⚠ ModerateRust erupts as raised orange-to-rust-red pustules on leaf undersides, releasing powdery spores — spread by air and water onto persistently damp foliage.
Leaf Rust — Pustules on Undersides
Commonly affects: Anthurium spp., Philodendron spp., outdoor-grown aroids, Colocasia esculenta
Small raised pustules — yellow, orange, brown, or rust-red — erupt on the undersides of leaves, with corresponding yellow flecks visible on the upper surface. As pustules rupture they release powdery spores that smudge onto a finger. Heavy infections cause yellowing, premature leaf drop, and a generally weakened, unsightly plant.
Potential Causes
- 1Rust fungal spores spread by air movement and water splash
- 2Prolonged leaf wetness and high humidity enabling spore germination
- 3Poor air circulation keeping foliage damp
- 4Introduction from infected plants, particularly those grown or moved outdoors
Solution Steps
- 1Remove and bag all leaves bearing pustules — do not compost, to avoid spreading spores.
- 2Sterilise tools between cuts and avoid brushing against infected foliage, which dislodges spores.
- 3Keep foliage dry by watering at the base and eliminating overhead watering and misting.
- 4Improve airflow and spacing to help leaf surfaces dry quickly.
- 5Apply a suitable fungicide labelled for rust to remaining foliage, repeating per label instructions.
- 6Isolate the plant until new growth emerges free of pustules.
Prevention Tip
Rust requires sustained leaf wetness to infect, so keeping foliage dry and air moving is the most effective prevention. Inspect the undersides of leaves on new and outdoor-exposed plants, where pustules first appear.
Related Issues to Explore
Anthracnose — Colletotrichum Leaf Blight
Both are foliar fungi favoured by leaf wetness — rust forms raised pustules, while anthracnose forms sunken dark lesions.
Read guide
Bacterial & Fungal Leaf Spot — Xanthomonas & Cercospora spp.
Leaf spot and rust both produce discrete lesions — rust is distinguished by its raised, powdery, coloured pustules on the underside.
Read guide
Powdery Mildew — Erysiphe & Golovinomyces spp.
Both are dusty fungal diseases — rust pustules are coloured and on the underside, while powdery mildew is a white coating on the upper surface.
Read guide
