CORVALLIS, Ore. – Oregon is famous for its beautiful roses. Ample rainfall and mild temperatures encourage leaf growth and flower production. But the same conditions that favor roses also favor their diseases. Black spot, rust and powdery mildew can challenge any gardener.
Simple adjustments in care, however, can improve the health of roses, according to Jay Pscheidt, plant pathologist for Oregon State University Extension Service.
First, it's important to understand the conditions that foster disease in roses.
A fungus that overwinters on infected plant tissue causes black spot. Spores are splashed onto newly emerging leaves by rain or overhead watering. If the leaf stays wet for 24 hours, spores germinate and grow into the leaf surface.
Rust is caused by several fungi whose spores are blown by the wind onto new, susceptible plant tissue. It is favored by the weather we see typically in spring and early summer: scattered showers followed by warm sun.
Powdery mildew hits Oregon roses during summer's driest time, particularly when dry days are followed by nights with high humidity.
Rather than targeting these diseases individually, Pscheidt treats the rose as a whole organism and recommends a year-round integrated approach to pest and disease control.
Choose disease-resistant plants. The disease reactions of many rose cultivars are listed in the OSU Extension Service's "Pacific Northwest Plant Disease Control Handbook," available at most county Extension offices and online.
Plant varieties with good resistance to all three major diseases; ask about them at your local nursery.
It might be hard to find a cultivar with disease resistance and the color and fragrance you desire.
However, selecting plants that have even moderate resistance can reduce the care required.
Provide air circulation. Take care to plant new roses far enough from walls, shrubs or each other to allow air to circulate and keep leaves dry. Prune stems from the center of the bush to keep the shape open.
Remove and destroy diseased plant material. Because all three of these diseases grow from infected tissue, sanitation is of primary importance. Rake and remove all leaves, dead flowers and plant debris from around the bush.
Keep leaves dry and nighttime humidity low. Water early in the day and avoid splashing the leaves.
Pscheidt and colleagues at OSU Extension have prepared a pamphlet that details rose diseases and their controls. It includes a year-round schedule of what to do to keep roses healthy. "Controlling Diseases and Aphids on your Roses," EC 1520 may be downloaded for free or call 1-800-561-6719 to purchase a printed copy.