The natural persistence and distribution of the proposed biological control agent Verticillium nonalfalfae on Ailanthus altissima in Virginia, USA

Reports of Ailanthus altissima stand declines in south‐central Pennsylvania resulted in the identification of the causal agent, a vascular wilt fungus, Verticillium nonalfalfae . Additional surveys throughout North America and Europe found that this disease outbreak was not an isolated event. One of these surveys identified and monitored six naturally diseased A. altissima stands in Virginia, USA, between 2009 and 2012. To help predict the long‐term outcome of infested stands, these naturally infested sites were monitored again from 2017 to 2018. While A. altissima were still present at all six sites, a reduction in the number of living A. altissima per hectare (from 604 to 66 overall) and A. altissima basal area per hectare (from 16.1 to 2.9 m 2 /ha overall) was consistently seen. At all but one of these sites, V. nonalfalfae was still causing disease on the remaining A. altissima . This reduction in A. altissima and persistence of V. nonalfalfae highlight the effectiveness of the pathogen at controlling this tree over long periods of time. A walking survey looking closely at all A. altissima stands along roads within 1 km of these sites found additional V. nonalfalfae infections around four sites. Since no correlation existed between tree infection status and distance from the known site, the patchy disease occurrence may indicate this pathogen is able to spread over longer distances or is more abundant than the previous survey suggested. Soil collected at these sites did not infect any A. altissima seedlings in a greenhouse study, suggesting that soil inoculum may not play an important role in V. nonalfalfae persistence. This research further supports the use of V. nonalfalfae as an effective biopesticide that can reduce A. altissima numbers consistently over many years.