Warm temperatures inhibit disease progression of <i>Verticillium nonalfalfae</i> in <i>Ailanthus altissima</i> , regardless of virulence

An isolate of the fungal pathogen Verticillium nonalfalfae sourced from infected Ailanthus altissima in Pennsylvania, USA (VnAa140) has demonstrated some variable efficacy as a bioherbicide, even though it was originally selected for its ability to consistently induce wilt symptoms and mortality in the host. We quantified the virulence of several VnAa140 isolates and tested whether host passage can restore virulence after attenuation. We also determined how warm temperatures impact fungal growth on media and influence its ability to induce wilt symptoms in the host. We found variable levels of disease caused by VnAa140, for which assessed values increased by a factor of 2.86 ± 0.44 after host passage while rates of A. altissima mortality likewise increased from 0.0 to 83.3%. These findings were confirmed on mature trees in the field by inoculating them with a highly virulent isolate and another formerly attenuated isolate. Additionally, 6-hr exposures to warm temperatures (27, 30, and 33°C) reduced the growth of VnAa140 on media. Disease severity in inoculated seedlings also declined when exposed to the same temperatures, and at 33°C, the V. nonalfalfae-inoculated seedlings were as healthy as the water-inoculated controls after eight weeks. VnAa140 can cause extensive disease in A. altissima, but it is constrained by warm ambient temperatures and excessive handling outside the host, limitations that are common to many bioherbicides. The success of Verticillium nonalfalfae as a bioherbicide to suppress invasive populations of Ailanthus altissima will be closely linked with the range of environmental conditions that are conducive to disease development.