Pathogens - Chestnut Blight



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Chestnut blight fungus (Cryphonectria parasitica (Murrill) Barr or Endothia parasitica)

DESCRIPTION:  C. parasitica is a fungus which infects American chestnut (Castanea dentata) trees through wounds in the bark.  Its spores can be spread by wind, rain, insects, birds and other animals.  Once inside the bark, the fungus multiplies rapidly, moving through connective tissues and making sunken cankers which expand and girdle the stem, eventually cutting off nutrient transport.  All tissue above the canker is killed, usually within one growing season. (25)

INTRODUCTION HISTORY AND DISTRIBUTION:  A native of Asia, the blight is believed to have been brought into the U.S. on resistant Japanese or Chinese chestnut trees imported as nursery stock for ornamentals.  Though first identified at the Bronx Zoo in 1904, an earlier introduction to the United States is more likely since the blight would otherwise have been unable to spread so rapidly. (25)  Because it had never before been exposed to this fungus, the American chestnut was highly susceptible; the species was quickly devastated throughout its natural east coast range, extending over the Appalachian hills and highlands from Maine to Georgia.  Even where all the American chestnuts have been killed, the blight fungus is still present today. (26) 

IMPACTS ON ECOSYSTEM:  The introduction of chestnut blight (Cryphonectria parasitica) to North America resulted in marked change in the forest structure of vast areas of the eastern United States.  In less than 50 years, the fungus killed 3.5 billion chestnuts, permanently changing the composition of the forest landscape. (25)  The blight effectively removed 25% of the area’s forest cover; the impact this had on ecosystem diversity, composition, and wildlife cannot be overestimated.  Chestnut blight is one of the most destructive plant diseases ever recorded. This fungus has forever changed the forest tree composition in the eastern United States. (2)  American chestnuts survive mostly as small understory trees that sprout back from the original root systems and are continuously attacked by the blight.  (26)  The disease does not affect the roots of the tree and each time a tree dies, sprouts grow from the old stumps of the diseased trees, but growth does not occur for more than 15 feet. (27) There has been little chance for resistance to evolve in American chestnut, since the sprouts that come up are often killed by blight before they become sexually mature. Since two flowering chestnut trees are needed for seed formation (they must be cross-pollinated), sexual reproduction has been drastically reduced. (28)

Wildlife species were decimated by this disease because of their dependence on the fruit of the chestnut.  The trees provided large and reliable crops of nuts that supported black bears, turkeys, squirrels, deer, and other creatures.  The trees’ disappearance is thought to have had impacts on these animals as well as contributed to erosion of thin mountain soils. (12) Although no vertebrates became extinct because of the loss of American chestnuts, seven moth species fed exclusively on American chestnuts and are now extinct.  Another 49 moth species also feed on American chestnuts, but because of broader diets, they are able to feed on related trees and shrubs, including the introduced Chinese chestnut. The structure of the forests significantly changed because of the blight, and other tree species became dominant. (29)

The loss of the American chestnut also had an enormous economic impact.  The tree was one of the best for timber, growing straight and often branch-free for fifty feet.  The spread of the blight resulted in billions of dollars from losses in lumber (27) and nut crops as well.  Chestnut was a central part of eastern rural economies and an important cash crop for many Appalachian families. (30)  The bark, rich in tannic acid, was ground and used to tan leather. (25)

CONTROL EFFORTS: Superficial spraying with a fungicide is useless, since the infection occurs deep within the tree below the bark. (27)  ‘Hypovirulent’ strains of the blight fungus were discovered in Italy in the 1960s; inoculation of these strains into existing (lethal) cankers have resulted in canker remission in many experimental situations so this is an option being looked into for future treatment of eastern chestnut trees. (26, 28)  There also exist strong genetic breeding efforts for the American chestnut tree.  Resistant Asian trees are crossed with susceptible American trees, and the partially resistant hybrids are crossed to American trees again.  One out of four of the progeny from these crosses have partial resistance, and these are crossed again to American.  Repeated back-crossing increases the percentage of American genes in the hybrids, and selecting for partial resistance insures passage of the resistance genes.  Several organizations, such as the American Chestnut Foundation, are participating in these breeding programs. (28, 30)

 

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