Can the invasive tree Ailanthus altissima be tamed with a native Verticillium fungus?
The invasive tree Ailanthus altissima is widely distributed in the eastern United States. It is allelopathic, a prolific wind-dispersed seeder, and displays aggressive clonal growth which can create dense thickets. Given its life history traits, traditional methods of control often fail. In 2003, the soil-born fungus, Verticillium nonalfalfae, was identified as the causal agent of large areas of dead and dying Ailanthus in Pennsylvania. Recently, the same fungus, native to North America, was isolated from dying Ailanthus in Virginia and Ohio. Pennsylvania researchers demonstrated that it selectively kills Ailanthus while not harming a wide range of native woody species. In 2013, we expanded the testing of this fungus in Ohio implementing greenhouse and field experiments to evaluate the susceptibility of Ohio seed sources of non-Ailanthus species to the fungus. However, our primary objective is to test whether V. nonalfalfae can eliminate Ailanthus thereby facilitating the reestablishment of native plants within degraded forest stands. In 2014, plots were established at five forested areas to characterize the response of stem-injected Ailanthus trees to the fungus; estimate its rate of spread; monitor for effects on non-target species; and assess the response of impacts on regenerating native and non-native vegetation.
Within the test plots, Ailanthus (>6 cm d.b.h.) represented 34% of the total basal area. Acer spp, (A. saccharum and A. rubrum) were the second most common group. The shrub layer was most often Lindera benzoin (60% of plots), followed by Rosa spp. (23%) and Elaegnus angustifolia (15%). Native tree regeneration was sparse, with tree seedlings representing only 9% cover within regeneration subplots. This suggests that native tree regeneration of any kind maybe poor due to the existing shrub layer. Preliminary stem-injection trials of Ailanthus with aqueous solutions containing fungal spores demonstrated that small trees (3.8-9.3 cm d.b.h.) display wilting symptoms within 2 weeks and die within 12 weeks. To date, no nearby woody or herbaceous plant species display any symptoms. These observations have been corroborated with our companion greenhouse inoculations of seedlings of oak, hickory, elm, ash and beech species in Ohio, as well as the >70 plant species tested in Pennsylvania. This native fungus shows great promise as a biocontrol agent of Ailanthus but further testing is needed to determine its safety. It has the added benefit of eliminating ecological concerns about introducing yet another non-native organism as a biological control agent.