He walked through the garden when he noticed an odd canker on the chestnut tree. While this could have been a scene from the recent past, chestnut blight was discovered in the New York Botanical Gardens in 1904. Dutch elm disease was first identified in the U. S. in 1930 and was likely introduced into Europe from Asia during World War I. Both of these have resulted in significant losses in both of these species of trees. However, in recent years, through a combination of identifying resistant individuals and breeding programs, there have been efforts to give both species a second chance.
Savannah Ferreira, the Forest Health Specialist for the Vermont Department of Forests, Parks, and Recreation, stated that both American Chestnuts (Castanea Dentata) and American elms (Ulmus americana) are “severely impacted by the diseases.” In their native ranges, they have provided food, timber, habitat, and erosion control. Chestnuts were one of the most important food-producing trees for both large and small wildlife. Ferreira said that as chestnuts became less prevalent, they were replaced by other (and sometimes invasive species).
Both chestnut blight and Dutch elm disease are caused by fungi. Chestnut blight is caused the fungus Cryphonectrea parasitica and was accidentally imported from Asia. Ferreira said that the spores find a wound in the bark and grow through the cambium layer, where tree growth takes place, preventing water and nutrients from reaching the upper part of the tree and subsequently killing it. The symptoms include reddish brown patches that develop into swollen or cracked cankers, which kill twigs, limbs and eventually the tree. Yellow or orange masses of spores ooze from fruiting bodies on or around the cankers. It is spread by the wind, splashing rain, people transporting infested wood, animals, and can also persist in the root system for years. Ferreira said that the fastest form of transmission was typically through root grafts, with wind as the next fastest. She said animals can pick up spores, but they would have to carry it to another chestnut tree.
Dutch elm disease is caused by three species of fungus in the genus Ophistoma. It is spread by the European elm bark beetle and the American elm bark beetle. The beetles seek out dead or weakened trees and excavate a gallery in between the bark and the wood where they lay their eggs. If the fungus is present, adult beetles can carry it on their bodies when they emerge. When they feed on the leaves of the tree, some of the fungus can get dislodged. It colonizes the xylem, which is an inner layer of the tree’s wood, and as with chestnut blight, prevents water and nutrients from reaching the tree, thereby killing the tree. The leaves wilt and turn a dull green to yellow or brown and drop early. Ferreira noted that elms coevolved with the American elm bark beetle and could tolerate them more easily than the European variety. In addition, she also said that elms were not the only host for them either, which put less pressure on the trees. In addition to natural resistance, predation can also play a role in keeping pests in check. Ferriera said that research may not be available, but many beetles are preyed on by woodpeckers as well as parasitic wasps and flies.
In recent years, there have been efforts to bring both species back through a variety of breeding programs. The American Chestnut Foundation (TACF) crosses the American chestnut with the Chinese chestnut (Castanea mallisimo) for resistance to chestnut blight. Chinese chestnuts coevolved with the disease and are therefore, more easily able to tolerate it. Deni Ranguelova, the New England Regional Science Coordinator for TACF, acknowledged that the original plan wasn’t “as evolved” as it could have been, but said the main focus was on tolerance of the disease rather than resistance. In addition to chestnut blight, the foundation also breeds for tolerance to Phytopthora root rot, which is caused by the soil pathogen P. cinnamomi. PRR, as it is known, attacks the root system below ground, though it is primarily found in the southern part of the country. The foundation uses computer models with DNA profiles along with field measured responses to the diseases to predict which trees will be the most tolerant of the diseases. The top 10% are planted in seed orchards with the remaining 90% planted in offspring test sites. The trees are then evaluated in a variety of environments for blight resistance and also used to update computer models for selection accuracy. The hybrids are then backcrossed with pure American chestnuts with the goal of disease tolerance as well as maintaining 70% American ancestry.
Ranguelova said that in addition to the seed orchards and evaluation sites, some trees have been replanted in the wild, but these are not as closely monitored. She said some Japanese chestnuts have been used in the crosses, but that Chinese chestnuts were more cold tolerant and better suited to the North American climate. She said the foundation only breeds chestnut trees, but noted that all diseases “were important” and she would “volunteer for other organizations.” She also said that the hybrids were still considered American chestnuts, officially designated as Castanea dentata, and the goal was to restore the trees to their native range and fulfill their ecological role.
Regarding TACF’s work, Ferreira said that hybrids would “hopefully” allow the return of chestnuts to their native ecosystems. She noted that there are resistant elms for sale in some nurseries, which are less susceptible, but are not immune. She said scientists are breeding both resistant elms as well as nonresistant ones with other species of elms. She also said that there are “aspects” of hybrids that may fulfill certain ecological roles, but only the American chestnut can fulfill 100% of its ecological role and the same hold true for the American elm. American chestnuts can reach heights of 60-100 feet. By comparison, Chinese chestnuts generally only grow to about 30 feet.
Unlike pests such as the emerald ash borer and the spotted lantern fly, chestnut blight and Dutch elm disease, have been in North America for over 100 or nearly 100 years, respectively. So, can something be learned from “established” diseases in order to address more recent invasive pests? Ferreira said yes and emphasized the importance of diversification in urban and forest landscapes. She said that when the focus is only on one genus or species, there is an “increased risk of large scale impacts by invasive species.” Ferreira also noted that the “gears were shifting from reporting diseased trees to non-diseased or potentially resistant trees.” She recommended treesnap.org as a place where such trees could be reported in order to further efforts of breeding resistant individuals.
Invasive pests and diseases have been a serious threat for a long time. However, an understanding of the importance of biodiversity as well as how species can evolve to combat pests can go a long way to giving some species a second chance. For related topics, see my articles on Asian jumping worms from August of 2024, Asian carp from February of 2023, the spotted lanternfly from November of 2021 and endangered plants from May of 2023.
The Nature Journal 