Marijuana Pests & Disease

Marijuana pests and marijuana diseases are one of the biggest problems encountered by marijuana growers and can cause huge amounts of damage to your plants if not treated immediately.  This guide will help you spot the most common marijuana pests and marijuana diseases found in your marijuana grow and tell you what steps you can take to help prevent and cure any problems you may face.

Prevention of Pest and Disease

As with most marijuana diseases, prevention is the best cure.  Good hygiene and a clean grow area are the best ways to ensure your marijuana plants stay pest and disease free.  Make sure you wash your hands before you enter your grow room and remove any discarded foliage or clippings.

Fungi thrive in warm and moist conditions and are an inviting food source for marijuana pests.  Good air circulation and ventilation help keep the conditions at an optimum.  More information on humidity, temperature and air circulation can be found here.

There are strains on the market that are more hardy and resistant to marijuana pests.  Growing weed strains like Northern Lights, Big Bud and The Cheese give you a greater chance of not encountering the problems highlighted below.

Types of Marijuana Pests and Marijuana Diseases

Spider Mites

Definition and associated problems?
Spider mites are the most common marijuana pests found in indoor marijuana growing and left untreated, will slowly drain the life out of your plant(s).

What to look out for?Spider mites and eggs
Mites look like little dots and can be found on the underside of leaves, leaving yellowish white spots on the top of the leaf.  You can detect the tiny spider webs by gently spraying a mist of water on stems and the undersides of leaves.  A loupe or magnifying glass will help you to identify yellow, white, brown, red or two-spotted mites and their translucent eggs.

How to treat it?
You can combat spider mites by spraying plants with neem oil or pyrethrum.  An alternative measure is to introduce lady bugs to your grow room as they lap them up.  Prevent an infestation by keeping your grow room clean and make sure all tools are cleaned and sterilized regularly.

White Flies

Definition and associated problems?
Just like spider mites, white flies are a common marijuana pests that will suck the life out of your plant, diminishing the plant’s chlorophyll and stopping its vigorous growth.

What to look out for?White Flies
White flies look like little white moths and live on the underside of leaves and cause similar damage to spider mite, causing yellowish white spots to the tops of leaves.  You can identify them by shaking the plant gently, as they fly out from their cover.

How to treat it?
There are a couple of ways to deal with a white fly infestation; apply pyrethrum every five to ten days, fit fly tape on top of plant pot, or introduce Encarisa Formosa (a type of tiny wasp).  Introducing two per plant should be sufficient.

Fungus Gnats

Definition and associated problems?
Fungus gnat larvae attack seedlings, roots or weak plants, causing plants to loose vigour and inviting in other marijuana diseases.  They are most commonly found in dark and damp conditions, so thrive in the NFT hydroponic set up.

What to look out for?Fungus Gnats
A Fungus Gnat Larvae infestation can get out of hand quickly, so keep an eye out for larvae that have short translucent bodies and black heads.  They will attack around the roots so check around the base of the plant and the growing medium.

How to treat it?
If you do have an infestation of Fungus Gnat, use Bactimos or Gnatrol which contain Bacillus Thuringiensis.  Drench your medium with neem oil or insecticidal soap if you’re growing in a soil or soilless medium.  Prevent an infestation by keeping humidity down and making sure the soil does not stay soggy, as these create damp conditions in which they thrive in.

Gray Mold (Botrytis)

Definition and associated problems?
Gray mold or Botrytis is the most common fungus or marijuana diseases to affect indoor grows and thrives in humid conditions, mostly infecting plants that are around two weeks from harvest.  Once it has started, Botrytis is fatal if not dealt with immediately and will reduce your crop to unsmokable dust if left to fester.  Airborne spores are everywhere and can spread like wild fire destroying every plant in your grow room in a week to ten days.

What to look out for?Grey mold marijuana
Grey mould is difficult to detect from the onset and appears on buds as blue-ish green hairs that are not too dissimilar to lint.  It quickly turns to dark brownish spots and leaves your buds feeling slimy.  If your stems are infected, they will turn yellow and all growth above the infection will wilt.

How to treat it?
Use sterilized scissors to remove infected buds at least an inch below the infected area, making sure you take it out of your grow room and destroy it.  Wash your hands and sterilize your scissor thoroughly afterwards.  The best cure is prevention; keep the humidity of your room to around 50% during flowering and make sure your grow room is clear of any discarded foliage.  Ensure your hands are washed and clean when coming into contact with your buds.

Damping Off

Definition and associated problems?
Also known as “Pythium Wilt”, damping off mainly attacks newly sprouted seeds and seedlings and is fatal.  It is caused by fungal infections that are already present in the grow area, excessive humidity and overwatering your plants.

