Grey Mould – Bud Rot Caused by Botrytis cinerea

In week six of flowering the main cola looks plump and healthy. Only one small yellow leaf sticks out of it. Bend the bud open and it is brown and mushy inside. This is bud rot, the flower rot caused by grey mould. The fungus Botrytis cinerea breaks the bud down from the inside out. Its spores almost only germinate where water sits on the tissue.

What is grey mould, and where do the spores come from?

Grey mould is caused by a fungus whose spores float in the air almost everywhere. Botrytis cinerea attacks more than 1400 plant species, including strawberry and tomato. Of 178 fungi isolated from rotting buds and stems of cannabis and hemp, 162 were Botrytis cinerea. That is a little over nine in ten. The spores are called conidia. A spore is 9–14 µm long and invisible to the naked eye. Only in large numbers do they look like grey dust, hence the name. The fungal threads inside the tissue are called mycelium. At first they are white.

The spores come from infected neighbouring crops, from plant debris left lying around and from rotting spots on stems. Outdoors the fungus survives in the soil as sclerotia. These are hard black grains of 2–5 mm. Air completely free of spores is hardly achievable in a grow. Prevention therefore aims mainly at keeping spores from germinating.

Remove fallen leaves and plant debris from the room or bed every day.
Do not place mouldy strawberries, tomatoes or ornamentals next to cannabis.
In a greenhouse, close the side walls when a neighbouring crop is visibly mouldy.
Filter the intake air of a grow room. A HEPA filter is a very fine filter that lowers the spore count.

How does the fungus get into the bud?

A spore almost only germinates in a film of water and usually enters at the base of the sugar leaves. Growers use this name for the small resinous leaves that stick out of the bud. Without free water almost no spore germinates, even at 98 % relative humidity (RH). On a moist culture medium 72–88 % germinated within 18 hours. Plant sap in the droplet speeds this up a lot. In rhubarb leaf sap 94.9 % of spores germinated within just 4 hours. In pure water it was 2.8 % over the same time. Cold only delays germination. At 5 °C it starts after 12 hours, at 20 °C after 3 hours.

A second way into the bud is through ageing brown stigmas. Growers call these hairs pistils. The fungus enters the stem through broken side branches and at the bud sites on the main stem. In outdoor hemp, caterpillars bore into the buds. Whether fresh cuts let Botrytis into cannabis has not been tested. In tomato a clean pruning wound loses half of its susceptibility within one day.

Cut broken branches off cleanly below the break straight away.
Outdoors, hang pheromone traps from the start of flowering. They lure the moths with scent and show when they fly.
Check outdoor buds regularly for bore holes.
Remove leaves cleanly at the stem without leaving a stub. This is shown in tomato and untested in cannabis.
Do not wet the buds. Because of the dew point risk, spray early enough for the buds to dry before lights out.

How do I recognise bud rot, and which stage am I at?

The first sign is usually a single yellowing sugar leaf in an otherwise green bud. It appears from week five of flowering. Infections from spores landing in weeks two to four often show only then. Why the rot only becomes visible from week five is not known. In humid conditions it then takes over large parts of the bud within three to seven days. After a week the whole cola can be lost.

A sugar leaf turns yellow, then brown, and wilts. From outside the bud looks healthy.
Inside, the tissue turns brown and watery. It falls apart wet between the fingers.
White mycelium grows through the cavities inside the bud.
A grey-brown mass of spores covers the bud.
The rot can move into the stem. The branch above it wilts or breaks.

Other fungi look similar. White cotton with black grains points to Sclerotinia, the cause of white mould. Pink-white mycelium points to Fusarium. This group of fungi can produce toxins. For what you do at the plant it makes no difference: every rotten bud goes. If the large fan leaves yellow from the bottom up instead, that usually points to senescence or a nutrient issue. The pattern is described in the article on nutrient deficiency.

From week five, check every day, starting with the densest buds.
Gently bend open buds with yellow sugar leaves and look at the base.
Below every infected bud, check the stem for brown, soft spots.
Cut every infected bud into a bag, as described in the last section.

