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Sulphur To Control Blackspot

Rosa ‘Lilli Marlene’ is another healthy rose that has stayed in my garden. Lilli Marlene is a floribunda rose with clusters of deep red blooms among dark green foliage.

In order to help combat black spot during the hotter weather I have been using lime-sulfur this year as a preventative. I also removed a large shrub that was near these roses, thereby increasing both sun and air circulation around the plants which is very important in controlling blackspot.

Black spot (Diplocarpon rosae) is a fungus disease that attacks roses, especially during hot humid weather. You know you have it when the leaves start to develop rough edged yellow spots that can spread throughout the entire leaf and eventually the rose can become completely defoliated.

Unfortunately the disease forms in the layers of the leaf so early prevention is important. Controlling black spot is a four fold process:

  1. Purchase disease ‘resistant’ plants.  Unfortunately Lilli Marlene is not particularly resistant to black spot but it is not one of the worst offenders in my garden.
  2. Prune out any signs of black spot early and destroy (don’t put it in the compost pile).  The spores overwinter in the soil so cleanup is important.  Consider cutting down the canes to the ground in winter if you have an infestation since the spores can also overwinter on the canes.
  3. Spray with dormant oil in late fall and early spring (before leaves appear) to suffocate overwintering spores (as well as eggs and overwintering adults of other pests on roses).
  4. Spray with lime-sulfur on all leaf surfaces as a preventative (every 7-10 days, mix as per directions).

Although some of the leaves have been attacked by a bug that has infested the entire bird sanctuary garden this year, I have sprayed the leaves with sulfur regularly and these roses are still very dark green.  I have also been spraying my peony leaves after having cut back all the peony flowers as I have experienced some powdery mildew the last couple of years despite cutting back and cleanup.  The sulfur seems to be helping control that problem as well.

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