Leaf
Rust
Orange-rust colored spore pustules on leaf undersides
Overview
Rust is caused by fungi in the order Pucciniales, named for the rusty iron-colored spore masses it produces on leaves. Rust fungi are obligate parasites requiring living plant tissue to complete their life cycle. Early symptoms include orange or brown spots on leaf undersides that rupture to release powdery spores. Severe rust causes extensive premature leaf drop, impacting photosynthesis.
Symptoms
Orange to rust-brown raised pustules (uredinia) appear on leaf undersides, rupturing to release rusty powder. Corresponding yellow chlorotic spots appear on the upper leaf surface. In severe cases, pustules coalesce into large patches and leaves yellow and drop early. Yellow to black spore patches may also appear on stems and petioles.
Treatment
Immediately remove and seal infected leaves in a bag for disposal (do not leave in pots or compost). Apply systemic fungicides (triadimefon or tebuconazole) every 7 days for 3 treatments, ensuring thorough coverage of leaf undersides. Reduce leaf wetness and avoid evening watering. Rotate fungicides with different modes of action in severe cases to prevent resistance.
Prevention
Do not purchase or introduce plants with rust spots. Keep leaves dry by watering at soil level. Increase plant spacing to reduce leaf-to-leaf contact. Thoroughly clean up fallen leaves and plant debris in winter. Apply preventive fungicides to susceptible varieties before the rainy season.
