If you were to step onto the village green of almost any town in the United States 100 years ago, chances are you would walk in the shade of the American elm; if you turned down Elm Street, you’d likely walk beneath rows of century-old elms, their vase-like canopies arching gracefully upward, their branches uniting to create a cool, green promenade on the street below. If you were to take a stroll through the woods at that time anywhere east of the Mississippi, you’d probably be walking through a forest dominated by the American chestnut, giants whose nuts nourished forest animals and humans alike, and whose wood was unsurpassed for furniture, home construction, railroad ties, and poles to carry wires for the newly invented telephone.

Together, the elm and chestnut were the unrivaled kings of the arboreal landscape of 19th-century America. Now, 100 years later, the chestnuts are gone, and it is a rare Elm Street that is still lined by elms. Both have fallen victim to fungal invaders from abroad.

The chestnut fell first, and hardest. The fungus responsible for chestnut blight was discovered in the New York Zoological Park in 1904, though it had almost certainly been imported earlier, probably on Japanese chestnut trees imported as ornamentals. Within 20 years, the fungus had spread across 80% of the chestnut’s range; within another twenty, four of every five chestnuts were dead, the hangers-on reduced to stumps whose shoots might continue to sprout and die back for decades.

In 1930, Dutch elm disease arrived in Ohio on logs imported from Europe, where the fungus had first been discovered a decade before. Carried by both the American and the newly arrived European bark beetles, the fungus was quickly and widely dispersed. Within decades, most of the elms in the country were dead.

Both fungi (members of the Ascomycetes, or sac fungi) grow into xylem, blocking water transport from root to leaf. Reproductive spores of both fungi enter through wounds or cracks in the bark. Infected chestnut trees are girdled by the fungus, killing the tree above ground, but preserving the roots. In contrast, the elm fungus infects both root and shoot, killing the tree completely. While chestnut blight relies on broken limbs or other wounds to create its entry sites, and on the wind to disperse its spores, Dutch elm disease has an unwitting ally in the two species of bark beetle, which bore under the bark to feed and lay eggs. Spores carried on their bodies germinate and infect the tree, eventually reaching the roots; the resulting systemic infection rapidly kills the tree. The dying tree attracts more egg-laying beetles, whose emerging young disperse the fungus ever more widely.

Despite the devastation wreaked by these fungi, the elm and chestnut still survive today. Sprouting chestnut stumps can still be found in Eastern woodlands, and in some island refuges there are even a few full-size trees never touched by blight. Elms have fared somewhat better, both because of their own prolific and early seed production, and their urban human protectors–intensive systemic fungicides can keep trees alive and healthy even where the fungus is endemic. Look closely at the base of a specimen tree in an urban park, and you’re likely to see a ring of quarter-size holes, scars left by low-pressure injector pumps. Protected from infection, cultivated elms may live for many decades, although the yearly accumulation of scar tissue eventually dooms them. In the wild, unprotected elms rarely live longer than 20 years.

In time, both trees might develop resistance to their fungal predators, but a number of concerned groups and individuals want to speed that process up. The Elm Research Institute has developed the Liberty elm, a new variety of American elm created by cross-breeding among six naturally resistant trees. Using vegetative propagation, they have raised and distributed more than a quarter million Liberty elms nationwide; so far, fewer than 30 have developed Dutch elm disease.

The American Chestnut Foundation is in the middle stages of an even more ambitious project: their goal is to breed a variety of resistant strains, enough to ensure success in the diverse conditions of the original chestnut forests. American chestnuts are first crossed with naturally resistant Chinese chestnuts, breeding in the resistance genes. These trees are then repeatedly backcrossed with Americans, diluting out the other less desirable characteristics of the Chinese chestnut. Four crosses leave the tree only one-sixteenth Chinese, resistant to chestnut blight but so similar to the American chestnut that only experts on the genus can tell the difference. The process is effective, but requires patience–the full series of crosses takes almost three decades to complete. The first group of trees is expected to be ready for distribution around 2010.

Neither group is ready to claim victory over these two fungal scourges, and both recognize the possibility that the fungi will evolve to thwart their best efforts. But after nearly a century in retreat, there is finally hope for the two giants of the American landscape, and the elm and chestnut may tower again over the streets and woodlands where they once were kings.

For more information, visit:

The Elm Research Institute

The American Chestnut Foundation

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