Treating Powdery Mildew on Rosemary
Answered by: Richters Staff
Question from: Nancy Cook
Posted: Before April 1998

What is the best way to deal with powdery mildew on rosemary grown indoors?

First, review your watering practices. Always water in the morning, not at night. Overnight water droplets on leaves are breeding grounds for the powdery mildew fungus. When watering, water the soil without drenching the leaves. With upright rosemaries this is easy to do, and with care you can do this for the prostrate varieties also. If low humidity is a problem mist leaves lightly and set pot on a tray filled with pebbles or gravel and keep water filled in the tray. The roots in the pot should not be in contact with the water in the tray.

If ventilation is good then the chance for disease is lessened; so consider moving your plant to a better ventilated area if air movement is a problem.

There are three treatments for powdery mildew that are safe to use on edible plants. Not all work in every situation so it is necessary to experiment.

1. Some amateur and professional growers have reported success with a homemade baking soda and soap spray. The recipe is: 3 tablespoons baking soda, 2-1/2 tablespoons horticultural oil, 1/2 teaspoon liquid soap mixed in a gallon (3.8 litres) of water. If horticultural oil (also known as summer spray oil) is not available locally experiment with a light vegetable oil; but test it first on a few plants.

2. A spray made from freshly brewed chamomile tea (cooled to room temperature) is effective on a variety of fungal diseases. We are using it on seedflats to combat damping off, another fungal disease. Pour 1 cup boiling water over 1/4 cup chamomile flowers and let steep until cool. Strain prior to use. The spray does not keep; brew fresh batches as needed.

3. If these treatments fail you can try sulfur spray or powder available from most garden supply outlets.

Each of these treatments will require repeat applications to bring powdery mildew under control.

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