Ash dieback, caused by the ascomycete fungus Hymenoscyphus fraxineus (formerly Chalara fraxinea), was first confirmed in Italy in 2013, in nursery stock intercepted in Friuli-Venezia Giulia. Within two years, field detections had been reported across Veneto, Trentino-Alto Adige, and Lombardy. By 2018, the fungus had been documented as far south as Tuscany, and monitoring data from the Italian national forest health network indicated a presence across all major ash-bearing regions of the country.
How the Disease Progresses
H. fraxineus is a leaf and shoot pathogen that overwinters on infected leaf rachises left on the ground. Ascospores released from these debris in early summer infect new foliage and shoots. The fungus moves from shoots into the vascular tissue of branches, producing characteristic diamond-shaped lesions in bark — often called "necrotic lesions" in the forestry literature — and wilting of current-year growth. In severely infected trees, successive years of shoot death progressively strip the crown of functioning foliage, leading to dieback that moves downward through the tree.
In Italy, the progression varies considerably by site. Trees growing on calcareous, well-drained soils in upland areas of Veneto and Friuli have shown slower crown decline than ash in poorly drained valley bottoms or riverine settings. The hypothesis, based on comparative monitoring work in Emilia-Romagna (published in the Italian forestry journal L'Italia Forestale e Montana, 2022), is that waterlogged roots reduce a tree's capacity for the compartmentalisation response that limits lesion spread in moderately resistant individuals.
Mortality Rates in Italian Populations
Pan-European monitoring data from the European Forest Institute estimates that in heavily affected regions, ash mortality over a 10-year period following first detection typically ranges from 50 to 80 percent of the standing population. Italian regional monitoring has not yet reached the timeframes needed for equivalent estimates in most areas, but crown decline assessments from Lombardy's provincial forestry services (2023) indicated that approximately 35–45 percent of monitored hedgerow ash in the western Po Valley showed crown die-back affecting more than 50 percent of the canopy.
These figures mask substantial within-site variation. At several monitored sites in the Oltrepò Pavese, individual trees showed minimal symptoms after a decade of confirmed fungal presence in the locality — a pattern consistent with findings in other European countries pointing to a low but non-trivial proportion of genetically tolerant individuals.
Effects on Hedgerow and Farmland Structure
The functional role of ash in northern Italian field boundaries means that its decline has consequences beyond the individual trees. Hedgerow ash typically formed the emergent layer above a mid-height structure of hawthorn, field maple, and elder. The loss of ash crowns opens the canopy, alters the local microclimate along field boundaries, and in some cases has accelerated the collapse of structurally weakened hedgerow banks. In areas where pollarding had maintained a dense thicket of upright rods, the shift to unpollarded growth followed by dieback has left exposed stumps and dead branch accumulation that requires management to avoid farm vehicle access problems.
Woodland edges in northern Italy — particularly in Friuli and along the Apennine foothills of Emilia — show similar patterns. Ash had a minor but consistent presence in mixed deciduous woodland margins alongside field maple, hornbeam, and downy oak. In areas of confirmed dieback, the gap left by ash is typically colonised by elder (Sambucus nigra) or bramble rather than by other native tree species, a succession dynamic that has been noted in similar contexts in central Europe.
Fraxinus ornus: A Different Situation
The manna ash, Fraxinus ornus, which grows across limestone areas of central Italy, the Gargano promontory, and parts of Sicily and Calabria, appears to be significantly less susceptible to H. fraxineus than F. excelsior. Infection has been detected on F. ornus in laboratory and controlled field conditions, but widespread disease-driven mortality in natural populations has not been documented in Italy as of early 2026. The reasons are not fully established; one line of inquiry focuses on the different phenological timing of leaf emergence in F. ornus — its later bud burst may mean that peak ascospore release coincides less reliably with susceptible tissue.
Monitoring and Regulatory Status
H. fraxineus is listed as a Union quarantine pest under EU Regulation 2016/2031, requiring member states to survey and report confirmed findings. Italy's national plant health authority (IPLA and the Ministry of Agricultural, Food and Sovereign Policies) coordinates annual monitoring through the national forest health network, which includes fixed observation plots and reactive survey capacity for newly reported cases. The most recent public summary from CREA (Centro di Ricerca Difesa e Certificazione, 2024) reported confirmed presence in all northern regions and in Tuscany, Umbria, and Marche in central Italy.
Regulatory requirements under the EU phytosanitary framework have shaped how Italian nurseries handle ash propagation material. Nursery stock must be inspected and certified free of detectable infection before sale, and imports of ash planting material from outside the EU require phytosanitary certification. This has affected the availability and cost of ash for replanting, a point addressed in the related article on Replanting Fraxinus in Mixed Farmland Systems.
Tolerant Individuals and Future Prospects
The detection of tolerant ash trees across European dieback zones has been one of the more significant findings of the past decade. Research programmes in Denmark, the UK, and Sweden have identified and begun propagating tolerant genotypes. In Italy, systematic screening of surviving trees in heavily affected localities has begun on a smaller scale, with initial work focused on Friuli-Venezia Giulia, where the disease has been present longest. The aim is to identify local provenance material that combines reasonable levels of tolerance with regional adaptation — a relevant distinction given that northern Italian ash populations show different genetic structure from central European ones due to postglacial recolonisation history.
Further reference: European Forest Institute ash dieback monitoring summaries; CREA Difesa e Certificazione annual reports; L'Italia Forestale e Montana vol. 77 (2022); EU Regulation 2016/2031 on protective measures against plant pests.