Upper surface of affected shrub rose showing round leaf spots with an orange center on yellowing leaves. Image: R. Benner, Penn State
Rose Rust is caused by the fungus Phragmidium, which only infects plants in the genus Rosa. Susceptibility varies by variety and cultivar. Unlike many rust fungi, this rust only has one host: roses. Rose Rust relies on a living host, and it will die quickly if no roses are present in the landscape. Rose Rust can weaken and distort rose growth or even kill roses.
In the spring look for round leaf spots with yellow or orange centers on the upper surface of the leaves. The lower surface of leaves, petioles, and cane tissue will have masses of raised, orange powdery pustules that are full of spores. The affected leaves may yellow and drop prematurely. The lower leaves and canes on the plant tend to be affected first, and the infection gradually moves upwards on the plant. Rose Rust can cause distortion, galls or lesions on canes. In the autumn look for black spores forming on the leaves and other parts. These are the overwintering teliospores that will spread the rust fungus next spring.
Sanitation and environmental management are important in preventing Rose Rust. Rust spores are spread through the air or from plant to plant by splashing water. The spores require moisture on the surface of plant tissue for 2 to 4 hours at 65-70°F to penetrate and infect the plant. Avoid working with the roses when the foliage is wet, and irrigate using drip irrigation or soaker hoses if possible. Space or prune plants to improve air circulation around the canopy and minimize drying time. If the plants will be in an enclosed space such as a greenhouse, use HAF fans and ventilate to keep humidity below 80%. In the spring and summer clean up and destroy any infected or fallen leaves, and prune infected canes as soon as they are detected. In the autumn collect and destroy any fallen leaves, and prune any infected canes to remove the overwintering teliospores.
Carefully inspect plants before you purchase or receive shipments. Do not accept plants that show symptoms of rust. If possible, quarantine the roses away from other roses for several weeks to make sure they are not infected. During periods of mild, wet weather, scout roses for signs of disease and pests.
Lower surface of rose affected with Rose Rust. Note the bright orange pustules. The pustules contain spores. Image: R. Benner, Penn State
Rose Rust can be difficult to manage on heavily infected plants, and it may be best to remove and destroy the infected plants. To dispose of infected plants or tissue burn them, bury them or seal them in a plastic bag and send to a landfill. On plants that are only very lightly affected, remove the infected leaves, canes or flowers (sepals can become infected) showing spots and/or orange spores. To protect unaffected plants/tissue where Rose Rust has been found, apply a protectant spray of a fungicide labeled for roses while mild, wet conditions occur. This rust does not survive well under hot, dry summer conditions, so the spread of the fungus should be reduced in the summer months.
Fungicides for rusts are preventative and not curative. Always read labels thoroughly before purchasing, applying, cleaning up and disposing of pesticides. Fungicides registered to control Rose Rust in Pennsylvania include (as of 2017):
- FRAC group M2: sulfur
- FRAC group M5: mancozeb
- FRAC group M5: chlorothalonil
- FRAC group 3: myclobutanil, propiconazole, tebuconazole, (some group 3 fungicides may have a PGR effect. Carefully read precautions on the label.)
- FRAC group 7: flutalonil
- FRAC group 11: azoxystrobin, fluxastrobin (field production and enclosed areas only), trifloxystrobin (field production and enclosed areas only)
- FRAC group 44: Bacillus subtilis strain QST 713
- FRAC groups 3, 11: triadifemefon and trifloxystrobin (Strike® Plus 50 WDG - in nurseries, garden centers and greenhouses only)
Homeowner fungicide choices include products with the active ingredients: chlorothalonil, mancozeb, myclobutanil, propiconazole or triforine.
More information on outdoor rose diseases.