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The Main Species Associated with Cankers and Dieback of Fraxinus Excelsior in North-Eastern Italy

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The Main Species Associated with Cankers and Dieback of Fraxinus Excelsior in North-Eastern Italy Article Diplodia fraxini and Diplodia subglobosa: The Main Species Associated with Cankers and Dieback of Fraxinus excelsior in North-Eastern Italy Benedetto T. Linaldeddu 1,* , Francesco Bottecchia 1, Carlo Bregant 1, Lucia Maddau 2 and Lucio Montecchio 1 1 Dipartimento Territorio e Sistemi Agro-Forestali, Università degli Studi di Padova, Viale dell’Università 16, 35020 Legnaro, Italy; [email protected] (F.B.); [email protected] (C.B.); [email protected] (L.M.) 2 Dipartimento di Agraria, Università degli Studi di Sassari, Viale Italia, 39, 07100 Sassari, Italy; [email protected] * Correspondence: [email protected] Received: 17 July 2020; Accepted: 11 August 2020; Published: 14 August 2020 Abstract: In Italy, after the first report in Friuli-Venezia Giulia along the border with Slovenia in 2009, ash dieback has successively been reported in Veneto, Tuscany and Trentino-Alto Adige. Given its alarming expansion in European ash formations along the sub-montane belt of north-eastern Italy and the limited information about the associated fungal microorganisms; since 2017, a study has been conducted in order to isolate and characterize the fungal species involved in the aetiology of the disease. The surveys were conducted in six ash–maple forests distributed along the Veneto and Friuli-Venezia Giulia pre-Alpine regions (Italy). In each site, the health status of ash trees was assessed and a sample (shoot or branch) with the typical symptoms of the disease was taken from ten trees to isolate the associated pathogens. The fungal colonies developed were identified using morphological features and DNA sequences. The 60 samples processed yielded a total of 109 fungal isolates belonging to 9 families including: Botryosphaeriaceae (62 isolates), Diaporthaceae (18), Nectriaceae (10), Didymellaceae (9), Helotiaceae (5), Diatrypaceae (2), Didymosphaeriaceae (1), Phaeosphaeriaceae (1) and Valsaceae (1). In particular, three species—Diplodia subglobosa, Diplodia fraxini and Diaporthe eres—were isolated with high frequency, while Hymenoscyphus fraxineus was isolated from only five plants distributed in four sites. The pathogenicity tests, conducted on 3-year-old seedlings, detached branches (3–4 cm diameter), and leaves of Fraxinus excelsior, showed that Diplodia fraxini is the most virulent species and the only one able to reproduce the symptoms observed in nature. Overall, the results obtained emphasize that several fungal pathogens are involved in the aetiology of the disease, many of which belong to the Botryosphaeriaceae family. Keywords: ash dieback; emerging disease; Botryosphaeriaceae; taxonomy; pathogenicity 1. Introduction Since the early 1990s, European ash (Fraxinus excelsior L.) has been impacted by a severe disease commonly known as ash dieback [1–3]. The disease was observed for the first time in north-eastern Poland [4], and has now become widespread in forest ecosystems and nurseries in over 20 European countries [5,6]. In Italy, the disease was detected in 2009 in Friuli-Venezia Giulia along the border with Slovenia and in a few years, it spread quickly to the Alpine valleys of Veneto and Trentino, and in ash formations of the Apennines [7–9]. Affected trees show a broad range of symptoms such as leaf necrosis, shoot blight, inner bark discolorations, sunken canker and epicormic shoots [3,5]. This range of symptoms is generally Forests 2020, 11, 883; doi:10.3390/f11080883 www.mdpi.com/journal/forests Forests 2020, 11, 883 2 of 13 attributed to infections by the ascomycetous fungus Hymenoscyphus fraxineus [10,11], an invasive species initially described in 2006 as Chalara fraxinea [1]. Since the discovery of this pathogen in Europe several researches have focused on its biology, ecology and genetics to understand the pattern and process of infections and its ecological impact [12–14]. Hymenoscyphus fraxineus is considered a hemibiotroph with an initial intracellular infection phase and late necrotrophic stage [15]. It is well-known for its ability to produce a broad array of secondary metabolites in vitro, including the phytotoxin hyfraxinic acid [16] and furanosteroid viridiol, a non-specific toxin with a wide spectrum of biological activities spanning from antifungal to antibacterial also produced by some saprotrophs, such as Trichoderma virens and Hymenoscyphus albidus [17–19]. Hymenoscyphus is a large genus within Helotiaceae, which includes principally saprophytic species able to colonize dead plant material such as leaves, petioles, twigs, fruits and wood in the litter [20]. To date, Hymenoscyphus fraxineus seems to be the only member of this genus reported as an