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Fact Sheets - Black Spot of Rose

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Pest Management Fact Sheet #5097

Black Spot of Rose

Bruce A. Watt, Extension Plant Pathologist

For information about UMaine Extension programs and resources, visit extension.umaine.edu.
Find more of our publications and books at extensionpubs.umext.maine.edu.

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.    

Yellowing of the leaf beginning to surround spots

Figure 1.

Acervuli- The tiny, dark, asexual spore-producing bodies

Figure 2.

Conidia- two-celled spores produced by the acervuli

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.
Traditional Fungicides for Black Spot Control
Fungicide Apply when first observed Examples of Trade Names
Trifloxystrobin 7-14 day schedule Compass
Chlorothalonil 7-14 day schedule Daconil, Bravo, Echo, Fungonil and others
Myclobutanil 7-10 day schedule Eagle
Maneb 7-10 day schedule Maneb
Mancozeb 7-10 day schedule Mancozeb, Stature, Dithane M45, and others
Thiophanate-methyl 10-14 day schedule Fungo Flo, Quali-Pro TM, Systec, Cleary’s 3336
Ziram 7-10 day schedule Ziram
Captan 7-10 day schedule Captan
Triforine 7-10 day schedule Funginex
Propiconazole 14-21 day schedule Banner Maxx
Alternative Fungicides for Black Spot Control
Fungicide Apply when first observed Examples of Trade Names
Copper products Coverage critical. 5-7 day schedule. Kocide, Tenn-Cop, Basicop, and others
Lime Sulfur Apply when dormant Lime Sulfur
Neem oil Preventative 7-14 day schedule. 70% Neem Oil
Potassium bicarbonate 10-14 day intervals Remedy Fungicide, Armicarb 100
Sulfur Coverage critical. 5-10 day schedule. Sulfur Dust, Wettable Sulfur, and others
Hydrogen dioxide Commercial only. See label ZeroTol

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
491 College Avenue, Orono, ME 04473-1295
1.800.287.0279 (in Maine)


Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 2010, 2013
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.

Call 800-287-0274 or TDD 800-287-8957 (in Maine), or 207-581-3188, for information on publications and program offerings from University of Maine Cooperative Extension, or visit extension.umaine.edu.


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