Skip to content
Pocket Botanist
Get the app

Guides

Rust

Disease

Rust is a common fungal disease that appears as powdery, rust colored spots on plant tissues. It matters because it can drain a plant of its energy, leading to stunted growth and leaf drop.

Rust on a plant
Photo: no rights reserved, CC0

Symptoms by part

Leaves

Small, raised, circular pustules that look like orange, yellow, or brown dust on the undersides or tops of leaves.

Stems

Elongated, slightly raised bumps or lesions that may crack open to release a fine, powdery substance.

Causes

  • High humidity levels around the foliage.
  • Poor air circulation in crowded planting areas.
  • Extended periods of moisture on leaf surfaces.
  • Warm temperatures combined with damp conditions.

Could be confused with

  • Leaf Spot · Leaf spots are usually flat, dark, and lack the powdery, raised texture characteristic of rust spores.
  • Nutrient Deficiency · Discoloration from nutrient issues typically affects the whole leaf color uniformly rather than creating distinct, raised pustules.

Treatment

  1. Prune away and destroy any leaves or stems that show visible signs of rust pustules.
  2. Clean your pruning shears with rubbing alcohol between every cut to avoid spreading spores to healthy tissue.
  3. Apply potassium bicarbonate to the foliage to alter the pH and inhibit fungal growth, follow the label.
  4. Apply sulfur to the affected areas to suppress further spore development, follow the label.

Without chemicals

  • Remove and dispose of all infected plant debris in the trash, not the compost.
  • Increase spacing between plants to improve airflow.
  • Water only at the base of the plant to keep foliage dry.
  • Use a rain cover or shelter for potted plants during wet weather.

Chemical options

  • potassium bicarbonate, follow the label.
  • sulfur, follow the label.

Preventing it

  • Choose plant varieties that are labeled as resistant to rust.
  • Water early in the morning so leaves have time to dry before evening.
  • Thin out dense foliage to allow better light and air penetration.
  • Clear away all fallen leaves and debris from the soil surface at the end of the season.

Season

The warm growing season, especially during periods of high humidity or frequent rain.

Spread risk

Moderate

Severity

Moderate

When to act

Act as soon as you spot the first orange or brown pustules to prevent the fungus from spreading to the rest of the plant. If the infection covers more than half of the plant, it is often best to remove and destroy the entire specimen to protect your other plants.

Think it's Rust?

This guide covers the general picture. Pocket Botanist looks at your actual plant from a photo and tells you what's really going on, how serious it is, and what to try first. Identification and a health check are free.

Download on the App StoreGet it on Google Play

Get the app · Or check it from a photo now, free

Questions people ask

Can I compost the leaves that have rust?
No, it is best to put them in the trash. Most home compost piles do not get hot enough to kill the fungal spores.
Will my plant recover after the rust is gone?
Yes, if you catch it early, the plant will usually grow new, healthy leaves once the environment is drier and the infection is managed.

Plants it affects

Back to all guides

This guide was generated with AI and is general information, not horticultural advice. Read about how we build these guides.