aggressive plant pathogen. The fungus is probably native to East Asia, where it occurs as an endophyte or latent pathogen on Fraxinus spp. [21,22]. In addition to H. fraxineus, various studies have demonstrated that other fungal species are potentially involved in the aetiology of ash dieback [4,23]. In particular, the plurivorous pathogens Diaporthe eres and Diplodia mutila have consistently been isolated from symptomatic ash trees in different countries and their pathogenicity towards European ash has been demonstrated [24,25]. Furthermore, another species Diplodia fraxini has been isolated in Italy, Portugal and Spain from declining narrow-leaved ash (Fraxinus angustifolia Vahl) trees showing the same symptoms reported on European ash [26,27]. The name D. fraxini was recently re-instated to accommodate several D. mutila-like isolates from Fraxinus spp. It is an aggressive pathogen known to produce host-specific phytotoxins towards ash [28]. Therefore, given the growing expansion of ash dieback in several forests in north-eastern Italy [29], and the still limited information available about the pathogens involved, a study was conducted to isolate, identify and characterize the main pathogens associated with the disease. 2. Materials and Methods 2.1. Study Sites, Field Surveys and Sampling Procedure Field surveys were conducted in six mixed ash–maple forests distributed along the pre-Alps of two regions—Veneto and Friuli-Venezia Giulia in north-eastern Italy—where ash dieback symptoms were previously observed (Table1)[29]. From spring 2017 to spring 2018 in each site, a circular monitoring plot (MP) of 10 m in diameter was established and geographical coordinates recorded with a portable GPS. At each MP, the number of European ash trees present was detected and their health status assessed based on the presence of typical ash dieback symptoms such as shoot blight, branch cankers and epicormic shoots (Figure1). Disease incidence (DI = n/N 100) was estimated as the number of symptomatic trees (n) out of the × total number of trees (N), whereas mortality (M = d/N 100) was estimated as the number of dead × trees (d) out of the total number of trees (N). In order to ascertain the causing agents of the main symptoms observed on European ash (shoot blight, brown inner bark discolorations and sunken cankers), at each MP, ten symptomatic trees were randomly chosen for sampling. From each tree, a single symptomatic sample of twig or branch was taken to be processed in the laboratory within 24 h (Table1). Basically, three types of samples were examined: shoots with dieback, branches with inner bark discolorations and branches with sunken cankers that in cross-section showed a wedge-shaped necrotic sector (Table1). Forests 2020, 11, 883 3 of 13 Forests 2020, 11, x FOR PEER REVIEW 3 of 13 FigureFigure 1. Main 1. Main disease disease symptoms symptoms detected detected on on EuropeanEuropean ash ash trees: trees: extensive extensive canopy canopy dieback dieback (a), shoot (a), shoot blightblight (b,c); (b branches,c); branches with with inner inner bark bark discolorations discolorations ( d,,ee);); active active sunken sunken cankers cankers on onbranches branches with with DiplodiaDiplodia fraxini fraxinipycnidia pycnidia (arrow) (arrow) (f–i); ( cross-sectionf–i); cross-section of branches of branches showing showing wedge-shaped wedge-shaped necrotic necrotic sectors sectors (j,k); epicormics shoots below a branch canker (l). (j,k); epicormics shoots below a branch canker (l). 2.2. Fungal2.2. Fungal Isolation Isolation and and Identification Identification Fungal isolation was performed from 60 symptomatic samples (Table 1). Samples were taken to Fungal isolation was performed from 60 symptomatic samples (Table1). Samples were taken to the laboratory, carefully checked for the presence of pycnidia or ascomata and then used for fungal the laboratory, carefully checked for the presence of pycnidia or ascomata and then used for fungal isolation. For each sample, the outer bark was cut away with a sterile scalpel and longitudinal and isolation.transversal For each sections sample, were themade outer to observe bark wasany dise cutase away symptom. with a Isolations sterile scalpel were performed and longitudinal from ten and transversalchips (5 sections mm2) of wereinner madebark and to observexylem tissues any diseasecut aseptically symptom. from Isolationsthe margin wereof the performednecrotic lesions. from ten 2 chipsIn (5 particular, mm ) of innerthe branch bark samples and xylem with tissuesinner bark cut discoloration aseptically fromchips thewere margin taken from of the the necrotic underlying lesions. In particular,xylem. All the chips branch were samples placed on with 90 innermm Petri bark dish discolorationes containing chips potato were dextrose taken agar from (PDA, the underlyingOxoid xylem.Ltd., All Basingstoke, chips
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