About 60% of the nut production of sweet chestnut (Castanea sativa Mill.) in Europe originates in Spain (FAOSTAT, 2022), mostly (91%) in Galicia (NW Spain). In September 2021, premature fall of immature chestnut burrs and nuts of C. sativa was observed in eight orchards of Pontevedra and Ourense (provinces of Galicia). Chestnut green burrs had turned brown and fallen off, and the nuts showed brown lesions in kernels and embryos. Some nuts had become mummified. Symptomatic samples of burrs and nuts, including mummified fruits, were collected. Small pieces of samples were surface-disinfected with 2% sodium hypochlorite, and plated onto potato dextrose agar (PDA) plates. Colonies were creamy white to gray or light brown, and presented a woolly to felty mycelium with a dense development in concentric circles. Brownish to black conidiomata, produced abundantly, were globose to sub-globose. Conidia were hyaline, oval, obovoid, fusoid and multi-guttulate, and 6.6±0.78 [5.07 to 9.01] μm x 3.2±0.43 [2.41 to 4.38] μm in size. These features matched those described for Gnomoniopsis smithogilvyi (Shuttleworth et al. 2012), syn. G. castaneae (Visentin et al. 2012; Shuttleworth et al. 2015). Genomic DNA was extracted from mycelium developed in 22 burrs and nuts, and 30 pure cultures. The rDNA internal transcribed spacer (ITS), fragments of the β-tubulin (TUB2) and the translation elongation factor-1α (TEF-1α) genes were amplified using ITS1F (Gardes and Bruns 1993) and ITS4 (White et al. 1990), T1/Bt2b (O'Donnell and Cigelnik 1997), and EF1-728F/EF1-986R (Carbone and Kohn 1999) primers, respectively. Two isolates (EFA 924A, EFA962.4A) were deposited in the Spanish Type Culture Collection (Paterna, Valencia), and their sequences submitted to GenBank (accession nos.: ITS: OM319846, OM319848; TUB2: OM417078, OM417080; TEF-1α: OM417081, OM417083). BLASTn analyses showed: for ITS and TEF-1α sequences, ≥99.7% identity to the ex-type of G. smithogilvyi (ITS: JQ910642, 474 matching bp; TEF-1α: JQ910645, 335-338 matching bp), and for TUB2 sequences, ≥99.1% identity to G. castaneae (LN999975, 446-453 matching bp). Pathogenicity tests were carried out on surface disinfected nuts of Castanea sativa `Raigona´. A superficial wound was made in the pericarp of each nut. A 2-mm mycelial plug of a 7-days-old culture of G. smithogilvyiwas then inoculated: twenty nuts with isolate EFA924A and twenty with isolate EFA962.4A. Twenty nuts inoculated with sterile agar were used as control. The holes were sealed with Parafilm®. Nuts were incubated in a moist chamber at 22±2°C. Two replicated tests were carried out. Four inoculated and four control nuts were inspected for the presence and progress of rot symptoms every seven days. Three weeks after inoculation, all remaining inoculated nuts were completely rotted, whereas all control nuts remained healthy. Gnomoniopsis smitholgilvyi was reisolated from all inoculated nuts, and it was not recovered from the controls. This pathogen causes chestnut brown rot on sweet chestnut worldwide (EPPO, 2022), causing also shoot cankers on chestnut (Lione et al, 2019). This is the first report of G. smithogilvyi causing chestnut brown rot on nuts and burrs of C. sativa in Spain. Future studies on the incidence of this pathogen and its impact on chestnut yield should be carried out in the main European producing countries, i.e. Spain, Italy, Portugal and Greece, where the disease has been detected and represents an emerging threat to chestnut production.

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