Emerging microbial tree pathogens in forestry and urban areas: an increased safety risk due to host expansion in a changing climate

Forest and urban trees are facing increasing pressure from emerging fungal pathogens, a trend strongly influenced by changing climatic conditions. Prolonged droughts, higher average temperatures, and extreme weather events reduce the resilience of tree species. Under stress, the host trees become more susceptible to infections from pathogens that were previously not associated with them. In recent years, stem and branch cankers have been increasingly documented, including cases of bleeding cankers in various urban tree species. Microorganisms are not confined to their original hosts but are constantly on the move. They can be transported by wind, rain splash, insects, birds, or even soil particles clinging to machinery and footwear. A further concern is the expansion of host ranges: fungal species typically associated with certain trees are now found on unexpected hosts, broadening their ecological footprint. This trend reflects both the adaptability of microorganisms and the stress-induced vulnerability of forest and urban trees. The combination of hidden colonization, delayed symptoms, efficient modes of transport, and host expansion underlines the growing challenge posed by microbial pathogens in the management and safety of urban green spaces.