Verticillium nonalfalfae, a biocontrol agent against Ailanthus altissima
Ailanthus altissima (Simaroubaceae - EPPO List of Invasive Alien Plants) commonly known as the tree of heaven is an invasive alien plant species in the EPPO region and native to Asia. It can invade a variety of habitats including managed and unmanaged grasslands, forests, riverbanks/canal-sides, rail/roadsides, wasteland, and urban areas. The wilt pathogen Verticillium nonalfalfae* (Sordariomycetes: Plectosphaerellaceae) was isolated from a dying A. altissima tree in Southern Styria (Austria) in 2011 and has subsequently undergone host range testing to evaluate its host specificity. Previous host range testing studies have shown the wilt pathogen to be highly specific to A. altissima. A further nine non-target tree species were testing using the V. nonalfalfae isolate Vert56 of strain G1/5, the particular strain used for biological control of A. altissima. The nine species were Acer negundo (Sapindaceae, A. platanoides, Castanea sativa (Fagaceae), Prunus avium (Rosaceae), P. serotina (Rosaceae), Quercus petraea (Fagaceae), Q. rubra (Fagaceae), Sorbus aucuparia (Rosaceae) and Ulmus glabra (Ulmaceae). All tested tree species exhibited vascular discolorations and the fungus could be re-isolated at varying frequencies (6—100%) from inoculated seedlings of all non-target tree species, although five species exhibited no external symptoms. Results confirmed high susceptibility of A. altissima to V. nonalfalfae, whereas A. platanoides, C. sativa, Q. rubra, S. aucuparia and U. glabra were considered as tolerant, and A. negundo, P. avium, P. serotina and Q. petraea were rated as possibly resistant due to the low rates of re-isolation.
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EPPO note: in several European countries (EU member states, Switzerland and the United Kingdom), Verticillium nonalfalfae is listed a regulated non-quarantine pest (RNQP).
Sources
Lechner Y, Maschek O, Kirisits T, Halmschlager E (2023) Further pathogenicity testing of Verticillium nonalfalfae, a biocontrol agent against the invasive tree of heaven (Ailanthus altissima), on non-target tree species in Europe. Phytoparasitica 51, 113-130.