Xylotrophic basidiomycetes are an essential component of any biogeocoenosis. It is due to these organisms that one of the most important processes occurs throughout the biosphere, namely the decomposition of organic substances and, first of all, cellulose, which is synthesized by producers. However, these phytopathogens bring significant losses to forestry, making wood unsuitable for industrial use. At the same time, sufficiently effective methods for diagnosing and assessing wood damage by heartwood rot, which are caused by xylotrophic basidiomycetes, have not been developed. The paper studies the peculiarity of the influence of these phytopathogens on the pigment apparatus of valuable small-leaved tree species using the methods of spectroscopy and electrophotocolorimetry, while comparing the rate of degradation of the main pigments of photosynthesis – chlorophylls a and b to pheophytin, as well as assesses the content of chlorophyll, and auxiliary pigments such as carotenoids and xanthophylls in healthy trees and trees with signs of damage by pathogenic xylotrophs. A significant number of effects of phytopathogens on leaf pigment apparatus of small-leaved tree species have been identified. Among those are rapid degradation of primary and secondary pigments, distortion of chlorophyll a concentration data caused by filter properties of photoelectric colorimeter and higher chlorophyll b concentration in affected trees in comparison to control sample. These methods and the results obtained can be actively used both in forestry to assess the phytosanitary state of small-leaved trees, and to further study the features of the interaction of xylotrophic basidiomycetes with woody plants.