Editorial: Foliar, shoot, stem and rust diseases of trees IUFRO 2022

pine wood nematode, the causal agent of pine wilt, a disease of global concern. In particular, they demonstrate that the fungal species, Beauveria spp. or their mycotoxin, beauvericin, have strong in vitro nematicidal effects. Staying with nematodes, McIntire (2023) begins to alleviate our lack of knowledge of the physiological effects of Litylenchus crenatae mccannii (the causal agent of beech leaf disease -BLD) on American beech (Fagus grandifolia). Specifically, this paper shows significant relationships between BLD symptom severity and leaf gas exchange and physiological leaf traits that lead to decrese growth, vigor, and long-term survival, all of which will likely compromise the significant ecosystem services by diseased American beech.Pathogens don't act in a vacuum, they are part of a microbial milieu, the phytobiome, which also includes non-pathogenic species, known as endophytes. Endophytes play an important role in disease defense strategies; however, how the plant's health status affects the endophytic communities is not known. Diez-Hermano et al. ( 2022) used metabarcoding to profile the fungal endophytic communities of four declining Mediterranean tree species and found indicator genera that were present only in declining trees. Declining Mediterranean trees are also discussed in Benigno et al. (2023), who report known and new harmful fungal and oomycete pathogens that are becoming more aggressive and widespread on Fraxinus species (ash) as a likely consequence of climate change, causing tree mortality, loss of natural regeneration and the retreat of some ash species from less favorable sites. This is a stark warning of things to come as climate warming shifts host-parasite interactions in favor of the infectious agent.Finally, while this research topic is all about diseases that affect above ground organs, we cannot forget that trees are unitary systems that are subject to diseases below ground as well, and those can have major effects on how serious above ground diseases can become. Diez-Hermano et al. ( 2023) investigated whether declining tree species in Mediterranean forests were associated with specific rhizosphere fungal communities. They found high overall diversity (674 genera) of fungal species but no evidence of known root pathogens in the declining areas. Paired with the study by Benigno et al. (2023), this suggests that, even though the two studies were conducted in different countries, Mediterranean forest decline may be due to a combination of environmental factors and pathogens acting above, rather than below, ground.Taken together, these papers contribute to many aspects of aboveground tree pathology while advancing our understanding of very critical groups of tree diseases, a body of knowledge that we will need to keep refining as we attempt to deal with an unpredictable world in this age of climate and global changes.

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