There are many reasons why trees die from the top down. Problems, including air pollution and drought, can attack a tree, regardless of its species. Other problems, such as fungal diseases, attack specific species of trees. Sometimes, the problem can be addressed; and once it is, your tree will be restored to health. Other times there is no cure, but even then, it is sometimes possible to extend the life of your tree.
Symptoms of drought include wilting, yellowing leaves, twig and branch dieback and death of the primary roots. Trees that suffer from drought usually die from the top down and from the outside branches in toward the trunk. Drought-stricken trees are also more likely to be attacked by insects and disease. If rainfall amounts are low, provide trees with a thorough watering every week, ensuring that the soil is soaked to a depth of 6 to 12 inches. Mulching trees will also help them retain moisture.
Pollution from acid rain, ozone and herbicides kills trees in a number of ways. Ozone interferes with a tree's ability to perform photosynthesis. Acid rain contains pollutants, such as sulfur dioxide and nitrogen oxide, that combine with sunlight or water to form acids that leach valuable minerals out of the soil. Too much herbicide can kill plants. Trees affected by air pollution usually die from the top down. Symptoms include thinning foliage and dieback of branches. Eventually, the tree breaks off or blows over, but the dying process can take 10 years or more.
Bark beetles, including mountain pine beetles and bronze birch borers, often infest trees that are suffering from other problems. Trees attacked by bark beetles usually die from the top down. Look for entry and exit holes in the trunk and branches and signs of woodpeckers feeding on the trunk. Small masses of pitch, mixed with sawdust and grass, may also be seen. Signs of tunneling will be visible in the wood when the bark is removed. Trees die from bark beetle attacks because they are no longer able to transport water due to the damage caused by tunneling. Fungi infections, spread by beetles, also inhibit a tree's ability to move water. Trees that are infested with boring beetles can't usually be saved. However, sprays are available to use on a preventive basis.
Dwarf mistletoe is a parasitic plant that lives off pines and conifers by inserting its root-like structures into the host tree's vascular system and drawing nutrients and water from it. The mistletoe kills very slowly, and infected trees die from the top down as the lower branches take more of the water and food. Early symptoms of mistletoe infection include swelling of the bark and growth of witch's broom. Other symptoms include yellowing leaves and dieback of branches. Severely infected trees should be removed, but mistletoe can be controlled on trees that aren't as severely infected by pruning infected branches. Ethephon spray can also be used to reduce mistletoe seed production, thus reducing the ability of the mistletoe to spread.
Oak wilt is a serious disease that kills thousands of trees every year. Although it can attack any oak species, red oaks are especially susceptible to it, and some trees can die within three weeks of becoming infected. This disease is usually fatal. Symptoms vary, depending on the species, but in red oak trees, symptoms include rapid wilting from the top of the tree down, along with discoloration of the leaves. Leaves look brown at the tips and margins but may remain green at the base. Complete defoliation can occur in a few weeks. There is no cure for this disease. Infected trees should be removed and either debarked or chipped and dried to prevent the fungus from spreading.
Cytospora cankers are a fungal disease that infects many hardwoods, including maple, aspen, willow and poplar trees. Symptoms include wilting and branch dieback. Cankers develop on branches and kill trees from the top down, often in just a year or two. Orange-colored fungal fruiting bodies can be seen in the cankers. Trees that suffer from drought are prone to cytospora. There is no cure for this disease. Prune infected branches as they appear.