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.
However, simple adjustments in rose care can improve the health of your plants, according to Jay Pscheidt, plant pathologist for Oregon State University Extension Service.
First, it is 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 different 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 "Pacific Northwest Plant Disease Control Handbook," published annually by OSU Extension Service and available at most county Extension offices and online at: http://plant-disease.ippc.orst.edu/
Cultivars with good resistance to all three major diseases are available, so ask about them at your local nursery.
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 when, how, and why to help keep your roses healthy. Controlling Diseases and Aphids on your Roses may be dowloaded from the Web, or call 1-800-561-6719 to purchase a printed copy.