Disease in trees causes a variety of different symptoms.  Some types of disease cause a slow decline and other types cause rapid death.  In both cases, identifying the pathogen is the first step to control.  Depending on the specific disease you may choose to chemically treat your tree, cut the tree down to avoid spreading the disease, or simply raking up fallen leaves to prevent the disease from returning next year.

Anthracnose

Apple Scab

Armillaria

Ash Yellows

Cedar Apple Rust

Decline

Diplodia Tip Blight (Dothistroma)

Dutch Elm Disease

Elm Phloem Necrosis

Ganaderma

Hawthorn Rust

Oak Wilt

Phomopsis

Rhizosphaera Needlecast

Root Rot

Slime Flux

Sphaeropsis Tip Blight

Sudden Oak Death

Verticillium Wilt