Black Spot of Roses

Black spot, a severe rose disease, is attributed to a fungus called Diplocarpon rosae that invades the leaves and significantly diminishes the vitality of the plant.

The term “black spot” aptly describes this fungal infection as it creates dark spots throughout the rose bush’s foliage. If not effectively managed, it has the potential to completely strip the rose bush of its leaves.

Although the disease itself doesn’t directly cause the death of the plant, the gradual loss of leaves weakens the plant over time, rendering it more susceptible to other stresses and damages.

The fungus exhibits significant genetic diversity, and new strains emerge rapidly.

DISEASE CYCLE

During the winter season, the fungus remains dormant in resting structures present on fallen leaves, as well as in inactive infections on young stems and buds.

The fungal spores that are produced germinate and spread through splashing water. To initiate an infection, these spores must remain continuously wet on the leaf surface for a minimum of seven hours.

After successful infection, the fungus progresses and forms fruiting bodies known as acervuli within black lesions. These acervuli subsequently generate spores that are dispersed and splashed onto new tissues, thereby facilitating the spread of the disease.

The progression of the disease is more likely to occur under cool and moist weather conditions.

SYMPTOMS

The symptoms of black spot disease can vary, depending on the specific rose variety and the strain of the fungus involved.

Typically, the affected rose plants exhibit rapidly expanding purplish or black patches on the upper surface of the leaves. In some cases, diffuse and radiating strands of the fungus may be visible in these patches.

Yellowing of the leaf tissue may occur around the spots, and the affected leaves often fall off, even if other parts of the plant remain unaffected. In other instances, the yellowing may not be apparent, but the infected leaves still drop.

Occasionally, the spots remain relatively small, and the leaves do not drop.

Furthermore, small scabby lesions of a black color may appear on young stems.

Severely infected plants may shed nearly all of their leaves, resulting in a significant reduction in their vigor.

DISEASE CONTROL

Chemical control

The following list includes recommended fungicides for the prevention and treatment of black spot disease in roses:

  • RANSOM 600WP: Mix 15 grams per 20 liters of water.
  • EXEMPO CURVE 250SC: Mix 15 milliliters per 20 liters of water.
  • DOMAIN 250EC: Mix 10 milliliters per 20 liters of water.
  • BRADLEY 500SC: Mix 10 milliliters per 20 liters of water.
  • DUCASSE 250EC: Mix 20 milliliters per 20 liters of water.
  • MEGAPRODE LOCK 525P: Mix 30 grams per 20 liters of water.
  • MILLIONAIRE 690WDG: Mix 40 grams per 20 liters of water.
  • PROVIDENCE 400WP: Mix 50 grams per 20 liters of water.
  • ABSOLUTE 375SC: Mix 10 milliliters per 20 liters of water.
  • EXPLORER 3SL: Mix 10 milliliters per 20 liters of water.
  • LONGSTAR 400EC: Mix 10 milliliters per 20 liters of water.
  • MONUMENT 400EC: Mix 40 milliliters per 20 liters of water.

These fungicides can be used to effectively control and manage black spot disease in roses. It is important to follow the recommended dosage instructions when preparing the fungicide mixture.

Non-chemical control

  • Gather and eliminate fallen leaves or bury them under a layer of mulch. Do not add them to compost.
  • Choose rose varieties that are resistant or tolerant to black spot.
  • Ensure you use planting materials that are free from the disease.
  • Avoid overhead irrigation to keep the foliage dry.
  • Prune the plants to improve air circulation and aid in the drying of the foliage.
  • Cultivate roses in an open, sunny location to facilitate the quick drying of the leaves.

Note:

  • When conducting spray treatments, it is recommended to enhance the effectiveness of the fungicide by mixing it with INTEGRA at a rate of 3 milliliters per 20 liters. INTEGRA serves as a sticker, spreader, wetter, and penetrant, improving the overall performance of the fungicide.
  • To prevent the development of resistance in the fungus towards any specific fungicide, it is advisable to alternate between multiple fungicides throughout the growing season.
  • Fungicides should be reapplied at regular intervals of every 7 to 14 days.
  • Timely and effective control of the disease is crucial in minimizing losses caused by the spread of the infection.

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