Host responses and pathogen strategies: tree decline and dieback etiology by bacterial and fungal pathogens

Trees provide oxygen, improve air quality, support biodiversity, regulate climate, prevent erosion, and offer habitats for countless species. Tree decline and dieback syndromes represent escalating threats to global forest ecosystems, driven significantly by pathogenic bacteria and fungi. This systematic review synthesizes contemporary understanding of the cellular and molecular mechanisms underpinning these diseases across diverse forest systems. We critically examine the pathogenic strategies of key microbial agents, focusing on their deployment of virulence factors, including cell wall-degrading enzymes, phytotoxic metabolites, and effector proteins. The review comprehensively details corresponding host responses, encompassing anatomical alterations (e.g., tylosis formation, gum deposition, necrosis), physiological disruptions in water and carbon relations, and activation of molecular defense signaling pathways. A central theme explored is the intricate interplay between pathogen virulence mechanisms and host susceptibility, particularly under environmental stressors like drought and nutrient deficiency, which critically exacerbate disease severity. Cellular analyses consistently reveal profound damage in compromised tissues, including apoptosis-like programmed cell death, mitochondrial dysfunction, and abnormal starch accumulation. By integrating molecular, cellular, and environmental perspectives, this review highlights conserved virulence strategies while identifying taxon-specific vulnerabilities, thereby revealing novel targets for advancing diagnostic tools and biocontrol innovations.