Pest Management Fact Sheet #5097

Black Spot of Rose

Bruce A. Watt, Extension Plant Pathologist

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Introduction

Black spot is one of the most common and important diseases of roses throughout the world. It is caused by the fungus Diplocarpon rosae. Black spot will cause a general weakening of the plant so that progressively fewer and fewer blooms are formed if the disease is left unchecked. Plants so weakened are increasingly subject to winter injury.

Symptoms

As the name implies, infected leaves show black spots especially on the upper leaf surface. The spots can be up to 1/2 inch in diameter and typically have fringed borders. Yellowing of the leaf begins surrounding the spots and the entire leaf may yellow and eventually drop off (Fig.1). Close inspection of the spots will reveal the presence of tiny, dark, asexual spore-producing bodies (acervuli) (Fig.2) which produce two-celled spores (conidia) (Fig.3). The fungus may also infect the canes where lesions appear purple at first and later black.    

Figure 1.

Figure 2.

Figure 3.

Environmental Conditions

As is true with most fungi, this fungus requires free water for infection to occur. The spores must be wet for at least 7 hours before they can germinate. A temperature of 65°F is best for spore germination and the disease develops most rapidly at about 75°F. Temperatures of 85°F and above inhibit the spread of the disease.

Survival and Dispersal

Acervuli form within two weeks of the initial infection. These structures release spores which are blown or splashed or otherwise carried to new tissues initiating new infections. The fungus survives the winter in fallen leaves and at infection sites on the canes. Spores will not survive in the soil and individual spores will not survive longer than one month.

Management

1. Rake and discard all fallen leaves which are the main source of spores in the spring.
2. Prune and discard any obviously infected canes.
3. Avoid wetting the foliage especially during dark cloudy days.
4. Grow plants in an open sunny location to promote rapid drying of the foliage.
5. Do not plant in dense plantings and avoid windbreaks to allow good air circulation.
6. Use resistant varieties for low maintenance plantings.
7. Remove infected leaves during dry weather to help retard the rate of disease spread.
8. Many fungicides are registered for control of black spot (see table). Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.

References

Horst, R.K. and R.A. Cloyd. 2007. Compendium of Rose Diseases and Pests. American Phytopathological Society, St. Paul, MN

Sinclair, W.A. and H.H. Lyon. 2005. Diseases of Trees and Shrubs. Cornell University Press, Ithaca, NY

When Using Pesticides

ALWAYS FOLLOW LABEL DIRECTIONS!

Pest Management Office
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Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.

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