Rust 🦠
At a glance
- Cause: Host-specific rust fungi spread by spores
- Tell-tale sign: Orange, yellow, brown, or rusty powdery pustules, often on leaf undersides
- Severity: Moderate — usually weakens and defoliates rather than instantly killing
How to identify
Rust often shows as pale yellow spots on the upper leaf surface with orange or rusty powdery pustules underneath. Rub a pustule and it may smear like colored dust. Many rusts are host-specific, so the exact rust on one plant may not infect a different plant nearby.
What causes it
Rust fungi spread by windblown spores and often need mild, moist conditions to infect. Some rusts require alternate hosts; others cycle on one plant group.
Treatment & management
Following Integrated Pest Management:
- Remove heavily infected leaves and clean fallen debris.
- Water at soil level and avoid wetting foliage.
- Improve spacing and airflow.
- Avoid high nitrogen flushes that create soft susceptible growth.
- Use resistant varieties where rust is a repeated issue.
Prevention
Scout leaf undersides in cool, damp seasons. Keep foliage dry and dispose of infected debris before spores build.
Affects (in this guide)
Many ornamentals, grasses, trees, and garden plants; less common on indoor plants but possible