Powdery mildew

The plant I am growing is nearly ripe but has white powder on most of the leaves. Can I bring it back to health or is it too late?
Kevo, Internet

Is there anything to do about powdery mildew on indoor plants that are almost done? I’ve never had this before, where does it come from? How can I prevent it from recurring?

Arcata, California

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.

Readers with grow questions (or answers) should send them to Ed at:
Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA
You can also email Ed at [email protected], and send queries via his website at www.quicktrading.com.
All featured questions will be rewarded with a copy of Marijuana Question? Ask Ed. Sorry, Ed cannot send personal replies to your questions.



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  17. Seagazer101 on

    I live in the same area as G., above, but my plants are outside. We have had an awful summer with fog almost every day. So, the humidity is indeed high, but it has rarely to never reached +60F while it’s been growing. I just removed most of the interior branches and all the obvious mildew, but still have a question. Will smoking this stuff give you a disease?