no. 2.946

Diseases of Roses in Colorado

Quick Facts...

• Colorado's dry climate and high altitude are not conducive to the development of most fungal diseases.
• Rose mosaic usually shows up as mosaic patterns or splotches of yellow and green on leaves.
• Powdery mildew is a common rose disease because it occurs in dry as well as humid weather.
• Rust appears on the undersides of leaves and other plant parts as orange, powdery pustules.
• Anthracnose appears as small, circular leaf spots with dark red margins.

Few rose diseases are found under Colorado conditions. The state's dry climate and high altitude are not conducive to the development of most fungal diseases. Viruses commonly associated with roses are not known to be transmitted by Insects.

Roses are becoming more popular in Colorado Gardens. As a result, people often plant them close together. This makes the environment among the plants more conducive to powdery mildew, rust and other fungi. However, most of these problems can be managed. It seldom is necessary to use fungicides if you are careful about plant selection, site location, soil preparation, fertilization, watering and pruning.

With two exceptions, fungi cause most Colorado rose diseases. Foliar fungal diseases are spread by air- or water-borne spores that survive on leaf debris. Nine diseases and their control are described below.

Rose Mosaic Virus

Figure 1: Symptoms of rose mosaic virus.

Rose mosaic, caused by a virus, is found worldwide. Symptoms vary but usually show up as mosaic patterns or splotches of yellow and green on leaves (Figure 1). Flowers may be mottled in color, a condition called flower breaking.

Leaf and flower symptoms may detract from the overall quality of the plant. Infected plants may be more sensitive to winter injury. Several rose growers in Colorado report problems with the virus. They say symptoms are more prominent in some seasons than others. This is because growing conditions change from year to year. Plant health, weather, varieties and other factors all can contribute to virus symptoms. No evidence exists that Insects transmit this virus. Transmission, therefore, appears to be limited to rooting cuttings and root grafting.

The only way to control rose mosaic is to remove infected plants. Because of limited effect on flowering, most people leave the plant alone and ignore the virus. Management of rose mosaic virus is through the use of virus-free stock.

Powdery Mildew

Figure 2: Powdery mildew.

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, Sphaerotheca pannosa, produces a white, talcum-powder-like growth on the top and bottom of the leaves and stems (Figure 2).

When the disease is severe, plants become stunted and leaves curl and drop. While other powdery mildew fungi can have several plant hosts, S. pannosa attacks only roses.

Generally, the most favorable conditions for powdery mildew are daytime temperatures near 80 degrees F with a relative humidity of 40 to 70 percent, and nighttime temperatures near 60 degrees. To manage powdery mildew:

1. Plant resistant varieties.
2. Remove and destroy infected shoots at the end of the season and rake up and discard infested, fallen, dead leaves.
3. Spray with sulfur dusts, neem oil (Rose Defense, Shield-All, Triact) or systemic fungicides [i.e., thiophanates (Clearys 3336, Halts Systemic, Greenlight Systemic), triforine (Funginex), triadimefon (Bayleton, Fung-Away, Intercept)].

Rust

Rust first appears on the undersides of leaves and other plant parts as orange, powdery pustules. As these pustules develop, they become visible on the upper leaf surfaces as orange-brown spots. Nine species of the rust fungus Phragmidium are found on roses. Rust can develop when temperatures are 65 to 70 degrees and moisture is continuous for two to three hours.

Considerable variation exists in plant resistance. It is not uncommon for only one or two rose varieties in a planting to get rust while others remain rust-free. It might be that a species of the fungus is present that can develop only on particular varieties. With nine rust species that can attack roses, there frequently are differences in resistance.

Management of rose rust depends on a variety of approaches. When used in an integrated system, these approaches can minimize the development and damage caused by the fungus.

