Three great online resources are below.

Arbor Day Foundation. Detailed information on dozens of commonly planted landscape trees that grow throughout the United States. Here you will find information on height and spread, soil and sun requirements, leaves and fruit, history, wildlife habitat, and more. http://www.arborday.org/trees/treeguide/index.cfm

Cooperative Extension System Offices. Great resources for tree identification, care, native species, pests, diseases, etc. Extension specialists available include certified arborists, nursery growers, landscapers, and orchardists. http://www.csrees.usda.gov/Extension/

Click to find your extension office

State Plant Disease & Pest Identification Labs
However, if your local Extension agent can’t fully answer a question, each state’s Cooperative Extension Service offer the advanced services of a Plant Disease & Pest Identification Lab. Remember to always check first with your local County Extension office about these diagnostic services. If necessary, they can assist you with the proper preparation of plant and pest samples for examination by your state lab. http://www.arborday.org/treeinfo/treehealth/coopextension.cfm