There is an army of naturally-occurring pathogens active in our forests, parasitizing living trees but ordinarily without decimating entire populations, reflecting a long-term evolutionary coexistence in native ecosystems between host and pathogen. However when you weigh in the introduction of pathogenic fungi and bacteria from foreign lands that come armed with genes never “seen” before by our native trees, losses can be severe. This post reviews the threat of disease affecting four native tree species and another soon-to-come posting will describe several others.
THE AMERICAN CHESTNUT
The American chestnut (Castanea dentata) once a mainstay of our eastern forests, anchored a food network that supported squirrels, chipmunks, turkeys, grouse, and other granivores as well as their primary and secondary predators. Many humans, both indigenous and immigrants, depended on chestnuts for food and timber and as livestock food.
The chestnut once comprised a quarter of all eastern hardwoods and occurred over nine million acres in North America! However a disease caused by an Asian fungus (Cryphonectria parasitica, until recently Endothia parasitica) was accidentally introduced to the New World in the early 20th century in a shipment of Japanese chestnut trees. The pathogen had a devastating impact on Castanea dentata. The blight spread rapidly as C. dentata showed no resistance to the fungus. By 1930, most trees east of the Mississippi river were dead. The tree was essentially written off. When not killed outright, as many millions of trees were, chestnuts became rare, surviving in forests they once dominated (especially in the widespread oak-chestnut forest type of uplands and dry slopes) as mere stump sprouts that were repeatedly killed back by the ravaging chestnut blight. The closely related Chinese and Japanese chestnuts are resistant to the blight and so have been widely planted. But their nuts are less tasty and their stature as mature trees less impressive. Furthermore they cannot successively compete with oaks, hickories and other American trees and so tend to die-out in forests.
THE TALL AMERICAN ELM
The tall American Elm (Ulmus americana), often described by admirers as stately or graceful, occurred naturally in eastern and midwestern river bottoms and rich forests. Extensively planted in parks and along streets of cities and villages, elms sometimes formed long rows that arched over the streets shading pedestrians and motorists. The largest tree of its genus and often surpassing all other floodplain trees, the American elm was severely set back throughout its range by a fungus (Ceratocystis ulmi) accidentally imported from Europe around 1930. The disease spread rapidly, extending into Canada and reaching California by 1975.
The elm disease fungus grows rapidly in the wood vessels through which water is transported from roots to the canopy, ultimately cutting off the water supply to the leaves. The fungus also appears to produce a toxin that directly kills tree cells. Fungal spores adhere to bark beetles that bore into the bark, carrying along the spores. Because the adult beetles are winged, Ceratocystis gets direct passage to other elms, a more efficient vector than the wind.
In recognition of their value, efforts are underway to revive both the chestnut and the elm. A chestnut variety that combines American chestnut growth and adaptation with resistance to Chestnut Blight derived from genes originating from the Chinese chestnut is showing promise in young trees planted in wild and managed environments. A modest number of nuts has already been harvested! In addition, nearly two decades after Cryphonectria was introduced to Europe, French pathologists isolated a less damaging (“hypovirulent”) strain of the parasite which carries a virus that can destroy virulent fungal strains. Although the weakened strain is less aggressive than the lethal one, and its spores are dispersed less widely, it offers some hope that the blight can be arrested.
Resistant varieties of American elm were introduced around 1996, offering hope the tree may recover as these natural descendents of wild American Elms so far show excellent resistance to Dutch Elm Disease in arboretum outplantings (www.elmpost.org). For both trees it will take decades extending across human generations to populate our landscape once again, and even if restoration programs are successful, the future trees will feature a novel genetic makeup and perhaps will not be seen again as natural forest denizens.
Less known than North America’s two most recognized tree tragedies, other native American tree species also are succumbing to pestiferous invaders. Butternut (Juglans cinerea) is an uncommon native tree scattered throughout rich woods in eastern North America from the southern Appalachian piedmont and mountains to southern Ontario and New Brunswick. Never common, butternut (also known as the white walnut but not to be confused withthe supermarket offerings which are English or Persian walnuts, J. persica) is esteemed for its tasty nuts that to many, are superior to the better known black walnut (J. nigra).
The species is subject to butternut decline (also called butternut canker) which has been observed only in the past decade in Wisconsin, Minnesota, Iowa, and Illinois, but has been known as early as 1967 in other American forests. In fact, butternut is now at risk throughout its entire range. Symptoms include trunk and branch cankers with subsequent crown dieback. The term canker refers to exterior patches of dead tissue that spread along the trunk and /or branches, representing the visible manifestation of fungus attack on the living tissues within. Trees injured by butternut decline do not recover. The principal pathogen involved is a species in the genus Sirococcus, a fungus which produces adhesive spores that gain access to living wood by attaching to burrowing beetles that enter limb wounds or bark openings. One Vermont study found that 17 beetle species can do this. The beetles also transport the spores to other trees by flight. Wind and rain and probably birds also are agents of dispersal. The origin of the fungus is uncertain but likely is Asia given the resistance of Asian walnuts to the fungus. Trees weakened by Sirococcus often become vulnerable to other fungal pathogens such as the aggressive root-rot fungus Armillaria. Click on FUNGI on this website to see photographs of an Armillaria colony parasitizing a dying red maple tree in my yard.
na.s.us/spfo/pubs/pest_al/bd/butter.htm ; ont-woodlot-assoc.org.
Flowering dogwood (Cornus florida) is a widely admired small native tree of the eastern U.S. that is frequently planted in yards in recognition of its large white (sometimes pink) flower bracts. It’s been known to be declining in large parts of its range due to attacks by anthracnose, a fungal disease believed to have originated in Asia. Leaf spotting and blotching and eventually leaf death are the visible symptoms that often progress to twig and even stem cankers. The disease was first recognized in the northeastern states but rapidly spread westward and southward. A 2012 study estimates that dogwood have decreased by abut 50% across the southeastern states!
In addition and independently of the anthracnose, dogwood are declining or dying in many forests that have recovered from past logging or clearing where overstory trees like sugar maple shade the dogwoods and compete with them for water and soil nutrients. This trend is believed to be a natural one so management interventions usually are not implemented to benefit the dogwood. In parks, yards and other spaces where dogwoods have been planted, and increasingly in wooded areas, however, anthracnose is arguably the major threat to survival of the species and efforts have been underway to use pesticides and sanitation (removal of diseased branches or whole trees) and other approaches to increase vigor of diseased trees. www.hort.uconn.edu ; news.uns.purdue.edu/html4ever/010427.Rabenold.dogwood.html.