The incidence of natural enemies of plants triggers in them a number of defensive reactions. In spite of many studies, the role of phenolic compounds in tree-pathogen interactions, has been still ambiguous. The aim of the present study has been to analyse the tannin content in the leaves of the black cherry (Prunus serotina) infested, to a different degree, by Monilinia seaveri. The observations of the health status of the leaves were carried out on marked trees for two years. The pathogen was identified based on the material obtained from laboratory cultures. The tannin content, expressed as (+)-catechin, was determined using the vanillin method. The greatest extent of disease development was observed at the end of July. It has been found that with disease symptoms covering > 25% of the leaf area, the (+)-catechin content in the diseased leaves was significantly higher compared to the healthy ones. The obtained results thus indicate, that at the damage level exceeding approximately 25% of the leaf area, mechanisms of the induced resistance are triggered in plants, that may prevent the further development of disease.

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