Bud Rot On Marijuana Plants

I-love-marijuana  grey mold

Bud rot (also known as gray mold or botrytis) is one of the most devastating fungal diseases to hit marijuana crops.  It thrives in cool temperate climates where a high level of humidity is present.  Gray mold can easily take out an entire cannabis crop within a week.  What’s most distressing is it is not selective in its choice of prey.  Gray mold will cause damping off of seedlings, killing the seeds in the soil or the young tender plants newly emerging.  Invading seedling stems at the soil line, the stems become brown, soft and rot, much like celery stalks that have been left in the fridge too long.  This causes the stems to topple over, thus the term ‘damping off’.

Gray mold is ubiquitous, meaning it appears anywhere and everywhere.  In more mature plants, gray mold attacks the stems, first appearing as a brown-gray mass of mycelium, or foundation, if you will.  The mycelium (thallus) then becomes covered with a mass of fungal spores. This ‘smothering’ action results in the plant turning yellow due to the deprivation of chlorophyll.  The stem of the marijuana plant can become encircled with the mold, weakening it to the point of becoming soft and full of cankers.  The stems often break at the canker sites.  All growth above the diseased stem will then wilt.

Oh, but gray mold is not satisfied with this damage!  Oh, no!  They will now attack the flowering sites of the cannabis plant.  Gray mold is not only non-selective, it’s greedy!  Large, female buds retaining moisture (and what female doesn’t!?) are the most susceptible.  First the leaves, then the pistils brown and wilt.  Gray mycelium then envelopes the buds, eventually morphing into gray-brown slime, rotting the buds just before harvest and rendering them useless.

Now you know why it is of the utmost importance to prevent this devastation from annihilating your marijuana crop.   Controlling the temperature in an indoor grow room can reduce the possibility of gray mold showing up by keeping the temperature above 70 degrees and ensuring the plants don’t become wet from humidity.  Change clothes before entering an indoor grow room if you’ve been outside, as spores can be introduced via clothing.  Keep your pets out of the grow room; they are known vehicles for transporting all kinds of critters you’d rather not have to combat.  Equally important is to sanitize pruning shears between clippings to avoid inadvertently transferring bacteria and fungi into the grow room.  The same precautions should be taken for outdoor gardens.


Areas with low light are most conducive to the introduction of, albeit unbeknownst and unwelcomed, the dreaded gray mold.  This is the beginning of infestation and it spreads prolifically from there.  Gray mold first sports a white powdery façade then turns to a smoky gray or brown color.  That’s when the true colors come out and fungal damage is well on its way.  The nasty mold takes on a velvety disguise and creeps into damaged tissue forming a wet, brown rot.  Again, picture celery that has been in the fridge too long; the brown, limp stalks that result from too much moisture is what you can liken to the appearance and effect of gray mold.  But gray mold is a far more formidable foe than soggy celery my friends!

As in any intrusion affecting life, whether plant, animal or human, prevention is the most effective form of control.  If you choose to grow your marijuana indoors, control the humidity level.  You have the advantage of controlling ‘weather’ conditions in this environment; keep humidity levels to below fifty percent.   This is easier accomplished indoors than out, where you cannot control nature’s ways.

Immediately remove any diseased or dead plants and destroy them.  Do not add diseased plants to your compost pile or you will pollute the mixture, rendering a diseased soil amendment unfit for any part of your garden.  Rather, place the contaminated plant material in a bag, seal tightly and discard.

Water your cannabis during the daylight hours.  Give your plants plenty of time to dry before the sun goes down.  If growing indoors or in a greenhouse, make sure the plants are dry before the lights go out.  Mold thrives in the darkness and looks for wet, unsuspecting hosts.

The cannabis buds are diamonds in the ruff and the sole purpose for growing marijuana. As with any disease, an ounce of prevention is worth a pound of cure.  There are some preventative measures you can take to divert gray mold from rendering your precious buds useless.

  • By spraying bacillus subtillis on your plants, you can aid in preventing bud rot.  It is recommended for food crops, of which cannabis is categorized, and is available online.
  • Potassium Bicarbonate (KHCO3) inhibits the growth of molds and fungi.  It is an organic treatment widely used on crops and edible plants because it is a naturally occurring compound in humans, animals and plants as a built-in defense mechanism.
  • Oil sprays made with neem oil or sesame oil form a barrier inhibiting mold germination.

When the efforts of prevention fail, all is not lost.  Bring out the armor and fight the battle.  Try these:

  • Remove diseased leaves and buds to avoid infecting other plants.  Clean pruners with rubbing alcohol or peroxide immediately after use.
  • Apply a Bordeaux mixture of copper sulfate and slaked lime.  This is often used in vineyards to control gray mold.
  • Copper soap or spray can be used up to the day of harvest and won’t burn the cannabis.
  • Make a mixture of one teaspoon baking soda per quart of warm water.  Spray on plants every seven to ten days.
  • Spray the plants with sulfur or use sulfur burners to create an airborne treatment.  Both are available at nurseries or your local garden center.

If you order marijuana seeds from my webshop and you get problems with pests or diseases, I’m here to help you. There are guides about plant care where all pests/diseases are discussed and you can contact me by mail. Please like or share this article

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3 thoughts on “Bud Rot On Marijuana Plants

  1. Hi…I have one out of 15 plants that is displaying a bunch of grey mold on the stem. Is there anything I can treat this with before it effects the actual budding process? Im about a month or so from harvest. Also all 15 plants are in the same location…is it normal for just one plant to get it….dont get me wrong….THANK GOD it only hit one!

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