Black Spot

Rose foliage infected by Black Spot. Photo by Ludwig Taschner
Rose foliage infected by Black Spot. Photo by Ludwig Taschner

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 due to loss of foliage. Plants so weakened are increasingly subject to winter injury and other diseases.

Description

As the name implies, infected leaves show black spots especially on the upper leaf

Black spot symptoms on top and bottom of rose foliage. Photo by Ludwig Taschner
Black spot symptoms on top and bottom of rose foliage. Photo by Ludwig Taschner

surface. The spots can be up to 50mm in diameter and typically have fringed borders. Yellowing of the leaf begins surrounding the spots (due to death of cells in an attempt to stop the fungus from spreading) and then entire leaf may yellow and eventually drop off. Close inspection of the spots will reveal the presence of tiny black spore producing bodies. The fungus may also infect the canes where lesions appear purple at first and later black.

Damage

Black spot causes defoliation that occurs when the fungus is allowed spread uncontrollably. First symptoms will occur on the mature [leaves] at the base of the plant, gradually spreading to the top. The main function of plant foliage is to photosynthesise, creating nutrition for the plant for further growth. Indirect damage is caused by sun heating up the sap within the denuded stem up to boiling point which may lead to stemkanker. You will see weaker growth and reduced bloom in the season following severe black spot damage.

Environmental Conditions

As with most fungi, this fungus requires free water for infection to occur. The spores

Black spot on rose foliage. Photo by Ludwig Taschne
Black spot on rose foliage. Photo by Ludwig Taschne

must be wet for at least 7 hours before they can germinate. A temperature of 18°C is best for spore germination and the disease develops most rapidly at about 24°C. Temperatures of 30°C and above inhibit the spread of the disease.

Life Cycle, Survival and Dispersal

The spore producing structures (acervuli) form within two weeks of the initial infection.

Closer view of infected area on underside of leaf with slightly raised, dark brown, spore-producing bodies ("acervuli").Photo by Paul Bachi
Closer view of infected area on underside of leaf with slightly raised, dark brown, spore-producing bodies (“acervuli”).Photo by Paul Bachi

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 do not survive longer than one month.

In spring, spores are splashed up onto newly emerging foliage during rains or irrigation.

Once the weather begins to stay consistently warm and humid, the spores germinate and infect the plant within one day. Visible symptoms (black spot and some yellowing) will be evident within five days, and reproduction and spreading new spores within ten days. The new spores will infect other parts of the plant, or be carried on the wind to any other nearby rose bushes.

Treatment and Prevention

NATURAL PREVENTION

  • Plant roses in full sun. They should receive a full six to eight hours of sun daily.
    Asexual Spore: two-celled spores of the anamorph fungus Marssonina rosae.Photo by Paul Bachi
    Asexual Spore: two-celled spores of the anamorph fungus Marssonina rosae.Photo by Paul Bachi

    Plants will grow more robustly and be able to resist black spot better. Shade causes slower moisture evaporation thus creating a breeding zones for black spot and other fungal diseases.

  • Plant roses in an area with good air circulation and space them well. Moisture evaporates faster. In addition the breeze will dry off the foliage.
  • Aerate the soil in winter. The roots of roses need an aerated soil; plants are stressed if water logging occurs and stunt new growth, thus being more susceptible to black spot.
  • Water correctly. In autumn, avoid over-head irrigation during the late afternoons and evenings. Water evaporates much slower in cooler evening and nighttime temperatures. Avoid wetting the foliage especially during dark cloudy days.
  • Choose resistant varieties. Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings.
  • The method of picking off diseased leaves to prevent spreading has become an old fashioned method due to the availability of new, disease tolerant roses and effective pesticides that should be used for major infestations.

PESTICIDES

Spot checks and preventative spraying are essential. Effective fungicides should be on the shelf in regions where this disease is prevalent such as Coastal regions and on Inland regions if summer rains are prevalent. Protecting the leaves by spraying is effective.

During ideal “black spot” weather condition, spraying on a bi-weekly basis is essential. The following fungicides are effective to a degree in preventing the spores to enter the leaves as well as killing spores on the leaves. The most common group contains the active ingredient Mancozeb. Of these are many fungicides registered under various trade names. Several fungicides are registered for control of black spot. Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.

We strongly recommend ‘CHRONOS’, a suspension concentrate fungicide with the active ingredient: Prochloraz zinc complex (imidazole) & Prochloraz equivalent.

The old remedy of treating black spot with a baking soda spray has been shown to be ineffective.

THE MOST COMMON

  • Dithane WG                 mancozeb
  • Mikal M                        mancozeb + fosetyl aluminium
  • Ridomil Gold                mancozeb + metalyaxyl

FUNGICIDES CONTAINING COPPER

  • CoppercountN                  copper ammonium
  • Copperoxychloride                copperoxychloride

FUNGICIDES ABSORBED BY LEAVES

(These have a partial curative action as they clear the blocked capillaries)

  • Proplant                  proparmocarb
  • Proparmocarb         proparmocarb
  • Benlate                   Benomyl
  • Chronos                  imidazole prochloraz zinc complex

When Using Pesticides always follow the instructions.

References

Insect Pests & Plant Diseases » Black Spot of Ros… http://umaine.edu/ipm/ipddl/publications/5097e/printpage/