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Organic Control of Plant Diseases


Black spot (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.


  • 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.

* Baking Soda Fungicide

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.

To one gallon 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 weekly 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.;


196 Moraga Way Orinda, CA 94563 Phone (925) 254-3713     Fax (925) 254-9168
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