Mites & mildews

July 2008. A question from Tonga: is there any information about organic pesticides for aphids, mites and powdery mildew.

An answer from a member stated it depends on whether you are talking about ‘organic’ by definition or are looking for purely natural products. For both mites and powdery mildews (by coincidence) sulphur is quite effective, but not on many cucurbits which are sensitive to it. Powdery mildew can be suppressed quite effectively (but not totally controlled) by spraying with a solution of ordinary dishwashing detergent such as Palmolive or Sunlight or the like. The rate is similar to that used for washing up. It certainly slows it down significantly. There is published literature on the method.

The reason it works is that powdery mildews do not like moisture and the detergent wets the spores and hyphae on the leaf surface. Mites also thrive in dry conditions so regular wetting with detergent may suppress them also.

Another member suggested that soap is excellent for aphids, all the commercial organic growers in Zambia use it, usually in combination with a chilli or onion tea repellent. Sulphur is allowed in organics, but starch solution is also good for powdery mildew. In the greenhouses, rose growers looked at using Trichoderma and bicarbonated soda to control powdery mildew. Mites are more difficult, but maintaining a high humidity is very effective. Some of the growers here switch to overhead irrigation at least once a week to achieve this; it also has the bonus of having a ‘wash-off’ effect.