Mold? Mildew? Fungus? What to do?

Hey all,
I’m growing in a greenhouse in the pacific northwest; we had warm weather, then a bout of cold, and now hot again. My humidity runs very high; I do have a fan to push air out the roof vents when above 85, but nothing to control humidity (kind of a lost cause up here anyway…cigars stay at 70% even when the humidor sponge is dry!).

I have several plants in buckets, all doing fine, and several in a raised bed. They have all grown like CRAZY (side topic: can I top or bend over the tops to stop growth? They’re growing out the roof vents at 7-feet-plus…). They are also mixed in with tomato plants, and one little auto-flower is definitely shielded from good sun and pretty short (4’). It is easy to miss this plant, and today I noticed it was about half covered in a grayish-white substance that reminds me a lot of tent caterpillars/web worms, except it’s not a web. I’m guessing mold or mildew, but don’t know for sure. I’ll add photos below.

Not being sure what it is, I tried searching the archives but didn’t find much; apologies for adding a new topic that is probably a repeat of previous ones!

In any case, I carefully cut out all of the plant that was affected (roughly 50%) and made sure not to touch other plants with it or shake, etc.

Any help much appreciated–I’m pretty concerned that this could take down the whole greenhouse!

Next year there will be a lot more space between plants…


It just occurred to me where I’ve seen this before–it reminds me of powdery mildew, which is a huge problem up here on things like squash, cukes, watermelon. Hmmm, I wonder if that’s what this is.

Also just found some articles on bud rot/gray mold, and they look more like my plant. Crap…


UPDATE: I went ahead and pulled the infected plant out of the greenhouse and potted it up, and found that my really tall AF of the same variety is also infected, but much less so. I think I’ll go ahead and harvest it also? Both are about 12.5 weeks since seeds were planted. They both have a few orange pistils but lots still green/pale too. However, I’m guessing if I let them go they’ll just get totally infected and I’ll lose the whole lot. One other AF is upwind and not infected, might do that one too. Trichomes are basically all still clear.

Now then, all the remaining plants appear to be uninfected, and are still in veg phase only. Perhaps some preventive care, since I’m sure spores have landed on them too? Any way to keep the infected plants going until a better harvest time, without risking the rest of the crop?

It is amazing how fast this took hold–less than a week ago that we had cool weather; I water every other day and take a pretty good look each time.

Thanks all, talk soon.


I would mix a ratio of 2 oz per gallon of neem oil with a few drops of Dawn dish soap in there and spray the heck out of the plants every 2 weeks until your 4 weeks into flower… :wink:
Unless the weather changes in your favor for some reason , it’s only going to get harder from this point forward when they start to flower and stack on weight …so hopefully you’ll get something figured out by then… you might have to get some big box fans in there and start pushing a hell of a lot of air around depending on how thick your growth is… :wink:

:v: :sunglasses:

Thank you. I discovered also that my greenhouse fan GFI had tripped, so the house had no fan for the cool wet weather. I bet that was all it took.
Will try the spray; we have hot weather coming so that’s good too.

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Just a quick update. After a few days of cutting out new mildew, it was clear I was going to lose the plants altogether. As a last ditch, I put in a room dehumidifier to keep humidity down. Being a greenhouse, it runs a lot, but it’s doing the job. I’ve pulled a few leaves with mildew/rot here and there, but not much at all. Pistils are turning red quickly now so hoping I make it to harvest with no more rot; some trichomes are changing too. Then I’ll keep running the dehumidifier as my non-AF plants start to bud, since I’m sure there are spores all over the place. Damn tomatoes are growing like Sasquatch, and the plants are out the roof windows every day now. Hard to keep it stealthy, but at least I’m legal! Just need to keep the teenagers out of there. :slight_smile:

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An update a couple of months later. I kept the dehumidifier going almost full-time (it’s a basic unit rated for about 1500 square feet I think) and it totally stopped the bud rot problem. I just let it drain into the gravel floor of the greenhouse so I don’t have to empty the tank. Literally not another incident since then, and all the plants I’ve just harvested were in contact with the infected ones, and also in very close proximity. Also, even here in Seattle, the inside of my greenhouse panels are bone dry–compared to them raining inside every morning otherwise. This solution worked beautifully! Thanks for all the help and tips, looking forward to trying the fruits of the labor!!!


Great news. I’m jealous. My first grow outside in the Northeast and I threw away tons of bud. If I decide to continue outside I will surely erectca greenhouse!