Here are some tips to identifying sick trees.
Trees in an urban setting are especially vulnerable to disease. Common causes of a diseased tree include damage from wind, extreme heat, cold or moisture; stress on the root structure from major changes in its natural environment due to development, including excessive backfill, soil compaction, hardpan or poor drainage; and insects and diseases. Mushrooms growing from the base of a tree or under its canopy may be a sign of root decay.
Unstable trees are more prone to fall in storms and cause property damage or even loss of life. Signs of structural instability include cracks in the trunk or major limbs, hollow or decayed areas, or excessive dead wood.
Trees naturally react to stress by trying to curtain their above-the-ground growth. Signs of a sick tree include a premature change in leaf color, on the whole tree or just a branch or area of the tree; misshapen leaves, thinness of the canopy and early loss of leaves. The presence of insects is not always cause for worry, some are beneficial in protecting the trees from harmful ones (i.e. ladybugs eat aphids).
Arborists are professionals trained to care for trees and shrubs by pruning, fertilizing, monitoring for insects and diseases, and consulting on tree related issues. When contacting a tree service about a diseased tree, make sure they have a professional affiliation such as Certified Arborist,
International Society of Arboriculture or National Arborist Association, as well as proper state licensing.