The biocontrol activity of two isolates of Trichoderma virens against blue mould of apple fruits caused by Penicillium expansum and their ability to induce biochemical defense responses in apple tissue were investigated. Apple fruit (Malus domestica) wounds were inoculated with 20μl antagonist suspension (107 conidia/ml) of T. virens and 4 h later with 20 μl of conidial suspension of P. expansum (105 conidia/ml). The apples were then incubated at 20°C for 8 days. Lesion diameters were measured 4 and 8 days after inoculation with the pathogen. Two isolates of T. virens were effective in controlling decay of apple fruits caused by P. expansum. Six days after treatment peroxidase activity increased by more than three-fold in apple treated fruit in combination of antagonist and pathogen, in comparison with in wounded, non-inoculated control tissue. Catalase (CAT) activity increased in inoculated fruits in combination of T. virens and P. expansum in comparison with healthy control at all days and maximum activity level was noted at 6 days after inoculation. The results indicated that T6 and T8 isolates of T. virens caused the increase in β-1,3-glucanase activity that reached maximum levels 4 and 6 days after inoculation with pathogen, respectively. The increase in β-1,3-glucanase activity was triggered by wounding although the level of increase was markedly lower than detected in treated fruits. Phenolic compounds accumulation showed the highest levels 2-4 days after inoculation and then decreased. The ability of T. virens to increase the activity of peroxidase, catalase, β-1,3-glucanase and levels of phenolic compounds may be one of mechanisms responsible for its biocontrol activity.
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