Here are the most common tree issues submitted to the UK Plant Diagnostic Labs by tree species.  If you don't know what's wrong with your tree, this could be a good starting place.

Arborvitae
Arborvitae

   

Dogwood

Dogwood

Ash leaf

Ash

   

Elm

Elm

Cherry

Cherry

   

Hawthorn

Hawthorn

Cypress
Cypress

   

Hickory
Hickory

   

Hemlock

Hemlock

   

maple

Maple

   

Sassafras

Sassafras

   

Honeylocust

Honeylocust

   

Oak leaf

Oak

   

Spruce

Spruce

   

Juniper

Juniper

   

Pine

Pine

   

Sycamore

Sycamore

   

magnolia

Magnolia

   

Redbud

Redbud

   

Poplar leaf

Yellow poplar