What you see
Leaves mottled or discolored

Possible Causes
Mosaic; nutrient deficiency, ozone, or sulfur dioxide injury; sooty mold; sunscald; yellows; insect pests

What you see
Leaves with yellow or brown spots

Possible Causes
Anthracnose; apple scab; bacterial spot; Botrytis blight; cedar-apple rust; downy mildew; Septoria
leaf spot

What you see
Leaves curled, cupped, or blistered

Possible Causes 
Bacterial spot; black spot; cherry leaf spot; late blight

What you see
Leaves, shoots, or fruit with white spots or patches

Possible Causes
Downy mildew; powdery mildew; salt injury

What you see
Leaves and stems wilted and dying

Possible Causes
Bacterial wilt; Dutch elm disease; Fusarium wilt; oak wilt; Verticilium wilt; 
waterlogged soil; lack of water

What you see
Leaves or stems with orange spots

Possible Causes
Rust

What you see
Stems with irregular swellings

Possible Causes
Black knot; cedar-apple rust; crown gall 

What you see
Stems with oozing slimy or gummy substance

Possible Causes
Cytospora canker; slime flux

What you see
Stems condensed into short, bushy "rosettes," plant stunted

Possible Causes
Peach rosette; nematodes

What you see
Stems of seedlings rotted at the soil line; infected seedlings collapsed

Possible Causes
Damping-off

What you see
Flowers or fruit with brown spots

Possible Causes 
Brown rot; Botrytis blight or fruit rot; 
fire blight; frost damage

What you see
Fruit with small, dark sunken spots

Possible Causes
Anthracnose

What you see
Apple fruit with green or velvety brown spots that turn into raised, 
dark, corky areas

Possible Causes
Apple scab

What you see
Fruit with water-soaked spots that turn brown and leathery at the blossom end

Possible Causes
Blossom end rot

What you see
Roots of young and old plants rotted; plants stuntedor wilted

Possible Causes
Root rot

What you see
Roots with irregular swellings

Possible Causes
Club root; crown gall; nematodes