explored in search of an alternative solution to the excessive use of cupric products. Antibacterial activity of aqueous extracts originating from three medicinal plants (PAEs) (Salvia rosmarinus, Thymus vulgaris, and Salvia officinalis), was evaluated against three E. amylovora strains (Ea2ON165028, Ea6ON165029, and Ea8ON165030) using a well diffusion assay. The three aqueous extracts inhibits the growth of E. amylovora by up to 60% compared to the control plates. Rosemary extract generated a total inhibition of growth (100%). In planta assays, two aqueous extracts of S. officinalis and S. rosmarinus were tested at 20% concentration. The highest effectiveness was achieved with rosemary extract (67.69%) against Ea2, and sage extracts (62.1%) against Ea8, as compared to inoculated and untreated controls. Besides, the antimicrobial effect of two aqueous extracts (T. vulgaris and S. rosmarinus) were investigated in field to manage fire blight disease in a naturally-infested pear orchard. The extracts were applied as foliar sprays (350 ml/tree). Significant disease reduction was observed on trees treated with rosemary aqueous extract. The disease incidence was estimated at 1 (infestation rate ranging from 5% to 25%/tree). When sprayed with the thyme aqueous extract, the infection incidence ranged from 1 to 2 (25%-50% /tree). We conclude that the aqueous extract of Rosemary was highly efficient in controlling fire blight disease.