Endophytic, canker‑inducing Botryosphaeriaceae causing flowering ash (Fraxinus ornus) dieback in central Italy

A widespread dieback of flowering ash (Fraxinus ornus L.), with high mortality of young regeneration, was observed in some woody areas of central Italy (Tuscany). Symptoms appeared at the beginning of the growing season as typical sunken, light-brown cankers on the stem of young trees, which died as soon as the cankers girdled the stems. Many plants reacted vigorously to infection trying to callus over the lesions. Starting from 2018, a study was undertaken to clarify the etiology of the disease. Three botryosphaeriaceous fungi were isolated with high frequency from symptomatic plants: Botryosphaeria dothidea, Diplodia fraxini and Neofusicoccum parvum; a Fusarium sp. was also sometimes isolated. Artificial infection tests were carried out in July 2022 on two-yearold Fraxinus ornus seedlings with the four fungal taxa. All the botryosphaeriaceous fungi produced cankered lesions, with B. dothidea which reproduced the typical elongated cankers observed in the field. It is hypothesized that environmental stress was the main factor weakening trees and predisposing them to aggressive attack and colonization by pathogenic Botryosphaeriaceae. In fact, the areas with higher disease incidence and severity were those characterized by harsh environmental conditions, with low moisture, prolonged drought, especially on poor and gravelly soils.