Natural spread of Verticillium wilt as effective constraint on Ailanthus altissima invasion

Ailanthus altissima (Mill.) Swingle (tree-of-heaven) is a widespread tree classified as an invasive alien species in the European Union. Recently, severe wilt decay from Verticillium spp. infections in A. altissima trees has been reported in both Europe and the USA. In Italy, the disease was first observed in Trentino-South Tyrol in 2017. Since then, the progression of infection foci has been monitored, and the pathogens involved have been isolated and characterized at the molecular level. Between 2017 and 2022, the number of disease foci increased to 171, affecting entire valleys in this mountainous region. The foci appear unaffected by environmental conditions and impact trees in urban, peri-urban, and wooded areas. Attacks targeted both individual trees and clusters of various size. Dead plants were detected in 138 foci, and in some cases, complete disappearance of A. altissima was reported. Infected or dead seedlings/saplings were also observed in 97 foci. Verticillium dahliae was isolated from 103 foci and confirmed through real-time PCR assays using the VertBT primer. No wilting symptoms were observed in other tree species near the affected A. altissima individuals. Inoculation tests on A. altissima seedlings confirmed the pathogenicity of the isolates, while vine and apple seedlings showed no effects. The natural spread of V. dahliae foci proved very effective, showing few limitations in controlling A. altissima. Therefore, reporting and monitoring the natural spread of wilt could represent a preliminary response to the EU’s request to combat the invasion of A. altissima.