Red Spots on Plant Leaves: Diagnostic Guide

Red spots on plant leaves — diagnostic decision tree for cedar-apple rust, leaf scorch, spider mite stippling, and fungal leaf spots, with confirmation and treatment for each.

The diagnostic decision tree

Step 1: Examine the underside of the leaf

Flip a spotted leaf over and examine the surface that was facing down. This is the most reliable diagnostic step.

• Orange, gelatinous, or hair-like spore structures on the underside: cedar-apple rust or a related rust fungus
• Tiny moving or stationary dots, webbing, or dusty residue: spider mites
• Nothing unusual on the underside, but spots are concentrated along leaf margins: leaf scorch (environmental or bacterial)
• Dark, raised pimple-like structures (pycnia) in the spots: fungal leaf spot diseases

Step 2: What plant is affected?

Host plant identity narrows the diagnosis significantly. Cedar-apple rust affects rosaceous plants — crabapples, hawthorns, serviceberries, quince. Spider mites affect a wide range of plants. Bacterial leaf scorch is associated with oaks, maples, elms, and certain shade trees. Fungal leaf spots are common on roses, tomatoes, and many ornamentals.