After growing weed for long enough, at some point you will encounter bud rot, mold, root rot, or all three. It’s important to know how to identify each of these common problems and how to fix them. These issues can stunt your plants and ruin buds, damaging your crop and reducing yields.

Smoking moldy weed is about as appealing as eating moldy bread. Apart from looking ominous, mold also stunts cannabis growth and renders the flower unfit for consumption. Unfortunately, mold is one of the most common blights to affect cannabis plants. Here, we’ll cover the telltale signs, how to prevent mold, and how to promote healthy growing conditions. 

Diagnosing and treating bud rot and mold on marijuana plants

There’s nothing worse than going through months of time and labor to grow weed only to discover mold on it. Also called botrytis, or bud rot, mold can be hard to see on your buds. 

What is bud rot?

Bud rot is a type of mold that develops in the dense cores of cannabis buds. It starts on the stem inside of the bud and spreads outward, and it’s difficult to detect in its early stages. It typically occurs on plants in the flowering stage.

What is mold, and why is it bad?

Mold is a member of the fungus family. Mold can grow both indoors and outdoors, only requiring damp environments with limited airflow to thrive. Once established, mold begins producing filaments called mycelium and forming airborne spores invisible to the naked eye. When these spores land on a moist surface, more mold begins to grow and the cycle starts again. Although mold represents a vital part of the ecosystem, it’s not something you want to see growing on your cannabis.

Problematically, mold spores carry toxic chemicals called mycotoxins. Mycotoxins are usually inhaled and can represent a serious threat to health in immunocompromised individuals. In some cases, the ingestion of certain mycotoxins, such as aspergillus, can even be fatal.  Smoking moldy weed should be avoided in general as it can lead to sinus and lung inflammation, undermining the potential therapeutic benefits cannabis may offer.

“Mold has been linked to many health challenges, including the worsening of asthma, coughing, wheezing, nasal congestion, sore throat, sneezing, and rhinitis,” says Steve Sakala, longtime cannabis cultivator and founder of Mana Artisan Botanics. “Chronic mold exposure can also lead to the scarring of the lungs or pulmonary fibrosis. There are also reports linking it to chronic fatigue syndrome and other long-term symptoms.”

What do moldy cannabis plants look like?

cannabis mold

While mold is often used as a catch-all phrase to refer to this unwelcome fungal visitor, there are distinctive types of mold. “There are many types of mold, but the two most common forms of mold found on cannabis plants are powdery mildew and botrytis,” explains Sakala. Another common variant is sooty mold. 

Picking out the signs of different molds in the early stages can help protect your crop. A black light or magnifying glass can also render it easier to pick out mycelium that’s getting established.

Botrytis (bud rot)

Botrytis is known more colloquially among growers as grey mold or bud rot. Grey mold can occur in both growing plants and dried buds, but is more common in outdoor plots after heavy rainfall or humidity. 

The fungus often originates inside dense, moist buds before becoming visible on the outside. Bud rot can enter the tissue and remain dormant until the environment becomes conducive to its spread, then it rapidly rots tissue. This particular mold produces enzymes and toxins that destroy the cell walls of cannabis plants. Research suggests that the mold triggers the plant host to induce its own cellular death.

Because it lurks in buds, it can be tricky to identify. “Bud rot is harder to spot as it usually develops in the interior of mature buds, progressively reaches the exterior, until it takes over the whole bud,” explains Josh Ferranto, cultivation expert and chief operating officer at Silver Therapeutics. 

Here are some signposts that point to bud rot:

  • Dying or yellowing leaves close to the cola
  • Colas that look limp or discolored
  • Patches of brown or grey that look lifeless
  • Plant tissue that crumbles to the touch and feels dry

The easiest way to confirm the presence of botrytis is to break apart suspicious-looking buds to check if the dark mold is concealed inside. 

Mold Exposure Limits Cannabis Seedling Growth

One of the most discouraging effects of mold exposure on cannabis production occurs during the early phase of growing. Seedlings are susceptible to a process called damping off, which can destroy a large percentage of a crop before it even has a chance to get off the ground. Exposure to gray mold spores causes damping off.

When a seedling is affected by damping off, it fails to grow a strong, sturdy stem. Fungi attack the roots and stem, prevent nutrients from moving upward throughout the plant. Signs of damping off include a soft, mushy stem and seedlings that droop over. In the beginning stages, it may look like overwatering, but lesions soon begin to form on the plant stem. Unfortunately, there is little that can save seedlings at this point, so growers will have to start over and hope that airborne fungi have not permeated the new soil.

Effects of Mold Exposure on Mature Plants

cannabis mold

In mature plants, the effects of mold exposure may include disease of the stems, leaves, roots, and buds of a plant. Each type of fungus causes a slightly different problem. In general, however, growers should look for any lesions or spots on plants. Lesions on plant stems and leaf spots may be indicators of mold exposure. Affected plants often give off a musty or stale odor when exposed to mold spores.

Mold Exposure Curtails Cannabis Yield

Unfortunately, one of the primary effects of mold exposure is that it is lethal for cannabis plants. This is particularly true for gray mold, which may destroy your entire crop in a matter of days. Even when plants do not die, however, yield may be severely curtailed. Furthermore, the effects of mold exposure are not limited to your grow room. Mold also can spread throughout a drying room, spoiling an entire harvest.

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