Managing Common Rose Diseases

By Mary Bernard, Master Gardener

Roses have long been considered one of the most popular ornamental garden plants. They are cultivated for their beautiful, fragrant flowers and are generally considered easy to grow. However, many popular rose varieties are also susceptible to several common fungal diseases: powdery mildew, rust, and black spot.

Powdery mildew is by far the most common rose disease in California. It is recognized by its white to gray powdery or fuzzy growth on leaves, shoots, sepals, buds, and occasionally on petals. It spreads from infected leaves during periods of low rainfall and warm, humid nights. Rose varieties vary greatly in resistance; landscape (shrub) rose varieties are among the most resistant. Plants grown in sunny locations with good air circulation are less likely to have serious problems.

Synthetic fungicides are available but generally must be applied to prevent rather than eradicate infections, so timing is critical and repeat applications may be necessary. In addition to synthetic fungicides, sodium bicarbonate in combination with horticultural oils has been shown to control powdery mildew of roses when used in a solution of about 4 teaspoons of baking soda per gallon of water with a 1% solution of narrow range oil. The best time to apply this solution to avoid problems with phytotoxicity is during cool weather.

Rust is favored by cool, moist weather such as is found in coastal areas of California and may also be a problem inland during wet years. Infected plants have small orange pustules on the leaf undersides; upper sides of leaves may discolor, and leaves may drop. Avoid overhead watering, and remove and destroy severely infected foliage to reduce the source of infection. During the winter collect and dispose of leaves remaining on the plants as well as those that have fallen off. Low levels of infection can be tolerated without significant losses. Preventive applications of fungicides can be used, but frequent applications may be needed and may not be justified in garden or landscape situations.

Black spot produces black spots with feathery or fibrous margins on the upper surface of leaves and stems. Small black fruiting bodies are often present in spots on the upper sides of leaves. There is no fungal growth on the undersides of leaves. The fungus requires free water to reproduce and grow, so leaves should not be allowed to remain wet for more than 7 hours (hose off aphids in the morning so leaves have a chance to dry by midday). A combination of sodium bicarbonate or potassium bicarbonate plus horticultural oil (as discussed under powdery mildew) or neem oil has also been shown to be effective for reducing black spot.

Prevention is the key to managing these diseases. Avoid overuse of fungicides through proper management techniques. Here are some tips for growing healthy roses:

· Select disease-resistant varieties.

· Encourage healthy plants - use proper watering, pruning, fertilization, and dormant-season care practices.

· Proper sanitation is critical - remove fallen leaves and other severely infested material promptly. Prune out infected stems during the dormant season. Dispose of leaf litter and prunings to avoid re-infection. 

· Provide good air circulation around bushes.

· Monitor plants regularly for signs of disease.

For more information on rose diseases, call the phone number listed below and ask for Pest Notes publication #7463 - Roses: Diseases and Abiotic Disorders (click on link to get the publication).

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM. You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM. The San Luis Obispo Master Gardeners website is at http://groups.ucanr.org/slomg/. Questions can be e-mailed to: mgsanluisobispo@ucdavis.edu.