Field-inoculated Ailanthus altissima stands reveal the biological control potential of Verticillium nonalfalfae in the mid-Atlantic region of the United States

Ailanthus altissima, perhaps the best-known example of an entrenched invasive weed tree in North America, was introduced to the Eastern U.S. roughly 240 years ago. The biological control of A. altissima has been a topic of interest since the discovery of a destructive naturally occurring Verticillium wilt disease of A. altissima in 2002. After nearly 20 years of research, an augmentative commercial release of this disease agent, Verticillium non-alfalfae, could be initiated in the near future. However, a few questions still remain: i) does the interaction of V. nonalfalfae with the less virulent V. dahliae inhibit the biological control effectiveness of V. nonalfalfae, and ii) do climate and A. altissima stand variables affect this biological control's efficacy? To help answer these questions, a three-year field inoculation study including 3,245 A. altissima trees in 13 sites across four hardiness zones of Pennsylvania and Virginia, U.S. was implemented. Disease progressed and spread at similar rates in A. altissima trees co-inoculated with V. nonalfalfae and V. dahliae as those inoculated with V. nonalfalfae alone, with no indication of disease progression changing in co-inoculated trees. Verticillium dahliae alone resulted in lower levels of disease, and no disease spread. Similar results were seen in a supplemental greenhouse inoculation study. Despite slight regional variation of disease progression and spread correlated to climate or stand variables, V. nonalfalfae always caused severe disease and spread rapidly to other A. altissima trees through the forested plots. Our results support the use of V. nonalfalfae as a biological control agent throughout the mid-Atlantic region of the U.S. regardless of stand and climate variables, and including sites where trees are already infected with V. dahliae.