Preliminary in vitro tests on inhibitory activity of distinct plant extracts toward bacterial pathogens of fruit and nut trees

Abstract

The present study was undertaken to test the inhibitory activity of several plant extracts against bacteria causing economically important diseases on fruit and nut trees. Essential oils derived from oregano (Origanum vulgare ssp. hirtum), common sage (Salvia officinalis), savory (Satureja pilosa), monarda (Monarda didyma) and an extract from clove (Syzygium aromaticum) flower buds were tested in vitro for their antibacterial activity towards two different strains each of Agrobacterium tumefaciens (At), strains AT4 and C58, Erwinia amylovora (Ea), strains 659 and 691, Pseudomonas syringae pv. syringae (Pss), strains LMG 1247T and 110, Xanthomonas arboricola pv. corylina (Xac), strains CFBP 1159T and 301, and X. arboricola pv. juglandis (Xaj), strains CFBP 2528T and 510. The plant products expressed selective activity in suppressing bacterial growth. Overall, most efficient appeared the essential oils from monarda (for strains At AT4, Ea 659, Pss LMG 1247T Xac CFBP 1159T and Xaj CFBP 2528T) and oregano (against all strains used). In 100% concentrated form these oils exerted on average 48-63% and 40-55% inhibition, respectively. The inhibitory effects of sage and savory oils varied significantly causing either no or from 20 to 60% inhibition. Clove extract was medium efficient against all pathogens providing 18-30% inhibition. Generally, most of the extracts (except clove extract and in some cases sage and savory oils) showed activity exceeding up to 30-40% compared to the inhibitory effect of the maximum concentration (500 ppm) of the antibiotic streptomycin. Monarda, oregano oils and clove extract were almost equally active in undiluted form and at dilution to 50% whereas sage oil and in part savory oil showed lower activity at 50% dilution. The differences in the response of tested pathogens (that were of different origin) suggested that their sensitivity to applied plant products may be related to the genotype. In support to this assumption are our earlier studies where the genetic diversity of the strains used was proven by analysis of housekeeping genes. The results reported here are preliminary and further work is being undertaken to perspective the observed inhibitory effects by in vivo tests in a larger scale.