Shellie, Bobbins and Battie,
White City, Oregon
It sounds as if your plants have been attacked by powdery mildew. This is a mildew closely related to fungus. The powder is the mildew's reproductive spores. It thrives in an acid environment in a temperature range of 60-70?F (15-21?C) with a humidity above 50%. The spores are floating in the air and there is no practical way to screen them out. Instead, try to change the environment so that conditions don't match the mildew's needs. This may require raising the temperature or lowering humidity.
There are also several safe and effective ways of controlling powdery mildew using minerals or organisms.
Most of the mildicides listed here are fairly new and they are all much less harmful than the old chemical formulations. You won't find most of them at your local store or garden shop, but they are available on the Internet. Many companies sell them there. All of these mildicides are used on edibles or herbs. Some are naturally occurring organisms and are exempt from registration. Others are registered for use on vegetable crops and are considered in the "caution" category, the category for the least dangerous registered mildicides.
AQ10: AQ10 uses a totally new method of fighting powdery mildew, a biofungicide. The active ingredient, Ampelomyces quisqualis, is a fungus that parasitizes the powdery mildew organism. It offers control over a long period of time.
Cinnamite: Cinnamite is an extract of cinnamon used as a miticide which is also effective as a fungicide. It is very easy to use, is effective and has a pleasant cinnamon odor. Studies show it is not harmful to marijuana plants.
Copper: Copper ions inactivate some fungal enzyme systems, killing the mycellium. Copper has been used for over 100 years, and is effective. A few brands of copper based fungicides are Phyton 27, Dexol Copper Bordeaux Mix and Kocide DF. There are many other brands available.
Neem Oil: Neem oil is pressed from the nut of the Indian Neem Tree. It protects against and kills mildew by interfering with respiration and collapsing the cell wall. Some growers claim that plants grow more vigorously when sprayed with dilute neem oil twice a month. There are many brands of neem oil available. Many of them are listed as organic.
Plant Shield: Plant Shield contains the organism Trichoderma harzianum strain T-22. This organism attacks fungi and mildews. It is used as a spray or dip. The organism seeks its food and forms a symbiotic relationship with plant roots, which it also protects.
Potassium Bicarbonate: Potassium bicarbonate collapses and desiccates the mildew hyphae. This is a very safe, very effective contact fungicide. Mildew do not develop resistance to it. The potassium in the formula is absorbed by the plant. Two brands are Kaligreen (registered in California) and Armicarb100.
Serenade: Is the fermentation product of a bacterium, bacillus subtillis, that inhibits cell growth of fungi and bacteria. It is very effective and easy to spray on or to use as a dip. It is a contact fungicide that kills only areas that it contacts. A wetting agent or spreader increases total contact.
Sodium Bicarbonate (baking soda): Baking soda leaves an alkaline residue on the leaves. The sodium collapses the powdery mildew cell wall and the alkaline environment discourages growth. Plants have a limited tolerance to sodium, so the residue should be washed off before more is applied. Used at the rate of 1/2 teaspoon per quart of water with a wetting agent.
Sulfur: Elememtal sulfur interferes with mildew cellular respiration. It has been used as a fungicide for more than 100 years. There are small packages available in the baking sections of supermarkets.
These new remedies make it much easier to deal with powdery mildew. They are all non-toxic and eliminate the problem fairly easily.
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