<div><i>Ailanthus altissima</i> is a highly invasive exotic tree species. In 2002, a vascular wilt was discovered in <i>Ailanthus </i>stands in Pennsylvania. Isolation from symptomatic trees revealed the causal agent as <i>Verticillium nonalfalfae</i>. Since its discovery, studies have investigated its potential as a biocontrol focusing on efficacy, host specificity and disease transmission. As efforts to commercialize <i>V. nonalfalfae</i> intensify, several key questions need to be addressed: (1) does the naturally co-occurring and less virulent <i>Verticillium dahliae</i>, which also infects <i>Ailanthus</i>, alter Verticillium wilt progression; (2) how do environmental conditions associated with hardiness zones influence the individual contributions of each of the two <i>Verticillium</i> spp.; and (3) does continued disease pressure in long-established disease epicenters select for tolerant / resistant <i>Ailanthus</i> genotypes that correlate with morphological differences? Preliminary data from stands with co-occurring <i>Verticillium</i> spp. may indicate that synergistic interactions are facilitating enhanced disease progression. With regard to putative tolerance / resistance, re-visitation to sites inoculated with <i>V. nonalfalfae</i> from 2006-2008, where the fungus continues to infect trees, revealed established asymptomatic intermediate size saplings. Re-inoculation, symptom development and death of these residual saplings would indicate these stems merely escaped the initial infection and, as such, are not more tolerant or resistant to the disease. Conversely, failure of disease development would support tolerance and invite genotypic and morphological (vessel size and tyloses formation) comparisons to help explain differences in susceptibility.</div>