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My roses have black spots all over their leaves, will they die? PDF Print E-mail

Black spot is caused by the fungus Diplocarpon rosae. The symptoms are black blotches of various sizes on foliage, often surrounded by yellowing tissue. Severe infections can cause defoliation, especially in warm, humid and warm, wet weather. Modern hybrid tea and floribunda roses tend to be most susceptible when grown in warm, humid, near coastal regions. It isn't fatal, but it does weaken plants and spoil their appearance.

Control

  • Water in the morning and try not to wet foliage. If foliage does get wet there’s a better chance that foliage can dry out, reducing the risk of infection;
  • Regular spraying with sodium bicarbonate (baking soda) is useful for preventing powdery mildew, black spot and rust on roses.

This fungicide was developed and successfully trialled during the 1990’s at the rose garden in the Royal Botanic Gardens, Sydney. It is now part of their integrated pest management of the rose garden because it doesn’t harm beneficial insects, such as tiny predatory wasps and ladybird beetles, that naturally control rose aphids.

Preparation

To one litre of water add two teaspoons of bicarbonate of soda, one drop of horticultural or vegetable oil and a drop of biodegradable washing up liquid. Shaking well before spraying thoroughly mixes the ingredients and helps to dissolve the bicarbonate of soda.

Spray the solution during the cool of the morning to reduce the risk of foliage being burned in intense sunshine before it dries. Wet both sides of leaves and stems until it begins to drip off. Reapply fortnightly or after rain, which rinses this non-persistent solution off. It is most effective when applied in the early stages of infection.

How it works

The detergent helps to spread the mix so it covers the leaf surface evenly. As the mix dries the oil sticks the bicarbonate of soda to the leaf. The bicarbonate of soda is the active ingredient, making the leaf surface alkaline. The spores of powdery mildew, rust and black spot don’t germinate well in an alkaline environment.

Supporting strategies:

  • Planting rose varieties that are tolerant of heat and humidity reduces the risk of powdery mildew and black spot. These include the modern roses such as floribunda roses ‘Iceberg’, ‘Brilliant Pink Iceberg’ and ‘Burgundy Iceberg’, some varieties of ground cover rose as well as old-fashioned (heritage) roses known as tea roses (distinct from the  modern hybrid tea roses), noisette roses and China roses;
 
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