Chestnut Blight, Dutch Elm Disease,|
and White Pine Blister Rust
These three forest diseases have historically been important. They have dramatically changed the way the present forest looks. All of them were introduced from other continents. Both the American chestnut and the American elm have largely been eliminated from forests by these pests. Research is being conducted to find methods to control the diseases and identify varieties of these trees that are resistant to the diseases.
Chestnut blight, caused by the introduced fungus Endothia parasitica, was first reported in the United States in 1904. In 1914, the survey of Maryland's forests reported that American chestnut trees were present in the forest. By the 1950 survey, nearly all of the American chestnuts larger than sapling-size had been killed by the chestnut blight. Young trees are not usually affected by the blight. Since American chestnuts produce seeds at young ages, and sprout prolifically, young trees can be found in Maryland. The fungus also attacks post oak, and causes minor injury to maple, hickory, and sumac trees.
The chestnut blight enters through wounds on trees or through the bark when it begins to split and furrow with age. Then, it produces cankers on branches and stems. Branch cankers kill branches causing leaves to turn yellow and wilt. Stem cankers girdle and kill trees.
Dutch Elm Disease
Dutch elm disease, caused by the fungus Ophiostoma ulmi, also known as Ceratocystis ulmi, was introduced in the United States in 1930 from Europe. It spread into Maryland in 1933. The European elm bark beetle spread the disease rapidly across the country.
The beetle spreads the fungus when it feeds on twigs of elms. The fungus also moves from one tree to the next through connecting roots. The fungus quickly grows and clogs vessels that transport water throughout the tree. The water supply to leaves is reduced and the tree wilts and begins to die. Infected trees may die within a few weeks or gradually over several years.
While most of the American elm trees died years ago, the presence of Dutch elm disease is still evident throughout forests in the Eastern United States. Individual elms producing seeds can be found. However, most saplings succumb to the disease. A disease resistant strain of American elm has been developed and further studies are being conducted.
White Pine Blister Rust
White pine blister rust, the fungus Cronartium ribicola, attacks white pine trees. It is not native to North America. It is thought to have originated on the Swiss stone pine in northern Asia. In 1906, the disease was first found in New York on cultivated currants. It is now well established throughout most of the range of eastern white pine.
White pine blister rust completes two phases of its life cycle in the bark of white pines. Three other phases of its life cycle occur on the leaves of Ribes plants, including gooseberries and currants. On Ribes the rust is harmless and dies every fall. On white pine trees it causes severe damage and lives from year to year. The disease cannot spread from pine to pine; it is transmitted to white pines by spores produced in the final stage of the rust on Ribes leaves.
While white pine trees represent a small component of Maryland's forests, white pine blister rust is a major concern to Christmas tree growers, as well as landscapers.