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A severe dieback of flowering ash (<i>Fraxinus ornus</i> L.) has been observed in north-central Italy in the last decades. Symptoms include typical sunken, light-brown cankers on the stem and branches; vascular discoloration; tip and shoot dieback; and foliage necroses. The disease was more evident at the beginning of the growing season, and more severe on young regeneration. Six <i>Botryosphaeriaceae</i> species were consistently isolated from symptomatic plant tissues: <i>Botryosphaeria dothidea</i>, <i>Diplodia fraxini</i>, <i>Diplodia subglobosa</i>, <i>Dothiorella iberica</i>, <i>Dothiorella omnivora</i> and <i>Neofusicoccum parvum</i>. <i>B. dothidea</i> and <i>D. fraxini</i> expressed higher aggressiveness and showed a widespread incidence, being the species most frequently associated with cankers; the other four species were less virulent and more erratic, occurring mainly on succulent branch tips and foliage. Isolates were characterized using morphological and molecular approaches (colony/conidial phenotyping and rDNA-ITS genotyping). Phylogenetic analysis provided congruent phylogenies depicting the relationships of the six taxa with the most closely related conspecifics. Pathogenicity tests on 2-year-old seedlings confirmed the higher virulence of <i>B. dothidea</i> and <i>D. fraxini</i>. Extensive, multi-year field surveys at different sites supported the hypothesis that climatic vagaries, mainly heat, water and drought stresses, impaired tree health and vigor, facilitating infection and pervasive colonization by these <i>Botryosphaeriaceae</i> species. Environmental stressors are thus the key factor bringing the six fungal pathogens together in a multitrophic interaction with <i>F. ornus</i> in a novel, lethal fashion.