What to look out for?Damping off
Sprouted seeds won’t emerge from the soil and seedlings will rot at the soil line.  Older plants will turn yellow and also rot at the soil line.

How to treat it?
Once it has started it is fatal making prevention the only cure.  Over watering is the biggest cause of this marijuana diseases so keep the moisture of your growing medium at the correct levels.  If you have suffered from a damping off infection, throw away the old medium and thoroughly clean your growing containers.

Green Algae

Definition and associated problems?
Although Green Algae will cause little damage by itself, it’s a breading place for marijuana pests like Fungus Gnats. Its presence means you are over watering your plants and that humidity levels are too high.Green Alage If you have Algae growing in your medium, you need to remove it quickly before it causes problems that could bring about the death of your plant.

What to look out for?
Algae is slimy in its appearance and thrives on light and is usually found on the surface of your growing medium.

How to treat it?
You can treat it with Algeacide, which can be added to your nutrient solution or water.

Downy Mildew

Definition and associated problems?
Downy mildew affects both vegetative and flowering plants and thrives on your healthy plants at temperatures below 75°F temperature in °C .   It spreads very quickly and will slow growth, causing foliage to die back, eventually killing your plant.

What to look out for?
Look out for pale yellow spots on the top sides of leaves, with a gray-ish spawn mirroring the spots on the leaf’s underside.

How to treat it?
Remove the plant and destroy it.  Removing the foliage is not enough and will cause subsequent grows to become infected.  Prevention is the cure with this one, so make sure you keep your humidity and temperature levels correct and that your grow room is kept clean.

Root Rot

Definition and associated problems?
Root rot is caused by a lack of oxygen being supplied to the roots either by overwatering or supplying them with un-aerated water.  It will stunt the plant’s growth, making it unable to feed and transfer nutrients to the rest of the plant.Root rot

What to look out for?
Roots that have started to rot turn a pale brown, eventually darkening.  Growth of your plant will slow dramatically and it will start to wilt.

How to treat it?
As well as keeping your medium clean, take measures to avoid over watering your plant or using a medium that has poor water drainage.

Powdery Mildew

Definition and associated problems?
Powdery Mildew is present for at least a week before it shows and, in the most extreme cases, can kill your plant if it is not dealt with.  High humidity levels and poor light penetration by having plants placed to close together are the main causes.

What to look out for?Powdery mildew
It starts off as small white spots on the top of leaves, progressing to a fine pale powder on leaves, shoots, and stems.  As it progresses further, leaves start to yellow and growth grinds to a halt.

How to treat it?
Spraying your plant with a fine layer Bordeaux mixture or baking soda spray stops the growth of powdery mildew.  Keeping your grow room clean, the humidity at the correct levels and allowing for good light penetration help to prevent an outbreak.

Marijuana pests can be a big problem in a grow environment. It isn’t just novice growers that marijuana diseases can affect, experienced cultivators are equally as susceptible to fall foul. Follow our guide and we should help steer you in the right direction.



21 comments on Marijuana Pests & Disease
  1. Marty

    Fly paper for gnats worked like a charm, thank you! I bought some Neem Oil to spray the top of the soil but probably didn’t need it. I drape the fly paper in a kind of grid on top of the pots near the soil.

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  2. realdream

    I hear all your pain. I have been there too often. I just got done with a nutient project that I have been working on for two years. My last crop finally had no pests. I can’t believe it worked. I was sarting to think it wasn’t possible. Now I can spend more time on lighing. It is all organic and non toxic.

    It has manure broken down with microbes, plant extracts, potash, and other minerals in it. I finally works. It is best used to prevent pests but will work to get rid of them if you are unlucky enough to get pest.

    Any input is wanted. I am a newb and just got my first crop to work ever. I am so happy with this now.

    Thanks to veryone with your posts as they are what got me to try this.

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  3. realdream

    We’ve been into deep water culture for basic veggies, medicinal and cooking herbs. We ran into all the typical indoor growing pests and have worked now for 2 years to resolve those problems organically. Recently we met a new friend who is very involved in medical MJ hydroponic growing. He had all the usual problems with pests through his grow cycle. He found out what we were working on and gave us a great opportunity to test what we’ve worked with on his crop. We have now done two crops with him and he has had no pest problems and we’ve improved his nutrient uptake and he has had no water change outs with two crops in the same solutions. Only water is added as the system runs low.

    We are sharing this with others to get feedback and opinions. We are working with organic manure, organic plant material and organic minerals. Just so everyone is clear, this is not a product on the market, but if it seems it will be we will be glad to do the right thing with the webmaster and pay for advertising or whatever requirements are needed.

