A critical analysis of the forest decline theories has been reviewed to estimate the role and involvement of invasive pathogens in these processes and demonstrate the existing differences in the interpretation of spread and the impact of invasive pathogens that can cause diseases of the trees. Experiment with detecting pathogenic Oomycetes organisms in the rivers and streams along the Polish-Ukrainian border proved that the water can be one of the pathways for Phytophthora sp. introduction demonstrating no borders for pathogens and water as a neglected source of these phytopathogens. Three Phytophthora species (P. gonapodyides, P. lacustris and P. cactorum) were detected in Ukrainian rivers presumably moving naturally towards the EU border. Species of the Pythium genus, and among them the most common ones like Py. lycopersicum, Py. sylvaticum, Py. citrinum and Py. terrestris, were detected in Polish rivers. All deleted species belonging to both geniuses cause damage to the fine root and trunk collars of many forest tree species. Seven Phytophthora species including invasive ones (P. bilorbang, P. cactorum, P. gallica, P. gonapodyides, P. lacustris, P. plurivora and P. polonica), and two other clades six taxa were detected from the various forest types with tree species showing indicative symptoms of Phytophthora infections. The main representatives of Pythium were (Py. unregulare, Py. intermedium, Py. litorale, Globisporangium mamillatum, Py. violae, Phytopythium citrinum, Phytopythium litorale). Four of the Phytophthora species (P. bilorbang, P. gallica, P. plurivora and P. polonica) have previously never been reported from broadleaf forests in Ukraine. The sequences obtained for all Phytophthora isolates were registered in GenBank (Accession no.’s MT420377–MT420411). The underbark stem inoculation tests on black alder and Silver birch seedlings demonstrated plants' susceptibility to five Phytophthora species (P. plurivora, P. bilorbang, P. polonica, P. gonapodyides, P. cactorum) and proved high pathogenicity impact of tested Phytophthora sp. on the host species. The bark necrosis and extension of discoloration in sapwood and pith in a stem region with the season-specific patterns were studied on naturally infected ash plants using a H. fraxineus-specific qPCR assay and light microscopy. This study confirmed that hyphal growth in the starch-rich perimedullary pith is of particular importance for both axial and radial spread of H. fraxineus, but that most of its biomass accumulates in sapwood parenchyma. Results of the study present new information that refines the current stem invasion model. It was also shown that mycobiome comparison between healthy and symptomatic European ash leaflets revealed no significant differences in the relative abundance of H. fraxineus but the overall fungal richness on European ash leaves declined significantly towards autumn along with an estimated increase in H. fraxineus biomass. Soils’ mycobiota associated with northern white cedar (Thuja occidentalis) was analyzed using Illumina MiSeq sequencing in Poland and Ukraine. A total of 11490 operational taxonomic units (OTUs) were obtained from the soil samples representing 950 taxa of fungi. The frequencies of Ascomycota were 28% in Ukrainian soil samples and 72% in Polish soil samples detecting a group of invasive soil pathogens that can cause different types of plant diseases. The abundance and diversity of soil mycobiota were discussed. The mycobiome of Thuja occidentalis, Juniperus communis, J. conferta, Buxus sempervirens, Chamaecyparis lawsoniana represented 49,65% of fungi taxa was found belonging to Ascomycota, 18% to Basidiomycota, 0,315% to Mucoromycota, 0,024% to Blastocladiomycota, 0,462% to Chytridiomycota, 0,107% to Glomeromycota and 0,260% to Rozellomycota. 18 newly invasive species that are listed as EPPO and CABI species were also detected. Two invasive boxwood pathogens Clonostachys buxi and Calonectria pseudonaviculata were isolated from the infected boxwood leaves and added to the list of invasive pathogens in Ukraine. The conceptual principles for preventing the spread of invasive pathogens into the forest and urban ecosystems were proposed and include 1) prevention, 2) early detection, 3) eradication, and 4) control.
