Discovering that your cannabis crop has developed bud rot is definitely going to ruin your day. Learning how to detect bud rot, and how to avoid it in the first place, are the best tools that a grower can have in his or her arsenal.

Bud rot is a generic term for a type of grey mold known as Botrytis cinera. It can attack blossoms when they are growing on the plant or not become present until they are hung to dry. Either way, swift action is required to try and mitigate the damage bud rot can cause.

Symptoms of Bud Rot

Bud rot starts inside of a bud, so it is not easily seen until it has started to spread. It favors dense colas and densely leafed plants.

The infected bud will generally start to show a white mold, which will turn to grey and eventually black. Collapse of the bud will coincide with the darkened appearance and it will be reduced to a mushy mess.

Causes of Bud Rot

Botrytis is spread by spore, like other molds and fungi. It thrives in conditions where humidity levels are high (over 50 percent) and temperatures are not too cool and not too hot.

Like other fungi, it will form where there is poor ventilation and circulation. It also needs ultra-violet light to survive.

Treatment of Bud Rot

There are very few chemical options for the treatment of bud rot. Treatment of bud rot is usually surgical. The first sign of an infected blossom calls for action. It should be excised with sterile cutting tools.

Each cut should occur between two and four inches below the infected bud. If there are multiple infected buds to remove, the cutting device used should be sterilized between cuts and again before being put away after use.

The infected buds should be bagged and removed from the area. If possible, they should be solarized under plastic to kill the pathogen before being thrown away.

Another non-chemical method of control of bud rot, as referenced above, is to cut off any available UV light. This is of course done at the expense of THC levels, but if there is an outbreak of botrytis, war needs to be declared to eradicate it.

If there is no risk of runoff, and it is used judiciously, Bordeax mix has been used effectively by some cannabis growers. It will need to be sprayed well before harvest though to avoid damage. This copper and lime mixture has been used effectively in the control of fungal pathogens on crops for almost 200 years.

In the world of agriculture, there are two other chemicals that are approved for post-harvest treatment on crops with Botrytis. These active ingredients are Fludioxonol and Pyrimethanil. Neither is approved for use on medical cannabis, nor are they allowed for organic food production.

Prevention of Bud Rot

If no other change is made, growing sativa varieties of sativa crosses will reduce the chance of developing bud rot. This is not to say they never get them, nor is it to say that indicas always get them. However, sativas are far less likely to develop bud rot based on the areas of the world they evolved in.

Changing conditions in the grow room, or rather maintaining proper conditions, will do more to prevent the development and spread of bud rot than by changing varieties grown.

Botrytis cinera like many other fungi, require the orchestration of environmental conditions to develop and thrive. By removing any or all of these conditions, Botrytis will be controlled or not develop to begin with. The conditions to avoid or those that favor bud rot are as follows:

  • High humidity: Grow room humidity levels should not exceed 50 percent.
  • Medium temperatures: Unlike mildews that like it hot (powdery mildew) and those that like it cool (Downey mildew), bud rot likes mild temperatures. By keeping the growing area at least 70F (21C) to 80F (26C), bud rot will be kept at bay.
  • Poor air circulation: mold thrives in stagnant air. Keeping horizontal air flow (HAF) fans running at all times and maintaining adequate ventilation will reduce the chances for bud rot to establish.
  • Dense foliage/Dense colas- While there may be nothing that can always be done about these, plants should be spaced far enough apart to achieve proper circulation. If there is reasonable risk for Botrytis to make an appearance, some preventative thinning of branches and foliage may be advisable.

All of these conditions enumerated above apply to cut and drying blossoms as well as those that are still attached to the cannabis plants themselves. Even when removed, cannabis blossoms still need their space.