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Black Spot of Rose

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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.

Black spot of rose               Black spot of rose

Black spot of rose

Symptoms   Black spot of rose

Dark-brown to black leaf spots develop on the upper leaves, which eventually become yellow and drop. Black spot can be distinguished from other leaf spot diseases by its fringed edges and dark black color. Raised, reddish-purple spots may also appear on rose canes. Warm, humid conditions favor its germination and growth. Leaves having age less than two weeks get more infected. 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.

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.

Black spot of rose

Fungus over winters on dead and fallen leaves and also remains intact with twigs and branches. Fungus produces spores in spring which get disseminated by rain splashes besides rain wind, irrigation water flow and human mismanagement.. Spores are produced throughout the growing season and cause repeated infection cycles.

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
Chlorothalonil 7-14 day schedule Daconil, Bravo, Echo, Fungonil and others
Maneb 7-10 day schedule Maneb
Mancozeb 7-10 day schedule Mancozeb, Stature, Dithane M45, and others
Ziram 7-10 day schedule Ziram
Captan 7-10 day schedule Captan
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

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