    For ourselves, we have gotten rid of spider mites, root aphids, leaf aphids, white fly and powdery mildew. The water solutions in the tanks are always fresh and clean smelling, even during harvest and uprooting. As a test our friend tried placing new clones straight back into the old water to see what would happen. He got a second crop with no health issues.

    After talking with various growers, we get the same comments; that they don’t believe us and after seeing all the discussions on various forums we understand why that opinion is there. Pests can be a cronic problem. All we do is foliar spray twice a week and add it to the nutrient solution in the water supply tank. Our new friend the grower really likes the results of the crop and likes that it is all organic and disease free.

    Sorry for rambling on, but we are very excited. We really would like your comments and responses.


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  4. Mitch

    How do you get caterpillars out from inside the bud section of the plant

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    Rating: +1 (from 9 votes)
  5. richard williams

    does neem oil on the flowers make those flowers unusable?

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    Rating: -6 (from 40 votes)
  6. Jan DeSipio

    Question please about the viability of growing Blue Dream outdoors in the Northern California (Mendocino) coastal climate. Also optimum harvest time? Thanks

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    Rating: -2 (from 10 votes)
  7. Green thumb

    Spider mite suck, Cheep safe way to kill mites, Take 1 clove of garlic 1 onion 2 cups of water spree and strain. Put it in a spray bottle spray your plants kills them dead first time. This well work, It will not make your cannabis smell like garlic+onion when smoked My first and second I had mite. 3rd grow no mite Pest aside free grow room. The Bud was very sticky lots of red hair Avalon Indica

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    Rating: +2 (from 12 votes)
  8. Richard

    For caterpillar .. Try using B.T. Dust .. It’s harmless to humans .. That said.. It best without anything.. Sometimes you got to do it. Hope that helps . Marsbar

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  9. Dave

    I have some kind of catipiler in my buds .what can I do to get rid of them please help

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    Rating: -11 (from 51 votes)
  10. Melissa Lemik

    I had a problem with beetles growing outdoor this year so I put some DE powder on the girls. Anyway I thought it was the remnants of that but over the past week it’s formed spots and it’s definitely powdery mildew. I need it gone yesterday. Any tips?

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  11. Melissa Lemik

    I had a problem with bettlea outdoor this year so I put some DE powder on the girls. Anyway I thought it was the remnants of that but over the past week it’s formed spots and it’s definitely powdery mildew. I need it gone yesterday. Any tips?

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  12. cyndifagan

    leaves turning brown curling up buds turning brown had same problem last year kills plant from bottom up looking through microscope see what looks like larve moving around like little worms or maggots cant see with naked eye someone please give me some kind of answer dont want to lose whole out door crop

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  13. Don

    The best way to get rid of fungus gnats is to destroy them in the larval stage using benecial nematodes. You can order them on-line from Home Depot if you can’t find them locally. I have used them on my tropical plants and they compleley wiped out the fungus gnats in short order.

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  14. kerry perkins

    fungus gnats come from the soil if you get a bag of play sand and pour a half inch on top of your co.ntainer has to be play sand it has finer granules it will not allow them to hatch cut them to bits

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  15. Rick

    To naturally get rid of gnats mix: 1gal. of water with 2tbsp each of sesame seed oil, dishwashing liquid (non-anti-bacterial ), white vinegar, and 1tbsp hot sauce. Kills em deader than a hammer. Treat every two weeks till eggs have hatched. I got this off youtube and it works.

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  16. cris

    I have been around and around with the gnat problem. They were killing my plants off rather quickly, and I believe they were purchased in the potting mix I bought. I tried neem oil, other insecticides, but what got rid of them was fly strips. I draped one around the pot and hung a few up, and I caught hundreds of them and saved my plants. Was such a relief!

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    Rating: +3 (from 11 votes)
  17. Tampa420

    I’ve got the gnats and I got concentrated neem oil. I have been using it for about a week and they are about 70% gone. Is there something I can add to finish the job. I’ve been following your directions for the start and everything works just like you say. I thank you from the bottom of my heart. I suffer from chronic pain and you have helped so much.

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    Rating: +9 (from 11 votes)
  18. Gwen Stockton

    Dr. Greenthumb out of Canada sells quality, guarnateed seeds. I’m pretty sure they’ve got some quality cheese strain.They’re expensive but I’ve had SUPERB luck with their femed seeds.

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  19. Paul Watkins

    I’ve had good luck

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  20. brian cook

    i wish to purchase the seeds for growing a good strain of marijuana (cheese). Please let me know how I can accomplish this. Thank you!!!

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