Determination of antifungal efficiencies of antagonist bacterial isolates and plant essential oils against citrus and avocado anthracnose disease agent Colletotrichum karstii

Colletotrichum karstii has been recently reported as an anthracnose disease agent on citrus and avocado trees growing in Hatay province. In this study, in vitro antifungal efficiencies of antagonist bacterial isolates, which were isolated and identified from leaves, shoots, fruits and roots of healthy citrus and avocado trees, and essential oils of medically important plants were investigated against Colletotrichum karstii. As a result of GC-MS analysis, the major constituents of the essential oils were determined as carvacrol in Thymbra spicata var. spicata (Tss) and Origanum syriacum (Os), terpinen-4-ol in Origanum majorana (Om), trans-anethole in Foeniculum vulgare (Fv), eucalyptol in Eucalytus camadulensis (Ec), linalool in Mentha x piperita f.sp. officinalis var. 'Aura' (Ma), α-pinene in Ferula communis (Fc), limonene in Lippia citridora (Lc) and Citrus aurantium (Ca) essential oils respectively. Among the tested essential oils, Tss, Os, Tsy, Fv, Lc, Ma and Om essential oils were determined as the most effective essential oils by completely (100%) inhibiting mycelial growth (MIC) at the concentrations of 4.0-20.0 µl/petri. Essential oils of Ec and Ca plants were determined as the least effective essential oils comparison to all essential oils (MIC at >50 µl/petri concentrations). Fc, on the other hand, was not successful in inhibiting mycelial growth at all concentrations tested. Among 70 bacterial isolates, obtained from the rhizosphere and endosphere of healthy trees, 50 isolates were selected to determine antagonistic potential against fungal isolates and identified by MALDI-TOF MS analysis. According to results of in vitro dual culture assay, Bacillus mojavensis KGT1ep, Bacillus subtilis KGT2ep, Bacillus amyloliquefaciens KGT12ep, Bacillus atrophaeus KGT7en, Bacillus cereus KGT13en, Pseudomonas chlororaphis KGT10ep isolates were found as the most effective antagonistic bacterial isolates inhibiting mycelial growth of Colletotrichum karstii by 72-83 %. Bacterial isolates show significantly varied as statistically in the production of siderophore, protease, hydrogen cyanate (HCN), ammonia and plant growth promoting (PGP) mechanisms such as indole 3 acetic acid (IAA) and phosphate solubilisation activities. It has been determined that one or more of the antagonistic mechanisms such as siderophore, protease, HCN and ammonia production are the factor(s) playing a role in the suppression of mycelial growth of the fungal agent.Twenty nine selected bacterial isolates produced varying amounts of IAA in nutrient media. Among 29 tested isolates, fifteen isolates has the ability to dissolve phosphorus from tri calcium phosphate on Pikovskaya Agar (PVK) petri plates. In conclusion, showing the potent antagonistic (siderophore, protease, ammonium production) and PGP (IAA and phosphate solubilization potentials) activities, bacterial isolates belonging to Bacillus spp and Pseudomonas spp., have the potential to be used as biological control agent against anthracnose disease. This is the first study which investigates the biocontrol efficiencies and mechanisms of action of antagonist bacterial species against citrus and avocado anthracnose disease agent Colletotrichum karstii.