Georgofili World

Newsletter of the Georgofili Academy

Will a fungus save us from the ailanthus invasion?

Imagine everything happened merely because of a mistake when (around 1750) the French clergyman Pierre d’Incarville brought to Paris from faraway China a stock of seeds that he thought were those of the prized lacquer tree (Rhus verniciflua) but were instead ailanthus seeds (Ailanthus altissima, sin. A. glandulosa). Thus begins one of the most important biological invasions of the plant world in the modern times. This neophyte was valued in Europe for its beautiful appearance (it can reach over 20-25 m in height with an elegant straight habit, and a trunk diameter of more than 1 m; its foliage is pleasing to the eye and is reminiscent of walnut and ash trees).  Its hardiness is incomparable (it has no acclimatization problems as regards climate or edaphic factors). It is easy to cultivate and grows exceptionally fast. It is perfect for beautifying streets and parks, reinforcing slopes, and colonizing difficult, unstable terrain. One episode that contributed to its dissemination was when the breeding of the classical silk worm (Bombix mori) underwent a health crisis for problems and another lepidopteran was introduced from the East: the ailanthus silkworm, Samia cynthia, which feeds on the ailanthus (by the zoologist Paolo Savi in Pisa in 1856). Thus, an alternative sericulture chain was set up, with this tree becoming even more popular. In Florence, a “Società Ailantina Italiana” was even set up to encourage the spread of this plant species. However, this situation did not last for long. While Pasteur solved the Bombix infection problem – which was pébrine, a disease caused by the protozoan Nosema bombycis, the quality of the silk produced by the Samia left much to be desired. However, by then this plant had established itself and conquered ever more new areas. In the meanwhile, it had also reached the USA (starting from Pennsylvania in 1784), imported by English settlers and even there it was appreciated. A second line of introduction also occurred in California in the mid-19th century at the hands of the Chinese who emigrated to work on the transcontinental railway (but also to search gold!). The Ailanthus is a true cult for the Chinese, who use all its parts in their traditional medicine. The list of health problems that find a speedy and satisfying solution in extracts of its bark, leaves, fruit, and wood is endless. Strangely enough, this topic has never been tackled in real terms elsewhere on the planet even if by now the complex pool of secondary metabolites present in the various organs is well characterized. Within a few decades, this species has been naturalized on all the continents, and become a problem everywhere. Its favorite habitat is such open areas as railway lines and communication routes, abandoned settings, and marginal stations with a high anthropic disturbance.
Ailanthus is by no means a “banal” tree and its true distinctiveness peculiarity lies in its capacity to grow side-shoots and root-suckers. Starting from the truck, surface roots, sometimes tens of meters long, are differentiated and from them suckers easily branch off. Removing the trunk triggers quick and stubborn differentiation reactions of buds from the stump and roots so that, paradoxically, “the more it is cut back, the more the production of new biomass is stimulated”. The average lifespan is estimated at a hundred years maximum but, exactly due to its capability to propagate via underground stems, suckers or stolons, it is actually considered “immortal” in the sense that its vitality does not end with the death of the original trunk, as it has yielded a pure clonal colony that, if undisturbed, can even cover a 4,000 square meter area. It has a marked allelopathic activity, which grants it further advantages as regards competition. Its hardiness is proverbial, making use of an exceptional phenotypic plasticity that lets it adapt to a wide range of thermal and water regimes. It is a wonderful example of a “water saving mechanism”, through modifications of water conductance of the roots and evapotranspiration processes. It easily deals with critical situations, such as those of simple cracks in stone materials. It is not interested in substratum characteristics (either natural or human-altered). It can grow in extreme conditions, like granite quarries. It is resistant to urban pollution and to herbivores and is actually free from attacks by pests, at least in Europe (the above-mentioned lepidopteran Samia cynthia has been a mere scientific curiosity for some time, a real “prize” for photo-naturalists.
Like few others, the Ailanthus is an invasive pioneer plant (it successfully rivals with the Robinia pseudo-acacia).  Not accepted by ungulates, it threatens the biodiversity of natural and anthropized forests, easily establishing mono-specific plant formations. Once introduced into environments with a high natural importance (e.g., the Isle of Montecristo), it rapidly replaces the indigenous flora, jeopardizing the conservation of native biocenoses, and forcing difficult (and usually useless) eradication campaigns. The presence of stands along traffic routes is unacceptable because of the problems linked to the reduced visibility and the invasiveness of the foliage and roots that damage road surfaces. In Tuscany, the Regional Law 56/2000 forbids the use of this species for reforestation, re-vegetation, and reinforcement works.
Researchers have focused attention on finding strategies to counter the growth of this invasive plant in such sensitive areas as archeological sites, railway lines, roadways, natural parks, and cities. Without a doubt, there are ecological, economic, and cultural reasons urging the adoption of effective measures of eradication. But how can it be done? Some key points: the ailanthus is like an iceberg, since most of the problem is underground! A simple (manual) eradication and/or a mechanical cut at the base of the trunk or girdling are totally ineffective (even if they are useful for neutralizing highly prolific seed-bearing plants). On the contrary, they stimulate the tree to rapidly produce an abundance of vigorous and aggressive young runners and suckers. The above-mentioned actions must be accompanied by the periodical use of systemic herbicides that can be transported to the root system and to compromise (alas, only partly) future vegetative renewal. However, recent regulatory measures have made these interventions impossible. On the one hand, the Tuscan Region (Resolution 821 of 4.8.2015) has forbidden all non-agricultural use of glyphosate-based products, the universally acknowledged ingredient indicated as the most suitable for such treatments. On the other, the National Action Plan for the sustainable use of plant protection products (D.M. 22.1.2014)  is leading to serious limitations in the use of pesticides on roads (Action A.5.5) and in population centers (Action A.5.6); more specifically, Point A.5.6.1 (Use of herbicide plant-protection products) says that “weed-killer treatments are banned and have been replaced with alternative methods in population centers… in the case of exemptions  plant-protection products cannot be used if the label bears the following expressions of risk … R41” (i.e., risk of severe eye lesions, precisely the one referring to Glyphosate). What can be done, then?
A careful examination of the abundant specialized literature on this subject points out an interesting novelty. In the USA, for some years now, a case of the ailanthus naturally dying -off has been under investigation (tens of thousands of trees in Pennsylvania and in some neighboring states). It has been attributed to the attacks of a microfungus, new to the sciences, and described as an (almost) host-specific strain of Verticillium nonalfalfae (originally referred to V. albo-atrum). This pathogen is a tracheomycosis agent (thus able to move within the vascular system to reach the suckers and to transfer from an infected individual to a healthy one by grafting root) and is specialized on the Ailanthus. It leads to the rapid death of the affected subjects and seems a perfect mycoherbicide candidate. In vitro breeding techniques and artificial inoculation protocols are now ready. Once the Verticillium infections have spread in a stand, the disease can progress independently, starting from the inoculation produced by diseased trees. The role of bark beetles in the spread of propagules has been determined.
It is easy to dream about the possibility of introducing this organism and using it for effective, economical, and eco-sustainable actions to finally counter the ailanthus’s otherwise unrestrainable advance. Moreover, the idea of using plant pathogens as contrast agents of infesting and invasive plants is as old as the science of plant pathology itself. Unfortunately, practical applications are rather limited, given the remarkable difficulties that the technique entails. Obviously, a rigorous risk assessment would be indispensable, aimed first of all at ensuring that the pathogen does not also infect the non-target plants. However, have we perhaps found a solution?