Rust
Rusts are fungi that produce powdery orange, yellow, or rusty-brown pustules, usually on leaf undersides or needles.
Rust at a glance
| Also known as | Cedar-apple rust, leaf rust, gymnosporangium |
|---|---|
| Affected parts | Leaves, needles, stems |
| Most at risk | Junipers + apple/hawthorn/quince/pear (cedar-apple rust); roses; many broadleaf |
| Main cause | Rust fungi, several alternating between two host plants |
| Severity | Mild–moderate on bonsai; disfigures and weakens foliage |
| Reversible? | Manageable; remove infected parts and protect new growth |
What to look for
- Leaves: Yellow or orange spots on the upper surface with raised orange/brown powdery pustules beneath; leaves may yellow and drop; on pome hosts, bright orange-yellow leaf spots.
- Whole tree: Powdery orange spores rub off onto fingers; infected foliage distorted and weakened.
Overview
Rusts are fungi that produce powdery orange, yellow, or rusty-brown pustules, usually on leaf undersides or needles. Many rusts need two different host plants to complete their cycle — the classic example being cedar-apple rust, which alternates between junipers (where it forms bizarre orange gelatinous galls in wet spring weather) and apple/crabapple/hawthorn relatives (where it spots the leaves and fruit).
Causes
Rust spores spreading in warm, wet, humid weather; presence of the alternate host nearby (e.g. junipers near apples/hawthorns); and wet, crowded foliage.
Commonly confused with
Sooty mold is black and wipes off. Leaf spot makes dark (not orange) spots. The powdery orange/rust colour and (on junipers) the orange jelly galls are diagnostic of rust.
Which species are affected
Junipers and apple/crabapple/hawthorn/quince/pear (cedar-apple and related rusts); roses; and various broadleaf species with their own rusts.
Treatment
Remove and bin infected leaves and (on junipers) cut out the galls before they release spores; improve airflow; avoid wetting foliage. Apply a suitable fungicide to protect new growth in the infection season. Where practical, separate the two alternate hosts. Usually cosmetic on bonsai but worth controlling.
Prevention
Don't grow alternate hosts close together; good airflow and dry foliage; remove galls/infected leaves promptly; choose resistant cultivars; sanitise tools.