Intraspecific Root Grafts and Clonal Growth Within Ailanthus altissima Stands Influence Verticillium nonalfalfae Transmission

Verticillium nonalfalfae, causal agent of Verticillium wilt, is being considered as a biocontrol for the highly invasive Ailanthus altissima in Pennsylvania. This soilborne fungus is extremely virulent on Ailanthus and rapidly transmitted from diseased to healthy trees within Ailanthus stands. The rapid transmission of the fungus could be facilitated by root grafts, but neither root graft formation in Ailanthus nor Verticillium transmission by root grafts in trees has been reported. Here, V. nonalfalfae transmission between diseased and healthy Ailanthus trees via intraspecific root grafts and clonal growth is evaluated. Using air-spade excavation, dye translocation, and root graft inoculations, functional root grafts were detected between Ailanthus trees and transmission of V. nonalfalfae across root grafts demonstrated. Inoculation of one Ailanthus parent stem resulted in 187 root sprouts showing Verticillium wilt symptoms 12 months after inoculation. This study revealed that clonal growth and root grafts, normally advantageous growth habits, leave Ailanthus stands vulnerable to widespread V. nonalfalfae infection. This study also broadens the understanding of the Ailanthus-Verticillium pathosystem, growth strategies of invasive Ailanthus, and epidemiology of Verticillium wilt within trees.