Types of diseases are Bacterial, Fungal and Viral.
Some examples are:
Bacterial Blight – small, dark, water soaked spots on foliage that enlarge, then brown and dry. Dried areas may drop out, leaving holes. Spots may be ringed with yellow or light-green.
Bacterial Leaf Spot – small brown or purple spots on leaf surface. Entire leaves may yellow, wither and drop.
Downy Mildew – pale-green or yellow areas on tops of leaves, bottom of leaves have light-gray, purplish, or white fungi. Leaves wilt, turn brown, and die.
Powdery Mildew – on tops of leaves as white or gray patches. Leaves become distorted and may drop.
Rust – underside of leaves have pinhead-size powdery orange or yellow spots. Upper side has pale-yellow or white patches. Deforms leaves and stems and causes early leaf drop.
Viruses – leaves and flowers are greenish yellow, distorted, leaves mottled or streaked, new growth is spindly and plants are stunted.
Wilt – infected leaves and stems wilt. Leaf margins may yellow and curl upward, followed by leaf drop.
Chlorosis – Iron deficiency, and lack of Chlorophyll on the roots and trunk of the damaged plant, yellow leaves, brown crispy in the summer. Use Ironite or for longer treatment use Ross Root Feeder and iron tablets.
To control most of the Diseases on plants you can use: Fung-Away, Fung-onil, Liquid Fungicide, Liquid Systemic Fungicide, Halt, Liquid Sulfur, Dusting Sulfur.