The North American crayfish Procambarus clarkii is the carrier of the oomycete Aphanomyces astaci in Italy.

Aphanomyces astaci (Saprolegniales, Oomycetes) is classified among the 100 world’s worst invasive species. This species is endemic to North America and has been introduced into Europe by imports of their hosts, the North American crayfish species. As a consequence, extensive mass mortalities involved several populations of the European crayfish. Here, we checked its occurrence in four Italian populations of Procambarus clarkii, the most widespread alien crayfish in Italy. Digital image analyses and image processing techniques were used to select micro-melanized areas in the subabdominal cuticle of 2-10 crayfish per population. All the selected areas tested positive for A. astaci ITS nrDNA specific primers; moreover, the obtained sequences clearly corresponded to A. astaci, thus revealing that P. clarkii is an active carrier of this oomycete in Italy. Decisions are to be urgently made to control the spread of both P. clarkii and A. astaci for the conservation of the indigenous crayfish biodiversity.