Rust

How to identify it

• Orange, yellow, or rusty-brown raised pustules, mostly on the underside of leaves
• Pale yellow or orange spots on the upper surface above the pustules
• Orange powder (spores) rubs off onto your finger

If left untreated

• Leaves yellow, dry, and drop as pustules multiply
• Heavy infection stunts the plant and weakens new growth

What it could be confused with

Fungal Leaf Spot — Leaf spot is flat brown/black spots with rings; rust is raised pustules that shed coloured powder, and they sit mostly on the leaf underside.

Nutrient Deficiency — Deficiency yellows tissue smoothly with no raised pustules or powder.

Pest Damage — Scale insects also make bumps but they are smooth and don't release orange powder; rust pustules are powdery and dusty.

What causes it

• Rust fungi (many species, usually host-specific)
• Spores spread by air, water splash, and contact

Plants commonly affected

• Pelargonium / geranium
• Fuchsia
• Chrysanthemum
• Roses (indoor/patio)
• Some ferns

How to treat it

Do this first

• Isolate the plant immediately — rust spreads readily
• Remove and bin all leaves showing pustules (don't compost)

Cultural fixes

• Keep foliage dry; water at the soil only
• Improve air circulation and reduce humidity around the plant
• Avoid overcrowding

Organic treatments

• Sulphur-based or neem fungicide spray on all surfaces, including undersides
• Repeat weekly until no new pustules appear

Chemical treatment (last resort)

• Last resort: a labelled rust fungicide (e.g. myclobutanil or tebuconazole). Follow the label exactly; ventilate; keep away from children and pets.

In India

• Sulphur dust/wettable sulphur and neem are cheap and widely available at Indian nurseries

How to prevent it

• Keep leaves dry and humidity moderate
• Space plants for good airflow
• Inspect leaf undersides regularly to catch it early
• Quarantine new plants before grouping

Will the plant recover?

Controllable if caught early. Pustuled leaves won't recover, but removing them and treating promptly usually clears it, with clean new growth.

Frequently asked questions

What are the orange spots on the underside of my plant's leaves?

Those are rust pustules, a fungal disease. They release orange powdery spores. Remove affected leaves, isolate the plant, keep foliage dry, and treat with a sulphur or neem spray.

Is plant rust contagious?

Yes, rust spores spread easily by air and contact, so isolate the plant right away and treat the undersides of leaves where the pustules form.

Can a plant recover from rust?

Yes, if you act early. Existing pustuled leaves won't heal, but removing them and treating the plant usually stops the spread and lets clean new growth come through.

Sources

• Royal Horticultural Society (RHS) — Rusts
• Missouri Botanical Garden — Rust diseases
• University of Minnesota Extension — Rust on houseplants and garden plants