Powdery mildew can sweep through a planting in short order resulting in dramatic reductions in fruit and vegetable yields. Likewise powdery mildew can so damage flowering plants that they become unsightly.
Some easy preventative measures can be taken long before signs of infestation occur. Give susceptible plants plenty of room so that they have good air circulation. Treat any early signs of powdery mildew to deter its spread to other plants in the yard.
Garden talk is full of treatments for powdery mildew. There is a possibility that you have been advised to spray affected plants with a 10% solution of skim milk, a solution of vegetable or dormant oil, a weak tea solution, a solution of dishwashing soap, or a solution of baking soda. All of the above and others have been shown to be mildly to totally successful in preventing an infestation or stopping the spread of the fungi responsible for powdery mildew.
There are many commercial fungicidal products that can be sprayed on plants to give some degree of protection from fungal diseases such as black spot and powdery mildew. Some gardeners resist the urge to use these products due to fears of potential harm to birds, beneficial insects and themselves. Fortunately some of the very best fungicidal products are safe and considered organic, although they are chemical in nature. Four safe and effective procedures are listed below.
Powdered sulfur can be made into an aqueous solution and applied to affected plants at 7-10 day intervals. Several drops of dishwashing soap or insecticidal soap can be added as a wetting agent. Spray on upper and lower leaf surfaces of all affected plants. This treatment will result in a milky residue that deters from the natural beauty of the plant. This solution should not be applied when daytime temperatures are above 32 degrees C (90 degrees F) as its application can result in some plant tissue damage at high temperatures.
Sodium bicarbonate (Baking Soda) or potassium bicarbonate are very effective in preventing powdery mildew infestation and/or eradicating infestations of powdery mildew that have become established. A solution of one teaspoon of baking soda per liter of water will be a very effective treatment and at very low cost. The solution may be even more effective when a few drops of dishwashing soap or insecticidal soap are added as a wetting agent. Spray on both the upper and lower leaf surfaces. This solution sucks the
moisture out of the fungal hyphae in a process referred to as osmotic shock. Repeat treatment every 7-10 days but more often during times of rain. A slight residue remains.
Cupric hydroxide in an aqueous (water) solution is highly effective in treating a wide variety of fungi including black spot and powdery mildew. Unfortunately, it leaves a very prominent blue/gray residue on plant surfaces that have been treated. Again, a few drops of dishwashing soap should be added as a wetting agent. It is important to stir the cupric hydroxide solution well. Filtering the solution through a single layer of cloth will remove any clumps non dissolved solids that could plug the nozzle of your sprayer. It is important to wash all remaining solution from the sprayer after treatment is completed to prevent any buildup of solids in the sprayer, its pump parts and nozzle.
Soap sprays made by mixing liquid dishwashing soap (in amounts from one teaspoon to one tablespoon per liter) with water have been used to prevent fungal infestations with great success by thousands of home gardeners. The soap acts as a desiccant on contact with powdery mildew. The application of soap sprays is more effective if used as a preventative measure than when used to eradicate powdery mildew after infestation has occurred. No spray residue is visible on plant surfaces after treatment.