Pictures?
I have never had this issue but I found this on the web:
Powdery mildew spores float in the air and are everywhere. They need the proper conditions to germinate: a surface that contains nutrients on which they can grow, temperatures over 60?F (16-20 C), high humidity and a slightly acidic environment. Once the spore senses these conditions , it germinates, sending out a hyhae which seeks out a stomata. It enters the tissue and lives off its nutrients, eventually killing the tissue. The downy mildew powder you see on the leaf is the reproductive organ of the plant, which is releasing spores into the air.
Once the spore germinates, it survives in a wider range of conditions. Lowering humidity does not eliminate the infection. There are several things you can do to eliminate it:
Plant Shield is a foliar spray which kills many types of foliar and root fungus. It is not a chemical but a microbe, trichoderma harzianum strain T-22, which feasts on the fungus. It is totally harmless to non-fungal organisms so it is safe to use. It takes 2-10 days to control the disease. It is available from ITS: 800-375-1684 or 303-661-9546.
Neem Oil is available in many formulations. It is quite effective on powdery mildew, with results noticeable in a couple of days. It is available in garden shops, nurseries and the net.
Armicarb or Kali-Carb are made from potassium bicarbonate. Potassium is a plant nutrient and the bicarbonate makes it soluble and alkaline. Powdery mildew cannot grow in an alkaline environment. The mold dies.
Baking Soda, bicarbonate of soda, is a home remedy. It contains sodium so using it too often will be harmful to the plant. Use one tablespoon per gallon and spray the leaves thoroughly.
Alkaline water controls powdery mildew. Use a water with a pH of close to eight. Some western waters have higher pH. The pH should be lowered to the low eights. The pH of acidic water can be raised using pH Up, available at garden shops.
To prevent recurrences, lower the humidity in the space. The humidity goes up in the evening because as the air cools the air's water-carrying capacity goes down. The same amount of water in the air results in higher relative humidity at lower temperatures. This occurs when the lights are off, a favorite time for spores to germinate. To prevent this, use a dehumidifier, which pulls moisture from the air resulting in a lower humidity.
Hope this helps and post some pics so I can see the brown spots.
