What is Botrytis?
Botrytis or Gray Mold is a pathogenic fungus (necrotrophic) that affects more than 220 plant species, including marijuana. Actually, it is probably the most common fungus in cannabis crops, either indoors, outdoors or in greenhouses, and it usually affects plants during their latest stage of flowering , without forgetting the drying process .
The most important factor regarding the appearance of Botrytis is the humidity level, which is essential for its growth. The higher the environmental humidity level, the more chances for our plants to be attacked by fungal diseases and pests.
The ideal temperature for botrytis growth is between 17ºC and 25ºC, though it can grow with higher temperatures. A great difference between day and night temperatures can also stimulate the appearance of Gray Mold, for humidity levels raise when temperatures drastically drop at the end of the day.
Botrytis Cinerea is a fungus that colonises healthy plant tissues (parasitism), infected tissues (opportunism) and also dead ones (saprotrophic nutrition).
Outdoors, Botrytis attacks can be devastating, especially in crops that may take a little longer to flower, as well as in those plants that develop hard and dense buds. During Autumm most nights are humid and cold, what greatly favours the appearance of fungal attacks.
Botrytis spores, dispersed by the wind, can remain latent for years in the environment until climate conditions are favourable for their growth.
Which are the symptoms of Gray Mold?
Any part of the plant can be attacked by Botrytis: roots, stems, leaves, etc. The first visible symptom is a change in the colour and texture of the plant. Leaves develop necrosis and dry out very fast. Stems become fragile, brown and ulcerated.
But the most common place to find this fungus is in the buds. They become pale-grey, dry out and develop a cotton-like substance in the interior part.
How to combat Botrytis attacks?
First, we must cut off any affected part of the plant so that healty parts don’t get infected. Then, we should ventilate the cannabis growing space so the humidity level decreases to the desired value. Carefully checking the plants ensures that we’ll notice any other attack until harvest time.
Infected parts should never be consumed since they can easily cause a lung infection.
After harvesting our plants, we thoroughly clean the growing space, pots, etc. with bleach to eliminate any traces of Botritys.
There are several fungicides to treat Botrytis on the market; one of the best ones is, doubtless, the Botryprot fungicide , 100% natural and highly efficient.
We can also use – sprayed on the plants – benefitial bacteria to combat Gray Mold, such as Trichoderma Harzianum, Ulocladium Atrum or Gliocladium Roseum.
How to reduce fungal attacks?
Most fungus, and of course Gray Mold, grow better with high humidity levels, so keeping the humidity below 50% during the whole flowering period highly reduces the chances for our plants of being attacked by Botrytis.
To do so, we can either improve our ventilation system (air extractor, interior fans, etc.) and/or use a dehumidifier.
We should also leave enough space between plants to promote better air circulation, so that humidity doesn’t remain inside our growing space.
Keeping the space clean is also important to prevent any kind of pest or disease. We should clean it often and remove any decaying plant debris.
If the plant has cuts (from pruning, for example) it will be more sensitive to fungal attacks, so keeping our plants clean and healty is very important for Botrytis prevention.
It is also preferable watering our plants during the first hours of light, since watering them at night raises too much the humidity level.
At the end of the floweirng phase, reduce the amount of water in each irrigation, since too much water can affect the flowers, which could rot. During this stage, wait until the substrate is completely dried before watering again.
Excess Nitrogen at the end of the flowering period can also favour the appearance of Botrytis, although it is a rare case because we usually don’t give nitrogen to our plants at this stage.
Since Botrytis usually attacks the biggest buds, pruning the tops of the plants can help, for a plant with multiple tops usually develops slightly smaller buds (where Botrytis doesn’t grow so easily).
Pruning the lower part of the plants is also advisable, since it is where we find the higher humidity level. Outdoors, avoid placing your plants in humid zones, it is always better placing them on the most sunny space.
Harvest the plants when they need water to reduce rot and fungal issues. Dry the plants in a well-ventilated space, but do not place the fans directly on the plants or the drying process will be too fast, reducing the organoleptic properties of the flowers.
Which are the most resistant strains to Botrytis?
From a general point of view, mostly Indica strains develop harder and more compact buds, thus being more sensitive to fungal attacks than Sativa flowers, which develop less compact and dense, allowing better air circulation and retaining less moisture.
However, we can find a few exceptions among Indica plants, like the White Domina marijuana or Pakistan Chitral Kush.
Here you have a list of marijuana strains resistant to fungus , like Green Poison, High Level or Philo Skunk / Gokunk from Philosopher Seeds.
In a future article, we’ll teach you a way of making a completely clean resin extraction using buds infected by Botritis.
Botrytis and noble wine
It is also interesting pointing out that Botrytis Cinerea is used by wine producers for their noble late harvest wines. Sauternes, Barsac or Montbazillac are only a few examples of wines produced thanks to the action of this fungus on the bunches of grapes. It increases the levels of sugar, developing rich and complex aromas highly appreciated by connoiseurs. The low yields of this type of crop explain the high price of these wines.