Rust
A fungal disease that forms orange, brown or yellow powdery pustules, mainly on leaf undersides.
Also known as: Leaf rust
Symptoms and identification
Small orange, rusty-brown or yellow raised spots on the underside of leaves, with pale yellow patches on top. Rubbing a pustule leaves rust-coloured powder on your finger. Leaves yellow and drop early.
Causes
Rust fungi spread by wind-blown spores and need moisture on the leaves to infect. Humid weather, overhead watering and crowded plants increase the risk.
Organic treatment
Pick off infected leaves as soon as you notice them. Spray wettable sulphur or neem oil every 7–10 days in humid weather.
Prevention
Water at soil level, space plants well and remove fallen leaves. Choose resistant varieties and avoid excess nitrogen.
Chemical treatment (use with care)
Use a fungicide labelled for rust (for example propiconazole or mancozeb), following the label.