Generally speaking, mildew is caused by the humidity being too high. Whenever I see the RH go above 60%, I turn on my dehumidifier. Now, I live very close to the ocean (maybe 500 ft), and along with that, I live in an area that has lots and lots of rain. I'm not saying that will cure your mildew, I am saying that's how to prevent it. There is another way that also helps, called a sulfur burner. You can buy them at grow shops, and it has a small burner with a pot on it that will heat the sulfur. If its used, you don't want to harvest within two weeks, or you might get a taste of the sulfur in your buds. However, I have LIGHTLY used it just 3 or 4 days before harvest without any negative effects. Yes washing down the room cannot do any harm, in fact it might just help. However if you watch your relative humidity, and use the sulfur burner occasionally, you will not need to buy a dehumidifier which can be quite expensive, depending on size. After spending a lot of money on small dehumidifiers at Lowes and such, I spent $1200 on mine and it will bring down the humidity in my grow room from 65% to 45% in an hour. Please keep this in mind on your next grow!

keylime