Ailanthus’ Achilles Heel: A Poisoned Arrow in a Wilt Fungus

Contact: Aaron Cook | Aaron.Cook@Maryland.gov

ANNAPOLIS, MD (August 01, 2016) – Tree-of-heaven (Ailanthus altissima), also known as ailanthus, shumac, stinking sumac, stink-tree, copal tree, or Chinese sumac, is a common alien weed in many areas of the United States. Tree-of-heaven is a native of central China. It was first introduced into the United States in Philadelphia in 1784, and later into Rhode Island and New York. Chinese immigrants also introduced it into California in the 1850s. Initially valued as a horticultural specimen, the species was later planted as a street and shade tree. By the 1840s it was commonly grown by nurseries in the eastern United States. Its ease of establishment, rapid growth, absence of serious insect or disease problems, and tolerance of smoke and dust made it popular for urban planting. Its tendency to spread by both seeds and root suckers was frequently commented upon, but planting continued well into the 20th century in places such as Baltimore and Washington. From these areas, tree-of-heaven has gradually spread and become a serious weed in urban, agricultural, and forested areas where it increasingly invades our forests, displacing more desirable native trees. Red and yellow clusters of developing fruit among the trees' pinnately compound leaves and stout smooth branches are easily distinguished this time of year. Because of this, and the new hope for a biological control of this troublesome species, Ailanthus has been chosen as the MISC "Invader of the Month" for August.

Due to Ailanthus' rapid spread, and tenacious root sprouting, particularly after cutting, chemical control of tree-of-heaven is the only effective management method, but it can be costly — as much as $3500 per acre. Land managers have been seeking natural or biological control agents for Ailanthus to provide cost-effective alternative control options. Beginning in 2000, observers noticed extensive, unprecedented wilt and mortality of over 10,000 Ailanthus trees occurring within mixed hardwood forests in south-central Pennsylvania. In 2002, Penn State University researchers isolated the same pathogen again and again from symptomatic Ailanthus seedlings and trees in areas having high levels of mortality. Two species of wilt-causing fungal pathogens, Verticillium nonalfalfae, and the more common but less virulent V. dahliae, were each isolated from small scattered patches of diseased Ailanthus. In subsequent greenhouse and field tests, V. nonalfalfae killed 100% of Ailanthus; V. dahliadid not do as much damage.

Verticillium wilts occur on numerous plants including crops, weedy plants, and landscape trees, but V. nonalfalfae has proven to be very specific and pathogenic to Ailanthus. Verticillium wilt fungus often evolves host specific strains that become more virile as they develop. Penn State scientists believe that the strain of V. nonalfalfae they isolated developed over time as more and more Ailanthus invaded mid-Atlantic forests. Very few native species present in the same areas as diseased Ailanthus showed symptoms or died from Verticillium wilt. The understory trees striped maple and devil's walkingstick were among them, but only 1% of those trees got sick. Even deliberate inoculation with the pathogen of native trees like chestnut oak, northern red oak, red and sugar maples, white ash, and yellow-poplar failed to produce wilt symptoms. Additional studies on over 100 different potentially susceptible native tree species have confirmed these results. Plus, native herbaceous and woody vegetation was found to regenerate quickly and reestablish predictable native communities in the openings created by Ailanthus mortality. The lethal effectiveness and specificity of the fungus – killing Ailanthus but not harming natives — prompted support for the use of V. nonalfalfae as a potential biocontrol for Ailanthus.

Photos: Dr. Donald Davis, The Pennsylvania State University, used with permission.

Currently, V. nonalfalfae has been identified only in Pennsylvania, Virginia, and Ohio, not in Maryland, although foresters have found V. dahlia, the less virulent wilt fungus, in stands of Ailanthus in Allegany and Washington Counties. In order to test the mortality V. nonalfalfae might cause in Maryland tree-of-heaven copses, we have to find the fungus here. It is illegal to bring the pathogen across state lines without a permit from USDA APHIS, so it cannot be deliberately introduced from outside Maryland. In an effort to help observers locate areas of Ailanthus infected by V. nonalfalfae, university and federal researchers have described the symptoms of infection to look for in Ailanthus trees (see resources below). These symptoms include rapid or sudden wilting foliage throughout the entire tree followed by defoliation as leaves die. Infected vascular tissue in the sapwood, just beneath the tree's bark, will be an orange-brown color compared to the white to cream color of vascular tissue in healthy trees. If you see stands of tree-of-heaven exhibiting these symptoms, please contact the Maryland Forest Service or the Maryland Invasive Species Council.

Note that Verticillium wilt rarely impacts single trees, instead infecting large distinct areas of declining, dying, and dead trees rapidly. The fungus spreads outward through root-to-root transmission from infected to healthy trees, and can persist in the soil for many years. If V. nonalfalfae is found in Maryland, it will likely be in a rather isolated area. Recent research by entomologists at Virginia Tech suggests that a snout weevil (Eucrytorrhynchus brandti) and V. nonalfalfae may be able to work together to suppress Ailanthus over long distances because the weevil can carry and transfer V. nonalfalfae to Ailanthus. Laboratory studies using the snout weevil showed that tree infection occurred from the insects' feeding on foliage, and from their overwintering in soil below Ailanthus trees. Utilizing a weevil promises long-range release and spread to Ailanthus stands otherwise unreachable for short-range pathogen dissemination.

For more information about other Invasive Species of Concern in Maryland, visit the Maryland Invasive Species Council or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:Ailanthus Verticillium Wilt Photoguide, U.S. Forest Service (USFS).
Potential For Using Verticillium As A Biocontrol Agent For Tree-Of-Heaven (Ailanthus Altissima), The Pennsylvania State University.

Researchers:Matthew Kasson, mtkasson@mail.wvu.edu; Dr. Donald Davis, ddd2@psu.edu

References:Philip D Pannill, Tree-of-Heaven Control (Hagerstown: Maryland Department of Natural Resources, Forest Service). 
Donald Davis, et. al., Potential for Using Verticillium Albo-atrum as a Biological Control Agent for Tree-of-Heaven (Ailanthus altissima), [Abstract]. 2010 USDA Research Forum on Invasive Species, GTR-NRS-P-75 
Mark J. Shall and Donald D. Davis, "Verticillium wilt of Ailanthus altissima: Susceptibility of Associated Tree Species," Plant Disease 93:1158-1162