Roses are a popular addition to Colorado gardens. Fortunately, the state's dry climate and high altitude help prevent the development of most fungal diseases. Yet many gardeners plant their roses close together, which produce an environment more conducive to the growth of fungi. Fungi cause most Colorado rose diseases.

Rose mosaic is a virus that is found worldwide. Its symptoms include mosaic patterns or splotches of yellow and green on leaves. Other symptoms are chlorotic ringspots and yellow vien banding. The coloration in flowers may be uneven.

Transmission of the disease is through rooting cuttings and root grafting, and the only way to control rose mosaic is to remove the infected plants. Many gardeners leave the plant and ignore the virus. However, infected plants may be more susceptible to winter injury.

Another disease of roses is powdery mildew, which occurs in both dry and humid weather. This fungus produces a white growth on the tops and bottoms of leaves and stems. Plants may become stunted, and leaves may curl and drop. To manage powdery mildew, remove and destroy infected shoots at the end of the season and rake up any infected leaves, or spray plants with sulfur dusts, neem oil, or other systemic fungicides.

Rust affects the undersides of leaves and other plant parts. It appears as orange, powdery pustules that develop into orange-brown spots on the leaf surfaces. Rust can develop when temperatures are in the upper sixties and moisture is continuous for two to three hours. To control rust, remove infected stems and leaves when it first appears. Carefully prune old canes to help eliminate carry-over from season to season.

The best way to avoid these problems with Colorado roses is to buy and plant disease-free plants. Choose resistant varieties, and avoid wounding plants during transplanting.

Plant roses in sunny areas with good drainage and ventilation. Do not place plants too close together, and avoid shady spots. Avoid overhead watering that leaves water on the leaf surfaces and increase the chance of disease. When using a fungicide, carefully read the label and follow all directions.

Laurie Wasmund is a Colorado Master Gardener. The Colorado State University Cooperative Extension volunteer network strives to enhance Coloradans’ quality of life by extending knowledge-based education throughout Colorado communities to foster successful gardeners. For more information, call the CSU Extension, Elbert County Master Gardener office at 303-621-3162.