I recently dealt with quite a large Powdery Mildew outbreak. I have it 99% handled now and it was fairly simple. Here is how I took care of it:
I have a hose that I have permanently run into my grow area. It has a little nozzle thing on the end I can use to temporarily shut it off and on while I'm working and the spigot is turned on. After the lights in the grow room turn off for the night, I go in there with a green flashlight and the hose turned on (fairly high pressure). I open up the nozzle just slightly so its spraying out a very small high pressure stream of water. I use the high pressure stream to spray off each leaf. You want it spraying fairly hard and you don't want it using tons of water, otherwise you'll end up with a massive mess all over the floor. So, you spray off all the leaves. Make sure the pressure isn't so high that you are snapping off branches and leaves. Spray off the plants as best as you can. It will make a bit of a mess on the floor, but you just use a towel to wipe that up afterwards. The next step is: hittin the girls with some neem oil. I use 2 to 4 teaspoons of neem oil extract/concentrate per liter of water. I bought "Garden Safe" brand neem extract from Lowes. I think it was less than 10 bucks. I'll include a pic along with other pics. I used to use stuff like AzaMax and/or Mighty Wash to control problems like spider mites. They were really hard on the plants and they were really expensive. Now that I'm using neem, I haven't had an issue in 6 months. I haven't seen a single spider mite, and it works fantastic for PM. No issues what-so-ever. I just hit the girls with neem once every few weeks, and my plants are happier and healthier than they've been in over 4 years. So, I use a "gun" that you can buy at Lowes that you use for painting... spraying paint onto walls. You can use any spraying device you want, but if you have a large garden, this investment will be worth every penny. I mix up my neem oil and spray away. You can do this a half hour or an hour after you rinse the girls off with plain water, but I usually wait until the next night, after they've had a chance to dry. You want to DRENCH the plants top to bottom with your neem oil mixture. I mean DRENCHED. You want the plants to look like there was just a rain storm in there for the last few hours or something. When you go into your grow room the next day, you'll see 90-100 percent of the PM gone (depending on how bad the case was). Repeat this whole process every 1 to 3 weeks as needed. Once you get rid of your powdery mildew, continue doing this every few weeks. It's called preventative maintenance and will save you so much heartache. If you are worried about tasting it on your bud, discontinue the program 2 weeks before harvest and rinse off with plain water one last time about a week before harvest. I'm telling you... If you do this and continue doing it as preventative maintenance, you will never have spider mites or PM ever again. It sounds crazy using moisture to get rid of a mildew substance. It's counter intuitive but it works. I swear to god. Lemme know how it worked for you. I will include some pics of some PM I found on my clones and used a small spray bottle to fix it. I din't use the plain water rinse first. I just went straight to a heavy dose of neem. Here's the before pics and the day after pics. By the way, I took the clones out from under the T5 fluorescent and just let them sit out in the middle of the basement for a few hours after spraying so they could dry without damage from the lights.
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If you look at the leaf in the middle of the left side, you will see some powdery mildew. It was actually worse than the pic shows, but I guess I took a bad pic.
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You can see I drenched these girls. I'd reccommend drenching them even more than this picture shows. Especially on big plants. you want them dripping wet. Sopping wet. DRENCH THEM.
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Here they are the next day. Nice and dry. Powdery Mildew free. Thanks to the magical Neem plant.
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Here's the neem I used. Its cheap and effective. I bought it at Lowe's hardware store, but I bet you can find it other places. There are tons of brands out there. This is just what I used. I've never tried any others, so I'm open to any other suggestions if you know a better brand.
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Here's a spray bottle with the neem oil mixture in it. There is about 250-300 milliliters of water and about 1/2 to 1 teaspoon of the neem concentrate in it. All concentrates are different, so always read the labels. Super interesting picture, huh?
I hope this helps. Lemme know how it went, if you try it. Feel free to ask me any questions, privately or in the forum.
Totally like real sh*t 
