Drying out of woody plants at Peter the Great Botanical Garden BIN RAS (Saint Petersburg) and distribution of Phytophthora and Pythiumspecies: monitoring of 2018

During the study period of 2011-2018, six species of phytophthoras and one species of pythium were identified at the dendrarium park of the Peter the Great Botanical Garden of the Komarov Botanical Institute RAS. During two vegetative seasons of 2017 and 2018, the strongest signs of drying out were registered in 91 species of arboreal plants (53 % - shrubs, 45 % - trees, and one species of lianas and subshrubs respectively), which belong to 73 species of 39 genera of 21 families. The age of the affected plants ranges from 14 to 200 years, 60 to 100-year-old plants dominate. Presence in soil and influence on plants of two species of phytophthoras were confirmed. These are Phytophthora сactorum (Lebert et Cohn) J. Schröt. and P. plurivora T. Jung et T. I. Burgess. The two species of phytophthoras were observed before, but under other tree species. Pythium ultimum Trow were found in soil under Tilia cordata, being an active root pathogen – here it was found for the first time. It is evident that woody plants show different resistance to pathogens.