Dieback of apple trees: a complex syndrome and an increasing problem in Northern Italy (Trentino region)

Dieback and blight symptoms on apple trees have been increasingly observed in Trentino (Northern Italy). Stunted growth, chlorotic leaves, cracking, necrosis of the bark and cankers appear mainly at the base of the trunk and on the graft union. Affected plants commonly die during the growing season. To identify agronomic and weather conditions promoting the syndrome, an extensive monitoring was carried out in the main fruit-growing areas of Trentino. Symptomatic apple trees were sampled and microorganisms colonising the affected tissues were isolated in order to identify potential causal agents. The highest incidence of the syndrome has been observed on young stressed plants grown in low fertile soils and cold areas with south-southeast exposure. Bacteria (Pseudomonas syringae) and fungi (Phomopsis spp., Nectria spp., Botryosphaeriaceae) were isolated from necrosis and cankers. Pathogenicity tests were conducted to confirm their role in the syndrome