Roses are very popular with gardeners and non-gardeners alike. According to one source, roses are the most popular flower in California. They are recognizable by most people and fairly easy to grow. I have always been fond of roses, being drawn to them because my name is Rose and my mother’s name was Rose. I have always had roses in my yard. Even though roses are generally hardy plants, there are a few diseases that are common to them. If you know about these diseases and can recognize them, you can help keep your roses healthy.
Most diseases that home gardeners encounter on their roses are caused by a fungus and can be prevented or treated by simple cultural practices.
Powdery mildew One of the most common fungal diseases of roses is powdery mildew. Powdery mildew is a white powdery growth that appears mostly on leaves. It can also appear on shoots, sepals, buds and sometimes on the rose petals. The disease occurs when air circulation to the plant is poor. Keeping the center of the rose bush open and vase shaped helps to encourage air flow and may help prevent powdery mildew. If you notice powdery mildew, you can try hosing your plant down. This disrupts the daily spore release cycle of the fungus and can help control the disease. Make sure to do this early enough in the day so that the foliage can dry before evening. Pruning and cleaning up leaves during the dormant season can also help since the fungus spores need living tissue to survive.
If nothing else works to get rid of the powdery mildew, try a horticultural oil. Read the directions on the label to avoid burning the plant by applying it at the wrong time of the day.
Some rose varieties are more resistant to powdery mildew. Landscape (shrub) varieties and roses with glossy foliage are more resistant to powdery mildew.
Downy mildew is another disease caused by a fungus. It differs from Powdery Mildew in that it is purple, red or brown instead of white. It appears on the leaves between the veins. The leaves then turn yellow and drop. The fungus only thrives during a short period of time in the spring and fall in our area because it requires a narrow range of temperature and humidity. This makes it less common in our valley than in other areas of California. If you see a lot of leaf drop, I would inspect the plant for the spots of fungus on the leaves.
Again, as with the powdery mildew, maintain good air circulation by keeping the center of the rose bush open and vase shaped and avoid overhead watering as preventative measures. If you do suspect downy mildew and have fallen leaves, rake them and dispose of them to keep the spores of the fungus from spreading.
Rust is also caused by a fungus. Rust appears as small orange spots on the undersides of the leaves. The upper part of the leaf may be discolored. Rust is more common in cooler areas but it can be a problem in our valley during wet years. We have had a problem with rust on one or two of the roses in the Ralph Moore Memorial Rose Garden (the Master Gardeners maintain this garden in partnership with the city on the corner of Main and Hall Streets in Visalia). The roses that are usually affected are in a shadier and wetter area of the garden. As with other fungal diseases, avoid overhead watering. Collect and dispose of leaves that contain the orange spots. Prune out the affected areas of the rose. There are fungicides available that can be used, but these are not usually necessary as the disease can be controlled with pruning and avoiding overhead watering.
Black spot is another fungal disease. Black spots with feathery edges on the upper surfaces of leaves and stems are an indication of black spot. This fungus requires water in order to reproduce. If you hose your roses off and don’t allow enough time for them to dry before evening, this can encourage the growth of black spot. If you have an aphid infestation and want to control them by hosing them off with a strong spurt of water, do it early enough in the day so the roses have at least seven hours to dry. Just as with the other fungal diseases, providing good air circulation, removing fallen leaves, and pruning out infected areas will help prevent this disease. You can use oils or fungicides if nothing else works but it usually isn’t necessary.
Take aways that will help prevent most fungal diseases on roses:
- Encourage good air circulation by keeping the centers of your rose bushes open and vase shaped
- Avoid over-head watering
- Prune out diseased areas and dispose of leaf litter
Diseases caused by viruses are most commonly spread from one rose to another during propagation practices. Most home gardeners don’t propagate their own roses. The virus usually causes the rose to be smaller, flowers to be fewer, and foliage unsightly or deformed. When you purchase a rose, look for roses that are virus free. There is usually a label to tell you that.
Rose mosaic disease is a common virus-caused disease. Leaves can have ring spots, line patterns, distortions or puckering. The symptoms vary and a diagnosis is difficult.
There is no cure or treatment for eliminating the viruses that cause rose mosaic disease. Replace infected roses if their performance is unsatisfactory. Purchase and plant virus-indexed plants—roses that have been tested and certified to be virus free.
Rose spring dwarf disease causes the new growth to look deformed and occurs in the spring. Leaves are curved back or short and show conspicuous vein clearing or a netted appearance. If you suspect that your rose is infected with this virus you should remove it and replace it with a rose that has been tested for viruses.
In conclusion, diseases caused by fungi can be cured by gardening practices of pruning and watering. Diseases caused by a virus are untreatable. The rose should be removed and replaced by a rose tested by be virus free.
For more information or pictures of these diseases, UCANR has an informative pest note: ipm.ucanr.edu/PMG/PESTNOTES/pn7463.html.
The Master Gardeners will be live to answer your rose-related questions at their Public Rose Pruning Demonstrations on Saturday, Jan. 22, 10 to 11 a.m. at the Ralph Moore Rose Garden at Hall and Main streets in Visalia, and from 10 a.m. to 1 p.m. at Woodlake Botanical Garden in Woodlake. They can also be contacted between 9:30 and 11:30 a.m. on Tuesdays and Thursdays to answer your questions at 559-684-3325, or visit their web site at ucanr.edu/sites/UC_Master_Gardeners.