The role of Verticillium dahliae in the dieback of Ailanthus altissima in Trentino - South Tyrol

Ailanthus altissima (Miller) Swingle, known as Tree-of-heaven, is a widespread invasive plant species intentionally introduced in Europe as an ornamental and shade tree. It is now widely naturalized and occurs throughout the warmer climatic zones, which has become the most dominant tree species along the edges of forests and roadways. The negative implications of A. altissima establishment include the replace of the indigenous plant communities associated with changes on the soil properties and on the composition of the litter fauna. Its eradication is difficult, as it has a high regenerative and reproductive capacity. For these characters, ailanthus has been included in the list of invasive alien species of European Union concern in 2019, so obliging European countries to do monitoring and active control to check its spread. A destructive naturally occurring Verticillium wilt disease represents a potential control of A. altissima populations. The causal agent of the disease was identified as Verticillium nonalfalfae Inderb. et al. formerly V. albo atrum Reinke & Berthold and V. dahliae Kleb in the USA, Austria and Spain. Whereas only Verticillium dahliae were isolated in Hungary and in Italy. In Trentino South Tyrol, Ailanthus stand ranging from regeneration to canopy trees displayed foliage wilting, vascular discoloration, and epicormical shoots on affected stems, followed by tree death were observed since 2017. V. dahliae was identified as the only causal agent (1) and the disease incidence has been monitored in the last six years to evaluate the potential of V. dahliae to control Ailanthus invasion in urban areas and natural environments. A field work was carried out in the whole range of ailanthus colonization trying to individuate the affected stands: 171 points with declining plants were observed with several already dead trees, with a constant increase of the foci during the years. The presence of V. dahliae was detected by isolation on media and using real-time qPCR approach with specific primer pair VertBtF/VertBt-R (2). V. dahliae was identified as associated with the decline in most of the affected stands (119), while in the other points samples were too old to permit isolation and molecular texts or otherwise affected plants were not approachable. The disease was present in almost all the valleys of the region, either in wood condition or in urban contest. Pathogenicity test confirmed the virulence of the obtained strains on A. altissima seedlings. The consistency of the data on the distribution of Verticillium wilt in Trentino-South Tyrol suggests a considerable capacity for the spread of the disease, which appears to be undergoing strong and rapid expansion. The spread of Ailanthus wilt is proving to be a potential natural method to contain the populations of one of the worst invasive plant species in Europa