Plant Problem Diagnostic
Rust: How to Identify and Manage It
The short answer
Rust diseases are caused by a group of related fungi and show up as bright orange, yellow, red, or brown pustules, usually as light spots on the upper leaf surface with rusty spore masses on the underside. Rust needs long periods of leaf wetness to infect, and its spores blow long distances. The least-toxic controls are cultural: pinch off and remove rust-infected leaves (but never more than about a third of a plant's foliage at once), water at the base to keep leaves dry, and space plants for airflow.
Planting rust-resistant varieties and not planting two hosts of the same rust close together also help; some rusts need two different host plants to complete their cycle. Rust often does little lasting harm, so a protectant fungicide is a later step. If damage is heavy or you cannot confirm rust, contact your local Extension office before escalating.
How to identify rust
Look for powdery orange, yellow, or brown pustules, most often on leaf undersides, with lighter spots showing through on top. Inner and lower leaves where humidity is highest are affected first.
- Rusty orange to brown spore pustules on leaf undersides
- Light-colored spots on the upper leaf surface
- Favored by long periods of leaf wetness
Least-toxic management, in order
- 1Remove infected leaves — Pinch off rust-infected leaves and remove them, but do not take more than about one-third of the foliage at once.
- 2Keep leaves dry — Water at the base of the plant to reduce the leaf wetness rust needs to infect.
- 3Airflow and spacing — Space plants and improve circulation so foliage dries faster.
- 4Resistant varieties and host separation — Plant rust-resistant varieties and avoid planting two susceptible hosts of the same rust near each other.
When to escalate
Rust rarely does serious damage to plant health, so escalate only when infection is heavy. A protectant fungicide can shield healthy tissue when the first spots appear. Because many different rust fungi exist and some need alternate hosts, confirm the diagnosis with your regional Extension office before treating.
→ Some rusts require two different living hosts to complete their life cycle; removing one host can break the cycle entirely.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Minnesota Extension — Rust in the flower garden — Rusty pustules on leaf undersides, leaf-wetness requirement, removing infected leaves, base watering, and resistant varieties
- University of Maryland Extension — Rust Disease on Flowers and Vines — Rust identification and cultural management, and that rust rarely causes significant plant damage