What Are the Orange Spots Under the Leaves of My Rose Bush?
What you might not have realized when you planted roses (Rosa spp.) on the West Coast is that your plants would become bull's-eyes for rust-fungus spores. Although it's active across the Rosa genus' hardiness range in U.S. Department of Agriculture plant hardiness zones 2 through 11, rust fungus attacks most frequently in cool, moist or foggy conditions. When orange spots freckle the undersides of your roses' leaves, the plants have rust and need quick intervention.
The Rust Fungus
Rust fungus (Phragmidium tuberculatum) overwinters in soil debris as teliospores. In Mediterranean climates, it also survives in the stems of previously infected roses as a fungal mat, or mycelium. In spring, aeciospores from these structures infest rose leaves and spread from new to older foliage. They eventually change highly infectious urediospores, causing large, reddish-orange spots on the leaves' undersides.
When to Watch for Rust
Roses remaining wet for two to four hours at air temperatures between 34 and 81 degrees Fahrenheit may contract rust. Rain, fog or even dew provides enough moisture to spread the disease. The fungus' infectious stage repeats every 10 days to two weeks while favorable conditions persist. It's most active, however, temperatures between 59 and 70 F. A week of temperatures above 82 degrees kills the infectious urediospores, notes the University of Illinois Extension.
Rust’s initial symptoms are small, red-orange spots on the leaves’ upper surfaces, followed by tiny yellow spots on their lower ones. Deepening to orange as they spread, the yellow spots swell with orange-yellow urediospores until they burst. The disease distorts and reddens new shoots and leaves older foliage dusty, wilted and desiccated. Without treatment, it may defoliate its victims.
Stopping the Spread
Rust is difficult to eliminate once it gains a foothold in your roses. To limit its spread during summer, prune infected leaves at the first sign of disease. In spring and fall, prune diseased canes. Dispose of all infected material in sealed bags and disinfect your pruning tools in a solution of 1 part household bleach to 9 parts water between cuts. In winter, remove the remaining leaves and clear and dispose of the debris around the plants. When you water, avoid wetting the leaves.
Rust prevention begins at planting. Give your roses well-drained soil, allowing 3 feet between small varieties and 4 feet between large ones for proper air circulation. Prune their central canes back regularly to open up their interiors so the leaves dry quickly. Consider protecting healthy plants with sulfur fungicide when conditions favor infection. Mix a solution of 2 to 2 1/2 tablespoons -- or the label's specified amount -- of powdered sulfur concentrate per 1 gallon of water in a pressure sprayer. Spray your roses when the temperature is lower than 80 F until the solution drips from all their surfaces. Wear protective clothing during treatment and when working around the plants for the next 24 hours. Always follow the manufacturer's directions when handling any insecticide.
- Fine Gardening: Genus Rosa
- University of California Statewide Integrated Pest Management Program: Roses – Diseases and Abiotic Disorders
- University of Illinois Extension Report on Plant Disease: Common Rust of Roses
- University of California Statewide Integrated Pest Management Program: Pesticide Information -- Active Ingredient, Sulfur
Passionate for travel and the well-written word, Judy Wolfe is a professional writer with a Bachelor of Arts in English literature from Cal Poly Pomona and a certificate in advanced floral design. Her thousands of published articles cover topics from travel and gardening to pet care and technology.