Tree-of-Heaven (Ailanthus altissima) is an invasive tree in much of North America that was introduced from China (not Heaven, as the name might imply) as a garden ornamental in the late 1700’s (see North American geographic range here). This tree is particularly difficult to get rid of, because a single tree can make up to 325,000 seeds in a year, can easily grow 6 feet in a year, and can spread, even if reproduction through seed is inhibited or the tree is cut down, through vegetative sprouting. Currently, the best control measures are chemical.
Biocontrols are biological phenomena (such as disease and predators) that are used to combat unwanted non-natives. Biologists are currently exploring potential biocontrols for Ailanthus, and a disease called Verticillium albo-atrum was recently discovered that might hold promise. Mark Schall and Donald Davis at Pennsylvania State University noticed a stand of tree-of-heaven dying in southern Pennsylvania, and molecularly identified the pathogenic cause to be V. albo-atrum. Symptoms of the disease include yellowing, browning, and wilting of leaves or branches, in severe cases resulting in leaf loss and potential death. The disease spreads through wind-borne spores, and, after infection, the fungus invades the cells that transport water, dehydrating the plant.
Verticillium fungi are known to infect and cause leaf wilt and sometimes death in more than 200 plant species, including common trees and crop plants. Verticillium wilt is common in landscape trees, but uncommon in North American forests. A potential biocontrol cannot pose risk to other species in order to be legal. In initial studies inoculating trees that associate with tree-of-heaven, Schall and Davis report that V. albo-atrum caused severe disease in tree-of-heaven only, with the possible exception of striped maple. The strain of this disease that is infecting Ailanthus remains promising as a potential biocontrol, but the risk it poses to other forest species remains questionable. Few diseases that specialize on a particular tree alone cause complete host eradication (because then they would eradicate themselves), so even if Verticillium is a successful biocontrol, other control measures will need to be used with it in order to exterminate the invasive tree.