Rust on a hollyhock leaf. Photo: Ann Whitman
Orange, yellow, or red spots on leaf surfaces are an easily recognizable sign of the fungal disease called rust. The spots are actually brightly colored fungal spores. As the disease spreads, it causes leaves to yellow and drop; flower and fruit production are reduced.
Various types of rust can infect a wide range of food plants and ornamentals. Often the lower leaves of a plant become infected first because the spores are splashed up from the soil by rain or irrigation. Rust is most problematic during periods of wet, humid weather. The fungus overwinters in the soil on plant debris.
Prevention and Control
- Plant rust-resistant varieties when possible.
- Avoid touching plants when foliage is wet.
- Use drip irrigation or soaker hoses rather than overhead watering.
- Mulch around the base of plants to slow the spread of disease.
- Remove infected foliage from the garden and destroy it.