Botrytis on Cannabis: How to Spot, Stop and Prevent Gray Mold

Botrytis, also called gray mold or bud rot, is a fungus that rots cannabis buds from the inside out, and it thrives when humidity runs high, air sits still and temperatures stay cool. Fungal problems are super common in cannabis grows, and if you don't act fast, you risk losing your entire crop. 

Fungi are living organisms that reproduce very easily through spores, and they show up in warm, humid, dark environments.

Fungal spores are floating around in the environment all the time, and once they find the right conditions, they grow and spread. The catch is that fungi like pretty much the same climate your plant needs to grow. 

Ideally, you take preventive steps so they never show up or spread. If prevention fails and fungus hits your grow, catching it early and moving fast to get rid of it makes all the difference in saving your crop. 

Botrytis cinerea, also known as gray mold or bud rot, is a common mold and the most common humidity-related disease in cannabis plants. It's a fungal pathogen that attacks a whole range of crops, like tomatoes, cucumbers, peppers, basil and roses, and of course cannabis buds.

Gray mold is especially common in both indoor and outdoor grows. It mainly hits buds during flowering, driven by high humidity and poor ventilation.

To keep your grow room from rolling out the welcome mat for fungi, keep it spotless and disinfected. That also helps keep pests and bugs away. You can wipe down every surface in the grow room with a water and bleach solution. Clearing out dead leaves helps keep fungus away too. After harvest, deep-clean the room before starting a new grow. 

What conditions cause botrytis?

Botrytis is a very aggressive fungus, and it thrives at temperatures below 26 °C (79 °F) with humidity above 50%. It can attack plants in both veg and flower, but it takes hold much more easily in flower. You'll notice it when buds start changing color and a fuzzy gray mold appears on them. If it has already hit your plant, you can cut away the affected parts or use an antifungal product. To treat botrytis, there are fungicides that need to be applied within 24-72 hours of infection.

Good ventilation and humidity control in the grow room are essential for a fungus-free environment. Humidity shouldn't go above 50%, especially during flowering. A dehumidifier helps pull excess moisture out of the air.

These factors are always easier to control in an indoor grow. Outdoor plants are harder to manage, and you need to watch out for rain in particular, since it leaves plants soaked and wide open to a fungal attack.

Botrytis is easy to spot because it turns buds gray or brown, and the leaves around them too. Touch a bud that's been hit by this fungus and it crumbles or falls apart. 

Lots of growers run fans to move air around the room and cut down the extra heat from the lights. Unfortunately, that can spread botrytis spores all over the space. Without some kind of air purification system, that can be a fatal move.

Prevention is always what saves the plant, but if the fungus has already taken hold, cut off the affected part and throw it away. 

How does botrytis spread?

Also known as "gray mold," botrytis is a fungal pathogen that can infect lots of plants, cannabis included. It's highly contagious and spreads fast in damp, cool environments. Botrytis spores can be in the air, in the soil or on infected plants, and they get carried around by wind, rain or insects. Once spores land on a cannabis plant, they can germinate and work their way into the plant's tissue, causing damage and gray mold on buds and leaves. Botrytis can be especially nasty around harvest, because buds hanging to dry are particularly vulnerable.

How to identify botrytis on cannabis plants

To spot this fungus on your cannabis plants, watch for these symptoms:

• Buds with gray or brown spots.
• White or gray fuzz on the buds.
• Yellowing or browning leaves that wilt and drop.
• A damp, musty or moldy smell.

Inspect your plants regularly so you catch any sign of infection and stop the fungus from spreading. If you suspect botrytis, separate infected plants from healthy ones to keep it from moving on.

You can also keep it in check by holding humidity in range, cutting out infected leaves and buds, and using organic fungicides made for this fungus. Keeping temperatures in a good range matters too, avoiding both highs and lows, because the fungus does best in damp, cool conditions.

How to prevent botrytis on cannabis plants

To prevent botrytis on your weed plants, follow these steps:

1. Keep good ventilation and airflow in your grow space.
2. Control relative humidity, which shouldn't go above 50% during flowering.
3. Prune properly so air can move through the plants.
4. Keep buds from sitting in direct contact with leaves or other parts of the plant.
5. Use fungicides made to prevent and control botrytis, always following the manufacturer's instructions.
6. Inspect your plants regularly for any sign of infection and remove affected parts right away.
7. Keep the grow area clean and free of plant debris that can hold moisture and become a breeding ground for botrytis.

Remember, prevention is your best strategy against botrytis on cannabis. With good ventilation, the right humidity and constant monitoring, you can keep fungal infections off your plants.

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