First report of Phytophthora megasperma and P. pseudocryptogea on Rosmarinus officinalis in Tuscany, Italy

In January 2023, a severe dieback of Rosmarinus officinalis L. plants was observed in a rosemary plantation at Casole d'Elsa, Siena (Italy). Symptoms, including root rot, browning of the collar with discoloration of the internal tissue, leaf blight and stunting, closely resembled the typical root and crown rot syndrome induced by Phytophthora species. The lateral flow immunoassay test specific for Phytophthora species applied in situ on R. officinalis on symptomatic tissue samples gave positive results. In order to identify the associated Phytophthora taxa, roots, stem and soil samples were collected and taken to the laboratory. The baiting method was used in the laboratory for priming oomycetes. Only one of the two cultured morphotypes, produced gametangia and oospores, confirming to be homothallic. The other one was heterothallic. DNA-based identification of these isolates revealed complete (100%) homology with respectively Phytophthora megasperma and P. pseudocryptogea for both rDNA-ITS and CoxI gene regions. This is the first report of P. megasperma and P. pseudocryptogea causing disease to R. officinalis in Italy. The pathogenic occurrence of these two harmful oomycetes on R. officinalis, could represent a new problem to the cultivation of this economically important aromatic plant species in the Mediterranean area.