Common Rose Diseases
By Laura Pottorff, horticulturist and plant pathologist with Colorado State
University Cooperative Extension
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 powder-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
- 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 attacks
leaves and canes, prevents blooms from opening and often causes flower petals to turn
brown and shrivel (below). 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 Bortytis Blight. Roses under stress will be highly susceptible to
- 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 from 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.
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 powdery 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
- Avoid overhead watering. Water on the leaf surface will increase the chance of disease
- If disease is severe, fungicide use may be warranted.
Photos: Judy Sedbrook.
For more information on Roses, see CSU Fact Sheet 7.416 Rose
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