| Summary: | During the summer of 2016, severe dieback was observed on young potted <i>Prunus lusitanica</i> (Portugal laurel) plants in a nursery in the Pistoia province (Tuscany, Italy). <i>Cylindrocarpon</i>-like isolates were consistently recovered from diseased plant tissues. The combination of morphological and molecular traits, including sequence data of histone 3 and β-tubulin genes (<i>HIS3</i>, <i>TUB2</i>) and internal transcribed spacers (ITS), allowed the identification of <i>Dactylonectria macrodidyma</i> (Halleen, Schroers & Crous) L. Lombard & Crous (asexual form <i>Cylindrocarpon macrodidymum</i>) as the causal agent of the disease. Pathogenicity tests reproduced disease symptoms observed in the nursery after six months fulfilling Koch’s postulates. <i>D. macrodidyma</i> is a soilborne plant pathogen and is to be considered of great economic importance on <i>P. lusitanica,</i> especially under favorable conditions such as stress and/or reduction of plant vitality. The increasingly frequent reports of the disease caused by the pathogen in various nurseries suggest that pot cultivation, together with prolonged drought periods, may play a role in favoring infections. To the best of our knowledge, this is the first report worldwide of the occurrence of dieback on <i>Prunus lusitanica</i> caused by <i>D. macrodidyma</i>.
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