Powdery Mildew starts out as whitish spots that spread quickly until the entire leaf is covered. The white powdery growth is a fungus that with time becomes gray to tan/brown felt like patches. Leaves may become stunted, curled, chlorotic and eventually wither and dry up.
Moderate temperatures and high humidity help develop the disease. The pathogen also favors hot days, cool nights, high humid (85%) conditions and the change of season. Each mildew pathogens is specific to its host, the mildew that attacks gerbera daisy will not spread to cucumbers.
Depending on the severity, spraying it with a baking soda formula is effective as a preventative when applied regularily. For active infections spray daily for a week.
1. Mix 1 TBsp each of baking soda and horticultural oil (dormant oil/vegetable oil) or a few drops of liquid soap to 1 gallon of water. Spray weekly making a new mix each time. It will not elliminate the disease but help control it.
2. Mix 1 tsp baking soda with a few drops of vegetable oil in 1 quart of water. Spray or paint on the leaves. Works on houseplants, cucurbits & roses (balck spot).
Another suggestion is a solution of 1/3 milk and 2/3 water and spray on plants. Use every other day.
I also heard of mixing 1 tbsp of pine sol to 1 gal of water as a mildew spray. Neem Oil is also affectective in controling infections. Use 1 oz.(2 Tbsp) of Neem oil and 1/ 1/2 tsp of dishwashing detergent to one gallon of water. Spray once a week for two weeks.
Use chemical sprays such as Benomyl (systemic fungicide), sulfur/fungicidal soap early in the growing season as a preventative or as soon as symptoms appear. A new organic fungicide that has been making waves is No Powdery Mildew™ which is a natural plant extract formulation that cures, treats and controls powdery mildew outbreaks. Always follow label directions, to make sure the product is approved for specific plants. The combination of Neem and baking soda is the safest control method. Once the disease takes hold, it is difficult to control.
Some cultural preventatives that can be done are to remove the infected leaves, do not crowd the plants, provide good air circulation and keep plants well watered and stress free. Grow resistant plants when available.