The natural persistence and distribution of the proposed biological control agentVerticillium nonalfalfaeonAilanthus altissimain Virginia, USA

Reports ofAilanthus altissimastand 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 diseasedA. altissimastands 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. WhileA. altissimawere still present at all six sites, a reduction in the number of livingA. altissimaper hectare (from 604 to 66 overall) andA. altissimabasal area per hectare (from 16.1 to 2.9 m(2)/ha overall) was consistently seen. At all but one of these sites,V. nonalfalfaewas still causing disease on the remainingA. altissima. This reduction inA. altissimaand persistence ofV. nonalfalfaehighlight the effectiveness of the pathogen at controlling this tree over long periods of time. A walking survey looking closely at allA. altissimastands along roads within 1 km of these sites found additionalV. nonalfalfaeinfections 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 anyA. altissimaseedlings in a greenhouse study, suggesting that soil inoculum may not play an important role inV. nonalfalfaepersistence. This research further supports the use ofV. nonalfalfaeas an effective biopesticide that can reduceA. altissimanumbers consistently over many years.