Biological control of Ailanthus altissima: Transmission, formulation, and risk assessment of Verticillium nonalfalfae

Open Access
O'neal, Eric Scott
Graduate Program:
Plant Pathology
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 02, 2014
Committee Members:
  • Donald Durwood Davis, Thesis Advisor
  • Tree-of-heaven
  • Ailanthus
  • root grafts
  • Verticillium wilt
  • Verticillium nonalfalfae
  • biological control
  • formulation
Tree-of-heaven (Ailanthus altissima), first introduced to the United States into Philadelphia in 1784, is a very aggressive and invasive weed in both forest and urban environments, especially within the Mid-Atlantic region. In 2002 to 2003, in south-central Pennsylvania, Verticillium nonalfalfae, a soilborne, vascular wilt pathogen, was found naturally killing thousands of Ailanthus trees. To date, V. nonalfalfae has been isolated from dying Ailanthus in Pennsylvania, Virginia, and Ohio. Verticillium nonalfalfae causes typical Verticillium wilt symptoms on Ailanthus, including wilt, yellow vascular discoloration, defoliation, and mortality. The fungus naturally spreads rapidly through Ailanthus stands from diseased to healthy Ailanthus trees. Due to its efficacy and relatively small host range, V. nonalfalfae isolate VnAa140 has been proposed as a biological control for the invasive Ailanthus in Pennsylvania. In this thesis there were two main objectives. The first objective was to determine if intraspecific root grafts and clonal root systems in Ailanthus stands play roles in rapid local V. nonalfalfae transmission. The phenomenon of root grafting has been reported in >100 tree species and can aid in the transmission of pathogens from diseased to healthy trees, for example, those causing oak wilt and Dutch elm disease. Ailanthus altissima has not been reported to form root grafts, nor has it been reported that Verticillium spp. can be transmitted by root grafts in trees. Understanding the transmission of this potential biological control is important for optimized deployment and assessing risk. Also, investigating root grafting in Ailanthus altissima will allow for greater understanding of its invasion strategy and biology. The second objective of this thesis was to develop a simple formulation and delivery system for V. nonalfalfae isolate VnAa140 and explore the use of natural V. nonalfalfae inoculum sources, while further investigating this potential biocontrol agent’s safety. The level of control achieved by V. nonalfalfae surpasses herbicide treatments currently in use; however it is not formulated for anticipated expanded use by land managers. The development of an optimized formulation and delivery system for V. nonalfalfae will allow for more effective and efficient control of Ailanthus than conventional herbicide control treatments. In addition, developing simple inoculation methods utilizing natural inoculum sources will allow land managers to use V. nonalfalfae without complete dependence on laboratory-prepared inoculum. Lastly, the non-target plant species composition and health was reassessed within the Verticillium wilt epicenter where V. nonalfalfae isolate VnAa140 was originally isolated. In this area of Tuscarora State Forest, V. nonalfalfae has persisted and killed mature canopy Ailanthus trees for over a decade. This assessment will help to further investigate this biological control agent’s risk, efficacy, and host-adaptedness to Ailanthus.