Disease

Rust

Pucciniales

Fungi that form raised pustules of yellow, orange or brown spores. Each rust is specific to a few hosts, and some need two unrelated hosts to complete their cycle.

Rust pustules on the underside of a geranium leaf Photo: Plantpathfinder, CC BY-SA 3.0, via Wikimedia Commons

Signs

Orange, rust-brown or yellow pustules, mostly on leaf undersides.
Powdery spores that rub off on a finger or a white tissue.
Yellow, orange or red spots on the leaves.

What favors it

A film of water on leaves from rain, dew or irrigation.

Prevention

Grow resistant varieties where available.
Water at the base and early in the day.
Space for airflow; keep junipers and apples apart for cedar-apple rust.

Often confused with

NutrientPotassium deficiency DiseaseBlack spot of rose DiseaseSeptoria leaf spot

Sources

Rust in the flower gardenUniversity of Minnesota Extension
Cedar-Apple RustMontana State University Extension
Rust Disease on ShrubsUniversity of Maryland Extension

Why →

Treatment by crop

Cultural, biological and chemical options in the order to use them, and notes written for each crop.

Identification is free. Treatment is on the Home and Pro plans.

Open in the key See plans

Questions in the key that lead here

Look the plant over, leaf undersides included. Are insects, mites, slugs, eggs, webbing or droppings on it?
No, only damage or discoloration
Where is the problem?
Leaves
What does the leaf damage look like?
A coating or growth on the surface
What is the coating like?
Orange, rust or brown pustules

Not a match? Start the key or describe what you see.

More photos

Orange rust pustules under a blackberry leaf Photo: John Tann from Sydney, Australia, CC BY 2.0, via Wikimedia Commons

More from university Extension: search Extension pages on rust, or find a plant diagnostic lab through your state's Extension.

Rust

Signs

What favors it

Prevention

Often confused with

Sources

Questions in the key that lead here

More photos

Other diseases