1. Use resistant varieties.
2. Remove infected stems and leaves at first appearance. Careful pruning of old canes helps to eliminate rust carry-over on the canes and promotes air movement and drying of leaves.
3. Use preventive fungicides [chlorothalonil (Daconil 2787)] or systemic fungicides [triadimefon or triforine (Funginex)].

Other Diseases

Anthracnose or spot anthracnose is a disease that appears to be more prevalent in Colorado after a dry spring. Hybrid teas, as well as old-fashioned varieties, are susceptible. Worldwide, this disease seems to be of little importance, as there is virtually no documentation of it in the literature. Symptoms occur as leaf spots, sometimes running together. They usually are less than 1/4 inch in diameter and circular with dark red margins. Newly formed spots are red or purple. Older spots have white centers with a dark red margin. A shot-hole effect may occur -- the spot itself drops out of the leaf, leaving a circular hole. Defoliation may occur if the disease is severe.

Botrytis blight mainly affects hybrid tea roses. The fungus attacks leaves and canes, prevents blooms from opening, and often causes flower petals to turn brown and shrivel. To diagnose it, look closely at cankered stems, brown leaves and flowers. If the fungus is present, the affected areas 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 are highly susceptible to this disease.

Figure 3: Black spot of rose.

Black spot disease in Colorado is more common on some of the old-fashioned rose varieties. Hybrid teas often don't show symptoms in this climate. The disease is characterized by nearly circular, black spots with fringed (not smooth) margins (Figure 3). The spots vary from less than 1/16 to 1/2 inch or more in diameter. Black spot is favored by wet weather and 65 to 75 degree temperatures. Severe infections cause defoliation.

Coniothyrium canker is not a common problem in this area. However, it can be found in greenhouses. Occurrences were reported on outdoor roses in the spring of 1991. Symptoms include a discoloration of woody tissue (canker), usually near the base of the plant. The fungal cankers eventually can girdle the plant. The disease can be serious on rose plants in storage, in nurseries (where plants are crowded together and foliage is wet), and on recently planted roses, especially if they are under stress.

Crown gall is caused by a bacterium that survives in the soil. The pathogen enters the crown or roots via wounds made by mechanical damage or insect feeding. Once inside the plant, the bacterium causes the cell tissue to proliferate and form a gall (a tumor-like growth) on the crown or roots. The tissue eventually girdles the rose and kills the plant. Destroy infected plants. The bacterium can survive in galls and plant residue in the soil for many years without a rose host. Do not plant roses in infested soil for at least five years.

Verticillium wilt is caused by the soil-borne fungus Verticillium alboatrum. The disease becomes evident when temperatures are hot (late June to August). Initial symptoms begin with the wilting of new leaves and yellowing of older leaves. Permanent wilting occurs after a few days and leaves eventually turn brown and die. This disease can be confused with other problems, such as insect infestations or environmentally related causes. Make sure Verticillium wilt is positively identified by an appropriate plant disease clinic before following control measures. To control the disease, remove and destroy infected plants. Because the fungus responsible for the disease survives in the soil, do not plant roses in contaminated soil for three years or longer.

Disease Controls

Simple maintenance or cultural practices can prevent most rose diseases. Fungicides are necessary only in severe cases.

• Buy and plant disease-free plants.
• Choose resistant varieties. Varieties are available with resistance to powdery mildew, black spot and many other diseases. See Table 1.
• 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 leaf surfaces will dry faster, preventing disease infection.
• Remove and destroy infected leaves and canes during the season.
• Avoid overhead watering. Water on the leaf surface increases the chance of foliar disease infection.
• If disease is severe, a fungicide may be warranted. Diagnose the problem correctly before applying a chemical. Carefully read and follow all chemical label directions.

¹ L.P. Pottorff, Colorado State University Extension plant pathologist, Integrated Pest Management Program, Jefferson County; and W.M. Brown Jr., Extension plant pathologist and professor, bioagricultural sciences and pest management. 3/06.

Colorado State University, U.S. Department of Agriculture and Colorado counties cooperating. Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.

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