Indoor Garden Powdery Mildew Problem

by Cody
(Caledonia, MI)

I will give you some info about our grow. I am flowering in a 8x12x9 room featuring two 1000 watt lights on a light mover. I have a 6" inline fan exhausting the stale air outside, and a squirrel cage fan bringing fresh air in from another room, venting through the lights, and coming back out the other side to both cool the lights and heat the room.

My room conditions almost always stay perfect. I have 70-73 degree temps during the day with 40-45% humidity. At night temps stay at 66-67 degrees with 35-40% humidity. We are about 2-3 weeks away from harvesting and were having a very difficult time battling powdery mildew. I have tried Green Cure, which is an organic foliar spray that has an active ingredient of potassium bicarbonate.

This did not work, the Mildew came back. I tried spraying water with a ph of 8.2 ( I read that powdery mildew spores can not live with a ph above 8.0) Not only did this not work, it made our problem way worse. My question is....What would you suggest I do to save my harvest. I am willing to pull out all stops to try and battle this disease and save as much of our harvest as possible. Thank You, Cody

Answer: Cody- I wish I had gotten to your submission earlier. Unfortunately, once powdery mildew is well established it is very difficult to save a crop without destroying much of it. My biggest concern would be every crop you grow in that grow room after this point. Spores can lay dormant for years until the find the right conditions, and you are sure to have many spores contaminating that room now.

You seem to have your humidity in check, which is usually the culprit. Cool temps can play a roll also, but 66-67 degrees shouldn't cause too much of a problem really. Pay careful attention to humidity in the few hours after lights out. As the room cools, the air cannot hold as much water vapor. The excess water vapor condensates out of the air and onto your plants, usually causing problems like this.

I do have a couple of suggestions. First, wipe everything down with a mild bleach water solution before you begin another crop in that room...about two tablespoons per gallon should be all you need. Also, try to find any possible sources of contamination around the grow room, such as a basement with a dirt floor that stays too humid or a mildewy bathroom- start with the room you are pulling your fresh air from, but check the entire house.

If you do find any suspect areas, try to eliminate them and keep them dry afterward. Hopefully you are aware of the importance of air circulation (not just air exhaust). Plants do not have lungs, so any oxygen and water vapor exhaled by the plants tend to hang in the air in and around the plants....especially if the plant is big and dense with many tight nooks and crannies. If you do not have enough air circulation in and around the plants, the humidity inside these nooks and crannies could be very high (in spite of your low room humidity).

This is what is known as "micro-climate". With oxygen and excess water vapor hanging around the plants, the plants will have a hard time breathing. Increasing the air circulation helps the plants to breath by removing the water vapor from the tight spaces and bringing in a little fresh CO2. So, the more you increase the air circulation the more resistant your plants are likely to become to the powdery mildew problem (or high humidity in general, for that matter). One 12 inch oscillating fan per 4 x 4 foot garden area is normally sufficient.

If you do not already have a digital thermometer-hygrometer with a min and max memory, I suggest you get one. If you only have a regular thermometer that you check manually, you may be missing the true low temps at night or the true high humidity level for the day. The one I use is a Sunleaves only cost about $25, it has a remote probe for keeping tabs on the hot spot in your garden, and the min/max readings are resettable (I recommend resetting it each time you check it).

Last but not least, a gentleman from California recently contacted my about testing one of his products. It is a product made of nano-silver particles and essential plant oils, and is effective as both a natural pesticide as well as a fungicide. Here are a few clips from one of the emails he sent me after I asked for more info about his product....

"We work with a colloidal chemist, who has been fine tuning his chemistry for, 30 + years. We take a colloidal Ag base, which is made up of colloidal micelles, 2- 4 nano meters in size and add the highest grade essential oils, we can source. The AG base extrapolates the natural insecticidal properties of the oils to eliminate pests. As the insect inhales the solution thru their spiracles, the product attacks the protective protein chains. We attack 6 different chains, but specifically the one which governs the chiatin is the fatal action. The bugs suffocate in around 45 seconds, we kill can eliminate cockroaches, if need be.
In regards to powdery mildew, we inject a sugar carb, which is reduced to 2-4 nanometers, into the spore resulting in blowing up the spore. At this point the majority (60%) of our California clients utilize the product for its powdery mildew efficacy.
The product, along with all others from our chemist, are plant based extracts. The plant is starting the process of having all products certified organic, but that's probably a ways off. Our Ld-50 for the concentrate base is a little more lethal than water, so you have to consume a huge volume to shut down kidneys."

Given what I know about nano-silver, the products sound very promising....not only for powdery mildew, but also for spider mites, thrips, and other indoor garden pests. Truth be told, I am waiting for samples from the guy right now, and I am pretty excited about trying it out. You can check out the products on his website,

The guys name is Dudley, and there is contact info on his site if you have any specific questions about this stuff. I sure hope this helps you out Cody, and Happy Growing!

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