When is the risk highest?

The risk is highest when water condenses on dense, ripe buds. This mainly happens as the air cools. When humid air cools down, it can no longer hold its water. The temperature at which droplets form is called the dew point. At 30 °C and over 90 % RH, cooling by 1.8 °C is enough. Inside a ripe bud the air is on average 15.4 percentage points more humid than the room. In a greenhouse, buds cooled from about 36 to 24 °C within 30 minutes after darkening. Night-time infection of cannabis is therefore plausible but has not been measured.

Cold on its own is not the problem. The fungus grows best at 15–25 °C and not at all at 30 °C. The plant also plays a part. Cultivars with many sugar leaves showed 30–70 % infection, one with clearly fewer showed 15–30 %. Large, dense buds are hit more often than small, loose ones. No resistant plant has been found in trials. Cultivars differ only in how often and how badly they are infected. Outdoors, infections pile up in months with cool nights and humid days.

Measure temperature and RH at bud height at night, not just at the controller.
If the night temperature is close to the dew point, cool down more slowly or dehumidify harder.
Soften the drop in temperature at lights out and keep air moving through the buds.
For humid rooms or autumn harvests choose cultivars with few sugar leaves and loose buds.
In months with cool nights and humid days, check outdoor plants every day.

How do I prevent bud rot?

Prevention means moving air through the canopy, keeping plants apart and using beneficial fungi early. Fans 25 cm above the plants blowing at 7 m/s cut infection by 81.2 % on average. Humidity inside the buds fell by 11.6 percentage points. One instead of two plants per square metre also reduced infection. It cost 18–23 % of the yield, though. Harvesting after six instead of eight weeks of flowering reduced infection as well. It can cost yield and potency.

Trichoderma is a beneficial fungus that colonises the bud before Botrytis does. It was sprayed in weeks two, three and four. This cut infection by 47–91 %. In the lab, beneficial fungi only worked when applied two days before the spores. No product that cures an infected bud has been documented.

From week two, aim fans about 25 cm above the canopy with an airflow of around 7 m/s across the buds.
With susceptible cultivars, set fewer plants per square metre and factor in the loss of yield.
Open up the foliage inside the canopy. This is reported from grapevine and untested in cannabis.
In late flowering keep 23–28 °C and RH at the low end of 50–70 %.
In the morning raise the temperature by no more than 2 °C per hour. This is recommended for tomato and untested in cannabis.
Spray Trichoderma preventively in weeks two, three and four, following the label.
Under high disease pressure harvest earlier and accept the loss of yield.

What do I do with infected buds, and what are the health risks?

Infected buds leave the room in a closed bag and are never used. Cutting them off can spread spores. In tomato the advice is to cut 5–10 mm beyond the visible rot. The fungus grows ahead of the visible damage. In a bud it spreads inside, often invisible from outside. Even the healthy-looking rest of a rotten bud is therefore not safe.

Botrytis spores can trigger allergies. During one outdoor harvest, 59 % of the fungal DNA in the workers’ breathing air came from Botrytis. Levels were highest during harvesting and trimming. Rotten buds can carry other moulds such as Aspergillus or Fusarium. Some of them form toxins that survive even irradiation of the buds. People with weakened immune systems developed aspergillosis after using mouldy cannabis. This is a fungal infection of the lungs. Whether Botrytis itself leaves substances toxic to humans has not been studied. How to dry the healthy harvest without new mould is covered in the article on drying.

Pull a bag over the bud first and only then cut below it.
Cut generously into healthy tissue; in tomato the advice is 5–10 mm.
On the same day check neighbouring buds and the stem below.
Wear non-latex gloves. There is no official advice on respirators; people with allergies still wear an FFP2 mask.
Take the bag out of the grow, because plant debris is a source of spores.
Never smoke, vaporise or turn rotten buds into extract, hash or butter.
Never cut out the rotten spot and dry the rest of the bud.