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A beautiful rose is a trophy in the flower Garden, but its health is dependent on weather and
the vigor of the plant.
A wet and humid spring season results in an increased incidence of rose diseases, especially
on weaker plants. Cold, dry winters also are hard on roses. At the very least, tough winters
weaken some rose plants, making them more susceptible to disease.
With one exception, most Colorado rose diseases are caused by fungi. The five diseases to be
aware of and their control are:
- Powdery Mildew is one of the state's most common rose diseases because it occurs in dry
as well as humid weather. The powdery mildew fungus produces a white, talcum-like growth on
the top and bottom of the leaves and stems. When the disease is severe, plants become stunted
and leaves curl and drop. Generally, the most favorable conditions for development of the powdery mildew are
day-time temperatures near 80 degrees F with a relative humidity of 40-70 percent, and
night-time temperatures near 60 degrees F.
- Rust first appears on the underside of leaves and other plant parts as orange powdery
"pustules." As these pustules develop, they become visible on the upper leaf surfaces as orange
or brown spots.
- Botrytis Blight affects mainly hybrid tea roses. The fungus attack leaves and canes,
prevents blooms from opening and often causes flower petals to turn brown and shrivel. To
diagnose, look closely at cankered stems, brown leaves and flowers. If the fungus is present, the
affected areas of plants often are covered with a grayish brown fuzzy growth. Cooler
temperatures, moisture and weakened plant tissue create conditions that invite Botrytis Blight.
Roses under stress will be highly susceptible to this disease.
- Blackspot disease is more common on some of the old fashioned rose varieties. (Most
hybrid teas have been developed to resist blackspot.)The disease is characterized by a leaf
abnormality of nearly circular black spots with fringed margins. The spots vary from less than
one-sixteenth to one-half inch or more in diameter. Blackspot also is favored by wet weather and
65-75 degree F temperatures. Severe infections will cause defoliation.
- Rose Mosaic, caused by a virus, is found worldwide. Symptoms vary, but usually show up
as mosaic patterns or splotches of yellow and green. No adverse effect on flower production has
been reported, but leaf symptoms may detract form the overall quality of the plant. Infected
plants may be more sensitive to winter kill. The only way to control rose mosaic is to remove
infected plants. Because of no adverse effect on flowering, most people leave the plants alone
and ignore the virus.
Disease Controls
Most rose diseases can be controlled or prevented with simple maintenance or cultural
practices. Fungicides are necessary only in severe cases.
- Buy and transplant disease-free plants.
- Choose resistant varieties. Varieties are available with resistance to powder mildew,
blackspot and many other diseases.
- Avoid wounding plants during transplanting.
- Plant roses in areas with good soil drainage and ventilation. Avoid shady spots and dense
plantings. This will improve air circulation so that leaf surfaces will dry faster, preventing
disease infection.
- Remove and destroy infected leaves and canes during the season. Do not compost diseased
material.
- Avoid overhead watering. Water on the leaf surface will increase the chance of disease
infection.
- If disease is severe, fungicide use may be warranted.
Diagnose the problem correctly before applying a chemical.
Contact the Colorado State University Extension office in Jefferson county for
diagnostic services and chemical recommendations. Call (303) 271-6620.
For more information on Gardening, contact your
local Colorado State University Extension office.
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