Oak trees grow naturally in deciduous forests and serve as shade and decorative trees in parks and yards. They often have long lives, but may be dying if their roots or trunks start to rot or their foliage dies off.
The U.S. Forest Service explains that large percentages of oak trees in the eastern United States often die at the same time as each other due to widespread disease outbreaks and environmental stresses. Individual oak trees also start to die from the combination of improper care, insects and diseases without a widespread outbreak of oak tree problems. Unhealthy oak trees usually take several years to die.
Oak trees often die when environmental factors or human activities weaken them and make them susceptible to diseases and pests. The University of California Cooperative Extension-Ventura County blames overwatering, excessive pruning, root cutting and root crowding, fire and drought for weakening oak trees. Diseases or insects, root rot fungi, trunk rot fungi and boring insects, must also be present in the soil or on nearby trees to infest new trees and cause them to die.
Thinning out heavily planted oak trees provides better ventilation for the trees and allows gardeners to remove dying and infected trees before their diseases spread to healthy trees. Horticultural oil sprays act as fungicides and insecticides, killing leaf fungi, insect eggs and some adult insects. Gardeners should remedy moderate root rot by reducing irrigation and allowing tree roots to dry out. Trees with severe root rot and trunk rot, however, are usually structurally unstable and warrant removal.