Range Expansion of a Verticillium nonalfalfae Isolate Suppresses Ailanthus altissima With Variable Results Along Environmental Gradients

Verticillium nonalfalfae has been studied for > 20 years as a potential fungal bioherbicide for Ailanthus altissima. A specific isolate from Pennsylvania, VnAa140, has desirable virulence properties and demonstrated host specificity. However, it has yet to be tested outside Pennsylvania, and the potential for regional variation in the host–pathogen interaction and possible environmental limitations are unknown. The range of VnAa140 was expanded for the first time by inoculating A. altissima across 10 sites in Virginia that spanned three physiographic provinces and four USDA hardiness zones with the intent of quantifying disease progression in inoculated trees and the spread of the pathogen throughout A. altissima stands. However, initial inoculations resulted in very little disease development. Reinoculations the following year induced much higher levels of disease and mortality, including 52.3% ± 4.2% increase in disease levels of inoculated trees and a 27.2% ± 10.2% increase in symptom development among uninoculated trees, a marker of pathogen spread. However, results ranged widely along macro and microclimatic gradients, with higher site average temperatures and warmer microsite conditions correlating negatively with disease levels. While VnAa140 remains a strong bioherbicide candidate for the control of A. altissima, it appears that variability by culture and attenuation of virulence traits also contributed to the different levels of host disease development we observed in the field, suggesting that procedures to prevent and screen for these issues will be required as the fungus is propagated for bioherbicide formulation on a commercial scale.