Epidemiology and Ecology of Verticillium nonalfalfae: Application and intervention efforts to successfully manage tree-of-heaven and restore invaded sites

Abstract

Ailanthus altissima, or tree-of-heaven, is a problematic invasive tree species distributed around the world. A. altissima was recently found to be susceptible to a soilborne vascular wilt fungus, Verticillium nonalfalfae, in both the United States and Europe. Research to date on its potential as a bioherbicide has been promising, though field inoculations have been limited in scope and conducted primarily with local isolates. A specific isolate from Pennsylvania, VnAa140, has been petitioned to the federal government for commercialization. We conducted inoculations across Virginia using VnAa140 to test its efficacy in an expanded range and, firstly, found that our isolate was significantly attenuated. We then conducted re-inoculations with a new isolate, which induced much higher levels of disease. However, higher disease severity and mortality were limited to a few sites, and we found that efficacy was negatively impacted by warm temperatures. We also revisited former inoculation sites to quantify the understory plant community in stands of declining A. altissima. These areas had a significant contingent of other invasive species, so we compared this natural regeneration approach with an active revegetation strategy in our own inoculated stands. The native species we planted established at low rates, but we did successfully increase native species richness and similarity to the reference communities. Finally, we compared the virulence of several isolates of V. nonalfalfae and tested whether host passage could restore virulence to an attenuated isolate. We found variable levels of disease as a result, but that virulence could indeed be restored and evaluated two of these isolates in the field with an additional round of inoculations. We also quantified the impact of warm temperatures on the growth of the fungus and its ability to cause disease, finding that even the most virulent isolate is highly sensitive to heat. V. nonalfalfae continues to have promise as a potential bioherbicide and a crucial component of intensive restoration efforts to invaded forests, but isolates must be carefully selected and maintained, its efficacy will be limited by certain environmental conditions, and mitigation of other invasive species will be crucial to the success of restoration efforts.