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Hazelnut (Corylus avellana L.) is a major nut crop worldwide with increasing economic relevance in Spanish agro-industrial systems (Tous, 2005). A survey conducted in October 2024 in approximately 20 ha in Tarragona area revealed an incidence of 30% of trunk diseases (TDs). Plants belonging to the cultivars Daria, Negret, Pauetet, San Giovanni, Tombul, Tonda Giffoni showed branch dieback with internal discolored wedge-shaped cankers, reddish conidia masses and, in the severe cases, broken branches and plant death. In Spain, this symptomatology, known as “solcuit”, was associated with the fungal species Cytospora corylicola. Twenty symptomatic samples were sterilized in 1% sodium hypochlorite, rinsed twice in sterile distilled water, plated on PDA amended with streptomycin sulfate (PDA-S) and incubated at 25 °C for five days. Pure cultures obtained by hyphal-tip transfer were classified as Botryosphaeriaceae-like (20% isolation rate) and Diatrypaceae-like (75%), according to colony morphology (Zhang et al., 2021; Stoykov & Alvarado, 2023). Both morphotypes were recovered from all symptomatic samples. Twenty-six Diatrypaceae-like (CVG2522–CVG2547) and 12 Botryosphaeriaceae-like (CVG2566–CVG2577) isolates were morphologically and molecularly characterized. Diatrypaceae-like colonies showed cottony, multiseptated, white to cream–white hyphae, producing hyaline, lunate and unicellular conidia measuring 6.1 ± 0.7 × 1.5 ± 0.3 µm (mean ± SD). Botryosphaeriaceae-like strains showed fast growing white–black mycelium, globose pycnidia containing aseptate, hyaline, ellipsoidal and unicellular conidia (12.4 ± 0.7 × 4.5 ± 0.3 µm). These morphological characteristics were consistent with the species Anthostoma decipiens and the genus Neofusicoccum, respectively (Zhang et al., 2021; Stoykov & Alvarado, 2023). The sequencing of ITS region (PX482467–PX482492) for Diatrypaceae-like strains and of ITS (PX482544–PX482555), tef1 (PX560460-PX560471) and tub2 (PX560448-PX560459) for Botryosphaeriaceae-like strains was performed (Linaldeddu et al., 2016; Martino et al., 2024). ITS sequences of Diatrypaceae-like strains showed 100% identity with the strains IPV-FW349 of A. decipiens (DC.) Sacc. The sequences of Botryosphaeriaceae-like strains exhibited 100% identity for ITS, 99% for tef1 and tub2 with the ex-type strains CMW9081 of N. parvum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips. Pathogenicity of two strains of A. decipiens (CVG2526, CVG2531) and two of N. parvum (CVG2574, CVG2576) was confirmed on 2-year-old potted plants of C. avellana cv. Pauetet. Ten plants for each strain were wounded with scalpel, inoculated with mycelial plugs (5 mm diameter) from 7-day-old cultures and kept in greenhouse. PDA-S plugs were used as control. After 4 months, the inoculated plants showed the symptoms observed in field while control plants remained healthy. Colonies of A. decipiens and N. parvum were reisolated and identified with ITS and tef1, respectively. A. decipiens was reported in Italy causing hazelnut TDs (Martino et al., 2024), while N. parvum was found causing dieback in Chile and fruit rot in Italy (Waqas et al., 2022; Lolas et al., 2024). To our knowledge, this is the first report of A. decipiens and N. parvum causing hazelnut TDs in Spain where represent a serious threat to hazelnut production.