Tom Volk's Fungus of the Month for May 1998

This month's fungus is Cryphonectria parasitica, the cause of Chestnut Blight.

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Me with the state record chestnut tree The American chestnut tree, Castanea dentata, once covered the eastern United States. They were the dominant tree in the forests and grew to large and majestic size. They were prized for their fruit (which could often be found roasting on an open fire) and for their beautiful decay-resistant wood. However, due to the influx of the chestnut blight, large trees as you see to the left are no longer found in the natural range of chestnut. ---but there are several areas where chestnuts can still be found. The tree you see with me to the left is the Wisconsin state record American chestnut tree-- something more then 3 ft (1m) in diameter. It is on the farm of Ron Bockenhauer near West Salem Wisconsin, about 20 miles east of La Crosse. This tree was planted about 1885, along with 9 or 10 others. Their seeds propagated a forest of a couple hundred acres containing more than 3000 (three thousand!) American chestnut trees. Up until about 10 years ago the trees were free from the blight because of their isolation. But now they are being infected with Cryphonectria parasitica.

pycnidia of Cryphonectria parasiticaCryphonectria parasitica has devastated the chestnut in North America. It was probably brought over from Asia in a load of trees that were brought to the USA in hopes of producing larger nuts. Asian trees are resistant to the blight, but the American chestnut is highly susceptible. Cryphonectria parasitica is a fungus that invades the tree through wounds of cracks in the tree's bark. Once the tree has been invaded, the fungus grows through the vascular cambium of the tree, eventually girdling the tree and killing it. The fungus also produces bright orange stroma (shown to the right) containing pycnidia (shown below), which contain asexual spores that are then spread by the wind to other trees. Infected saplings will die within a year, but large mature trees can take several years to succumb to the disease. Like many other fungal pathogens, introduction of a fungus into an area where it has no co-evolved with its host makes for a more virulent and devastating pathogen. Remember, successful pathogens do not kill their host-- they want to keep their source of nutrients available to them for as long as possible.

microscopic view of stroma of CryphonectriaCryphonectria parasitica, formerly known as Endothia parasitica, is a member of the Ascomycota, producing its sexual spores (meiotic spores) in a sac called an ascus. To the left is a stroma that contains several perithecia, which bear asci with ascospores inside them. The spores re released through the long neck of the perithecium. Many other forest pathogens are member of the Ascomycota, including Ophiostoma ulmi, the cause of Dutch elm disease, and Ceratocystis fagacearum, cause of oak wilt. There are some Basidiomycota that cause tree diseases also, including Cronartium ribicola, cause of white pine blister rust, and Armillaria species, the incitant of shoestring rot.

tree climber inoculating hypovirulent strainSo, what is being done to try to save this massive and beautiful stand of American chestnuts in Wisconsin? A group of researchers from the Wisconsin Department of Natural Resources, West Virginia University, Michigan State University and Cornell University have set up a project to inoculate hypovirulent (less than virulent) strains of Cryphonectria parasitica into and around the cankers to try to contain the spread of the disease. The hypovirulent strains do not kill the trees and also prevent infection by virulent strains. There's some complicated genetics going on, and this is not as easy as it would first appear. Since the cankers can occur anywhere on the tree, professional tree climbers (shown here) are called in to help inoculate the higher disease centers. It is an ambitious project and many volunteers gather every year to try to save these trees,
So along with many fungi that are useful to us, there are some fungi that cause problems for humans by killing off plants we are interested in. In this case humans brought in the pathogen inadvertently from another continent. The pathogen kills American chestnuts but not the Asian ones. Breeding programs are underway to try to breed resistance to the pathogen into North American stock of the tree to try to repopulate eastern forest, but it has been a very slow process. Maybe someday we will all be able to enjoy a chestnut forest in the same way our grandparents did.

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