Rust on roses — symptoms and treatment

Severity: medium

What is Rust

Rose rust is caused by Phragmidium species. Multiple rust species affect roses with similar symptoms, which is good news for the gardener: you do not need to know which one you have, because they look alike and are handled alike. Severity is rated medium — rust does not usually kill a rose, but it strips the plant of leaves before the season is over, and a rose without foliage cannot support its flowers.

Symptoms

Rust announces itself from underneath, which is why it is often found late. Orange powdery pustules erupt through the undersides of the leaves — small raised spots that break the leaf surface and shed a vivid orange dust that smudges on a finger or a sleeve. From above, the first thing most people notice is different: yellow spots on the upper surfaces, sitting directly over the pustules below. Anyone who sees the yellow spotting and turns the leaf over has the diagnosis in a second. Premature leaf drop follows as infection builds, and reduced flowering comes with it — the direct cost of a plant that has lost its working leaves early.

• Early: orange powdery pustules on leaf undersides
• Early: yellow spots on upper surfaces, above the pustules
• Advanced: premature leaf drop and reduced flowering

Turning leaves over is the whole diagnostic trick here. Black spot also causes yellowing and leaf drop, but its lesions are black and sit on the upper surface; rust puts orange dust underneath.

Causes and conditions

Rust spores are produced in enormous numbers and carried on the wind, so the disease can arrive in a garden with no visibly infected rose nearby. Germination needs leaf wetness, so damp, mild weather, heavy dews, frequent rain and overhead watering all favour it, and anything that keeps foliage wet or air still — crowded plantings, a congested unpruned centre, a sheltered corner — extends the wet period the fungus depends on. The fungus also carries over on infected leaves and debris left around the plant.

Treatment

Remove infected leaves — cultural

Throughout the season. Pick off and destroy infected leaves. Clean up fallen debris. Improve air circulation by pruning. Picking off pustuled leaves early, while infection is confined to a few, removes the spore source before it multiplies — and the fallen debris underneath is where the fungus waits for next year, so clearing it is not tidiness but control.

Myclobutanil — chemical

At first symptoms. Apply fungicide at first appearance of rust pustules. Repeat every 10-14 days. Timing decides the outcome: the fungicide protects leaves that are still clean and cannot clear pustules that have already erupted, so the first orange spots are the moment to act. Pesticide registrations vary by country — check local approval before use.

Prevention

• Inspect the undersides of the leaves regularly — rust is only obvious there.
• Prune to open the bush so air moves through and leaves dry quickly after rain and dew.
• Water at the base and keep the foliage dry.
• Rake up and destroy fallen leaves and debris around the plant, and again at season's end.
• Space roses generously rather than crowding them into still, sheltered pockets.

Frequently asked questions

Is it contagious to my other roses? Yes. Rust spores travel on the wind in huge numbers, so a few metres between bushes counts for little. What actually helps is removing infected leaves early, clearing the debris and opening the plants up so foliage dries.

Will my rose flower again after rust? Reduced flowering follows leaf loss, so a badly rusted plant gives less this season. Deal with the leaves and the plant generally recovers its vigour — the flowers follow the foliage.

When should I spray? At the first appearance of rust pustules on the leaf undersides. Sprays protect clean leaves; they cannot undo pustules already there, so early detection is worth more than a heavier programme later.