Urban and wild trees in the United States have often been subjected to diseases and destruction by pests, microbes, drought, storms and fires. But instances of certain pests or microbes affecting a particular species of tree on a large scale and threatening to wipe them away are more problematic.
Chestnut blight is caused by Endothia parasitica and have almost wiped out chestnut trees from the eastern hardwood forests. The spores of this fungus are spread by winds, birds, rain and insects. They attack the tree through any external injury; the cankers kill the cambium and cracks are formed on the stem. Decades of research has not resulted in any appreciable result in curbing this disease.
Butternut trees, also known as white walnut, are mostly found in north eastern forests of America. The wood from butternut trees is used for making furniture. The fungus that causes butternut canker is called Sirococcus clavigignenti-juglandacearum. It attacks the tree through the buds or an opening on the surface caused by injury. Cankers are formed that girdle the stem; small branches die and ultimately the tree dies. The fungus can survive on the dead trees for two years and the spores are spread through rain, wind and animals. Resistant varieties of butternut have not yet been developed.
Dutch Elm Disease
The American elm tree is a popular urban tree because it is fast-growing, has a long life and withstands urban pollution. Dutch elm disease has had a devastating effect on this tree. The fungus affects the vascular system and stops the water flow to all parts of the tree. The leaves on the top of the tree start yellowing then turn brown and die; the bark also becomes discolored. Selecting resistant varieties, applying fungicides, removing affected tree parts and trees,and avoiding the spread of the spores through insects are some of the ways to manage the disease successfully.
Oak trees have been extensively affected in the eastern forests of United States by a fungal strain called Ceratocystis fagacearum. The fungus spreads from the affected tree to other trees through the root connections. Leaves turn from yellow to brown at the onset of this disease and start to wilt. How fast the tree dies depends on the type of oak. The disease can be managed by preventing injuries to the healthy trees, removing affected tree parts, isolating affected tree’s root system, and employing chemical treatments.