Identification of the antifungal activity of Trichoderma longibrachiatum T6 and assessment of bioactive substances in controlling phytopathgens.

Biological control with microbial antagonists is considered an alternative approach for controlling plant diseases. Trichoderma species are one of the potential fungal biocontrol agents in suppression of soil-borne pathogens. However, the mechanism and characterization of Trichoderma spp. in inhibiting different phytopathogenic fungi are largely unknown. In this study, we investigated the antagonistic potential of the endophytic fungus Trichoderma longibrachiatum T6 as a biocontrol agent against different phytopathogenic fungi and the associated antagonistic mechanism with bioactive substances. We found that the fermentation and crude extract of T. longibrachiatum T6 had a broad spectrum and potent activity inhibiting the growth of eleven phytopathogens evaluated, and of which, the inhibitory rate against Valsa mali reached 95% at 5 days after incubation. Ten fractions and six sub-fractions of bioactive substances were obtained on silica gel G chromatography and Sephadex LH-20 columns. One of the sub-fractions (coded sub-Fr.4f) exhibited highest inhibition against the pathogen V. mail, with the inhibitory rate of 80.64% at Day 5 of the treatment. Four key chemical inhibitors were identified: (i) 1, 2-Benzenedicarboxylicacid, bis (2-methylpropyl) ester (DIBP) (C16H22O4); (ii) (Z)-octadec-9-enoic acid (C18H34O2); (iii) 1, 2-Benzenedicarboxylic acid, mono (2-ethylhexyl) ester (MEHP) (C16H22O4); and (iv) (Z)-13-Docosenamide (C22H43NO), using spectroscopic and nuclear magnetic resonance data. Two fungicidal compounds DIBP and MEHP provided significantly greater antifungal activities than the other compounds in the inhibition of the V. mail growth. There was a significant linear relationship between the monomer compounds MEPH or DIBP and the inhibitory rates of V. mail; at the concentration of 200 μg mL-1, the inhibitory rate reached over 86% or 78%. We conclude that the strain of T. longibrachiatum T6 can serve as an effective biocontrol agent against V. mali and the mechanism for this function was due to the secondary metabolites with effective bioactive substance.