While roses are among the most popular ornamental flowers in the world, they tend to be prone to a number of diseases. Rose rust is a common affliction and can cause serious damage to the roses. Caused by a fungus, the disease affects leaves and stems and may damage roses, particularly cultivars that are more susceptible to infection, although the infection is usually more of a nuisance than a threat.
Pathogen and Host
The fungus Phragmidium mucronatum causes rose rust. It is more prevalent in the western United States. The disease infects rose plants for extended periods of time without killing them. All varieties of roses, both native and cultivated, are prone to the disease, although some cultivars are more resistant than others.
The fungus will survive cold weather as resting spores. In the spring, the spores are spread by wind to healthy plants and are encouraged to grow by wet conditions. Once the spore lands, it grows into the plant tissue, infection the rose. It then spreads, feeding on the cells within the plant. The rust will eventually reproduce creating more spores.
The first symptom is powdery, bright orange blisters that form on the underside of the leaves. As they grow, yellow to orange spots will appear on the surface of the leaves. The leaves may curl and flower sepals and young stems may become distorted. Defoliation of the plant can also occur.
Infected plant parts should be immediately removed and burned. Be sure to disinfect pruning shears or other tools after each cut to prevent further spread of the disease. Chemical fungicides are relatively ineffective for treating the disease once it has occurred, though they can be used as preventative measure. Mancozeb, myclobutanil, penconazole and triticonazole are effective at controlling rose rust.
Buy roses from a reputable grower, nursery or garden center that will guarantee disease-free plants. Its a good idea to quarantine new plants for a several weeks to determine if they have rust or not. Check the plants regularly and remove any parts of the plant that exhibit signs infection. If the disease progresses, remove and destroy the plant. It is also important to practice good agricultural hygiene, such as disinfecting work benches and tools at the end of the growing season.