Bud Rot in Weed: How to Identify and Prevent Them

Sophia Delphi May 12, 2022 - 7 min read
Fact Checked
Image of rotting weed bud

The conditions aren’t everyone’s cup of tea. But many people love the summer months when temperatures are warm and the humidity is high. They call it “beach weather.”

Here’s who hates that type of weather: cannabis growers with outdoor crops.

Growers don’t think of “seasonable” temperatures and high humidity as beach weather. They know that it’s the perfect environment for mold growth.

Many plants can suffer from mold infestation, but several types of mold commonly attack cannabis plants. One is powdery mildew, which attacks leaves and sucks out the plant’s nutrients.

Another, which is even more troublesome, is called bud rot. The name is appropriate; it develops in cannabis buds and, you guessed it, causes them to rot. It can hit indoor grows, too.

The potent, dank weed crop you’ve been anticipating for months? It can be gone in a flash.

Here’s the background on bud rot, and what to do if it hits your plants.

What Causes Bud Rot?

Bud rot is a mold that’s caused by fungus.

The fungus is known as Botrytis cinerea, and it can infect hundreds of different species of plants. When it hits most crops the resulting mold is called “gray mold,” but it’s usually referred to as bud rot on cannabis plants.

The mold may sit on a plant, dormant, for a long time. But when it attacks, it uses enzymes and toxins to destroy the structure of the plant, usually killing it quickly. Even a single mold spore can start the process.

Botrytis cinerea may cause a separate problem called root rot, but that disease is more often blamed on other types of fungi. In most plants invaded by Botrytis, the result is bud rot.

The mold is particularly difficult for growers to deal with because it normally develops inside the buds of a weed plant, where it can’t be easily seen. Over time, it infects the outside of the buds and quickly spreads to all plants in the vicinity. Needless to say, one affected plant puts all of a grow at substantial risk.

Even worse for indoor growers (if it could get any worse), bud rot is likely to target the largest, fattest, stickiest buds on the plant.

It’s a problem everyone with weed plants should be familiar with, so they can take fast action if necessary.

How to Identify Bud Rot?

Bud rot is most likely to develop during a plant’s flowering stage. As we’ve mentioned, the signs are virtually impossible to detect at first because the initial damage is being done on the inside stem.

The first visible sign is usually a partial discoloration of buds and the sugar leaves surrounding them. The rot can appear as gooey pockets of gray, dark brown, or purplish-black inside a cluster of flowers; if you touch the area, it may crumble.

Sugar leaves may look like they’ve been infected with white powdery mold, but the powder quickly turns dark. Plants’ main colas are quite susceptible to bud rot, turning darker than the surrounding plant and shriveling, with their sugar leaves turning yellow.

Not long after that, the buds will also seem to be covered with white or gray powder (which is mold spores), and webbing will be seen covering them.

At this point, your plant is lost. All you can do is carefully remove it from the area so the mold can’t be spread to the rest of your grow. There may certainly be a temptation to try to remove just the affected part of the plant, but that’s almost always a fool’s errand once the powder or webbing is visible. Bud rot has spread, and consuming it could be dangerous.

All of this may happen in just a few days.

However, there’s still hope for the plant if you catch bud rot very early and remove the moldy flower. That means being vigilant. Many growers say the first sign they notice is that “there’s something wrong with the plant.” At that point, there’s no time to waste.

It calls for an immediate inspection of the buds with a magnifying glass or jewelers’ loupe. If you break apart a flower that you suspect has bud rot, you should see the discoloration and possibly white powder on its inside. You may be able to smell the telltale musty odor as well.

If you’re at this stage, or if you’ve just started to notice the buds turning darker in spots, you can try gently removing the infected buds (so you don’t spread mold spores). Put them directly into a bag with as little motion as possible, remove the bag from the grow, and disinfect the tools you’ve used.

Then, keep a very close eye on the plant; if the bud rot remains, the plant has to go. If the plant is near maturity and everything seems OK, you may want to harvest the remaining flower early.

It’s easy to see why experienced growers fear bud rot — and why they do everything they can to prevent it.

How to Prevent Bud Rot

Since bud rot (and fungi in general) prosper in warm, humid, moist environments, many of the preventative steps you can take are obvious ones.

For Indoor Grows

  • Relative humidity in the grow room should be kept as low as possible. 50% is good, 40% is even better in the flowering stage when bud rot is most likely to develop.
  • Keep temperatures below 75-80° and lower them slightly at night, so the plants can cool down.
  • Ensure good air circulation; exhaust fans and circulation fans are your plants’ best friends. Make sure air can circulate behind and between colas.
  • Regularly prune plants to help airflow, and space plants to avoid creating large “canopies” where moisture will develop and humidity will rise.
  • Don’t overwater plants. That will boost humidity levels in the air.

For Outdoor Grows

  • Prune plants regularly and space them well, to ensure as much airflow as possible. Use cages or stakes to make sure plants don’t grow together.
  • Water in the morning, so the excess water can evaporate before it creates extra humidity in the plants’ environment.
  • Cover plants with a tarp to keep them dry, and shake them in the morning to get rid of accumulated dew.
  • Consider planting autoflowering strains that can be harvested before the humid/rainy season arrives, if your area’s climate is an issue.

For All Grows

  • Choose mold-resistant strains. Durban Poison and Afghan Kush are two to try.
  • Pay close attention to the plants’ nutritional needs; plants in good shape are least likely to be hit with bud rot.
  • Defoliate regularly.

Sooner or later, all growers will have to deal with bud rot. Those who pay close attention to their plants and take quick action will be the ones who will be fully able to enjoy the harvest.

Bud Rot in Weed: FAQ

Q: Can you apply fungicides to plants after they’ve developed bud rot?
A: Sadly, the fungicides that are effective against powdery mildew and similar issues won’t help with bud rot. You may see some products sold that are supposedly effective against Botrytis cinerea, but they’re actually preventatives that commercial growers use during the vegetative stage. They won’t help in the flowering stage when bud rot usually strikes.

Q: Should I only worry about bud rot during the flowering stage?
A: Not exactly. You should use the best practices we’ve mentioned throughout the cannabis life cycle, to minimize the chances that the spores reach your plants and sit dormant until the time is right to strike. You also have to worry about the possibility of bud rot appearing during the drying and curing processes after harvest; pay attention to temperatures, humidity, moisture, and crowding, and check the plants regularly for signs of the disease.


Williamson, B., Tudzynski, B., Tudzynski, P., & Van Kan, J. A. (2007). Botrytis cinerea: the cause of grey mould disease. Molecular plant pathology, 8(5), 561-580. [1]