| Tree-of-heaven [Ailanthus altissima (Miller) swingle], is an aggressive exotic invader of disturbed areas in urban and rural ecosystems throughout North America. Since Ailanthus was introduced into North America in 1784, the proliferation of this botanical pest has continued unchecked and Ailanthus is now found in nearly all 50 states of the USA. The need for a natural control of tree-of-heaven is of utmost importance, but the tree has few pests. However we recently observed several stands of Ailanthus that were wilting and declining in south-central Pennsylvania. Isolations revealed that two species of Verticillium, V. albo-atrum and V. dahliae, were associated with the disease. Inoculations of Ailanthus potted seedlings in the greenhouse, and canopy trees in the field, showed that both species were pathogenic on Ailanthus, but that V. albo-atrum was much more virulent and aggressive. It is now known that Verticillium albo-atrum is the primary pathogen causing extensive wilt of the invasive tree species Ailanthus in south-central Pennsylvania. The potential for using V. albo-atrum as a biocontrol agent is discussed.;Inoculation studies in both the greenhouse and field showed that Ailanthus was extremely susceptible, but that the host range did not extend to northern red oak, chestnut oak, red maple, sugar maple, yellow-poplar, and white ash were not susceptible hosts. However, striped maple exhibited wilt symptoms following inoculation, and V. albo-atrum was isolated from one wilted striped maple in the field. Ailanthus trees inoculated with V. albo-atrum died within the same growing season. In contrast, some Ailanthus trees inoculated with V. dahliae survived fro 2 or more seasons. Soil persistence as melanized hyphal resting structures may not play a significant role in the survival of the pathogen in south-central Pennsylvania, due to the highly acidic forest soils, which are unfavorable for persistence of V. albo-atrum in the soil. The pathogen likely survives within infected leaves on the forest floor, or possibly symptomless carriers.;Survival can occur within overwintering infected Ailanthus trees, on fallen Ailanthus leaves, or in soil, and possibly in symptomless hosts. Primary inoculum, in the form of germinating resting structures or conidia, is formed in spring, and inoculation takes place at time of leaf emergence. Dissemination may involve wind disseminated leaflets, seed transmission, and/or ambrosial beetle transmission. Dissemination by root grafts needs to be studied. Ailanthus altissima roots are predisposed to V. albo-atrum by wounding. During colonization, phenols are deposited in outer xylem parenchyma within one week of inoculation indicating that initial fungus colonization is circumferential. Colonization then proceeds both upward (rapidly) and downward (slowly) until host mortality occurs. In the field, rate of spread from tree to tree is rapid. Since 2000, approximately 10,000 Ailanthus trees have died in southern Pennsylvania from Verticillium wilt, likely mainly due to V. albo-atrum.;During the early 1980s, Ailanthus invaded an oak-dominated forest that had been logged and subsequently dominated stand openings, encompassing nearly 40% of stand basal area. In 2000 the stand was attacked by V. albo-atrum from an unknown source. By summer 2007, the pathogen had killed more than 7000 canopy Ailanthus trees in that stand. In severely affected parts of the stand, the entire overstory component of Ailanthus was removed and approximately half of the understory Ailanthus seedlings or sprouts were killed. Following decline of Ailanthus, the canopy composition is beginning to revert to a native state, as residual trees left from the salvage harvest expanded their crowns and young red maple and black birch trees are recruited into the overstory. The large openings caused by dying Ailanthus were filled by seedlings of red maple, sweet birch, and striped maple.;In conclusion, we determined the causal agents of Verticillium wilt of the invasive Ailanthus tree in southern Pennsylvania to be caused by V. albo-atrum and V. dahliae. Of these, V. albo-atrum is far more virulent and aggressive on both seedlings and canopy trees of Ailanthus. Within the forest, dissemination of Verticillium likely involves windblown leaves, seeds, and ambrosial beetles. Host range is very limited. Verticillium albo-atrum did not require wounded roots to incite disease on Ailanthus seedlings, though root wounding did accelerate symptom development. In naturally infected dense stands of Ailanthus, V. albo-atrum is moving rapidly, and is drastically altering forest composition, indicating a high potential for use in biocontrol applications. |
