The intriguing issue of Verticillium wilt of Ailanthus altissima in Europe: a new report from Tuscany, Italy

Ailanthus altissima is a unique aggressor plant, invading forests, displacing native vegetation important for wildlife and biodiversity. Chemical and mechanical control is almost unworkable, as ailanthus will re-sprout, requiring multiple treatments and making large-scale, long-term eradication expensive and impractical, leading to consider the biological control. Ailanthus is a ‘healthy’ plant, almost absent from phytopathological inventories, apart some wilt problems sometimes reported. The other cases may by brought back to mere scientific curiosities. The earliest report of a wilt affecting ailanthus and ascribable to Verticillium is from Paris, in 1894; the case was explored in detail in 1931. Other cases refer to 1915 in Beaver County (PA, USA) and the late 1920s and early 1930s in New York, Philadelphia and Roanoke (VA, USA). Goidanich described a ‘tracheoverticilliosi’ in Liguria (Italy) in 1935. Since then, European reports are absent until the late 2010s, apart isolate cases from Austria and Greece in 1998. Since long, our group is conducting cursory observations in ailanthus populations in several Italian regions and in 2016 identified sprouts near Leghorn (Tuscany, Italy) showing wilt symptoms. Canonical procedures were followed to isolate and identify the pathogen. Several isolates were collected and identified as V. dahliae based on microscopical features. A ITS region has been amplified and the sequence has been deposited in GenBank with the accession number MK474459. BLASTN analysis shown 100% similarity with V. dahliae (e.g. acc. # MK093977, MH392569, MG910491). To confirm pathogenicity, inoculations of ailanthus seedlings were performed with the root dipping technique or with stem-inoculations. All inoculated individuals showed wilt symptoms in a couple of weeks and the pathogen was reisolated from them, the resultant colonies being identical to the original isolate, so satisfying Koch’s postulates. In the meantime almost simultaneous reports of ‘new’ Verticillium wilts are curiously emerging in European ailanthus populations, namely in Austria, Hungary and northern Italy, all of them attributed to V. dahliae or V. nonalfalfae. This last species is responsible for ailanthus lethal epidemics in Eastern USA since the early 2000s. Studies are in progress to perform a Pest Risk Analysis of our isolate and to explore the opportunity of employing it as a specific mycoherbicide to counteract the otherwise unrestrainable advance of the ailanthus plague.