Sooner or later everyone growing marijuana indoor or outdoor will have to deal with the ravaging affects of a Botrytis cinerea outbreak. This necrotrophic fungus is commonly referred to by marijuana growers as gray mold or bud rot, and once infected can destroy your entire garden in a matter of days.
Description and Identification
Botrytis usually first attacks marijuana plants from the inside of the thick buds a few weeks before harvest. Because the infection begins near the stem, it can be difficult to detect until it has become well established. Often the first telltale sign of Botrytis is a single leaf protruding from the kola beginning to wither and dry out. Botrytis can be spotted by bending larger kola slightly to reveal the interior of the bud and stem. If your buds are infected you will see either a gray, white, or blue-green mold with hairs growing on the inside of the bud. In wet and humid conditions this will turn the bud to slime. In dryer climates the affects of botrytis appear brown or rust colored and will crumple when touched.
Botrytis can also attack leaves, stems, and seedlings causing damping off. It should be noted that even dried and stored marijuana is not safe from botrytis and should be inspected regularly. If an outbreak is left unchecked, botrytis can spread to all of your plants and entirely destroy your garden in as little as a week.
There are many genetic varieties of Botrytis cinerea but all persist through the winter months in the form of sclerotia or mycelia. In the spring, both the sclerotia and the myscelia produce conidiophores which grow millions of asexual spores (conidia) that are then spread by wind and rain.
Because of prevalence of Botrytis cinerea spores in nature, this fungus affects many species of plants besides marijuana. Vineyards are often infested with Botrytis where it can either rot bunches of grapes or change the grape chemistry, making them actually more suitable for certain types of wine. Botrytis also can cause significant damage to strawberry plants and tomatoes grown in greenhouses.
In dealing with Botrytis, nothing could be truer than “an ounce of prevention is worth a pound of cure”. While Botrytis is probably the most prolific mold found in marijuana buds, there are many different kinds of fungal outbreaks that could affect your garden. Luckily, most types of fungus thrive in the same environment, so in working to prevent one, you can effectively decrease the potential for other fungal outbreaks as well. Botrytis thrives in contaminated environments with humid air and poor circulations. A combination of cleanliness and climate control measures can be taken to decrease its prevalence.
Unfortunately botrytis spores are nearly impossible to completely eliminate from your grow room or garden. If you are growing marijuana indoors this begins with properly constructing your grow room.
If possible remove all carpet, curtains, fabric and trash from the grow room that could harbor botrytis spores. Cover wood or cement floors with linoleum that can be regularly mopped clean with at least 5% bleach and water solution. If you don’t glue the linoleum down, make sure that its edges are folded up around the baseboards of the room to prevent any spills from seeping under. Thoroughly wash the walls and ceiling with a bleach solution, and then repaint the room with a fungus resistant white paint. Covering floors with plastic or hanging plastic on the walls in not advised as this can trap moisture and create a fungus friendly environment.
Once all of your equipment is in place and wiped down with a bleach solution, treating the grow room with a high dose of ozone can help kill any remaining spores. When the room is filled with plants, you’ll need to make sure dead leaves and debris are never allowed to remain in the room. I’ve been in more than one grow room where the grower removed foliage but left it in the room’s garbage can to rot and spread mold spores.
For those growing marijuana outdoors, remove as much dead foliage from the garden as possible. Keep any grass or ground-cover as short as possible so that it does not trap moisture or prevent air circulation. If possible cover the ground around the plants with landscape cloth to prevent weeds and maximize airflow.
Humidity, Ventilation, and Circulation
In addition to keeping the grow room or garden as free of spores as possible, you must also maintain an environment healthy for marijuana plants, but inhospitable to botrytis. Fungi that affect marijuana plants thrive in conditions with overly wet and soggy soil, humid air, and poor air circulation. These problems can generally all be fixed through adjusting your watering schedule and proper ventilation.
It is very important to maintain a humidity level in your grow room under 50% and a temperature that is above 70 degrees F. To accomplish this, pay careful attention to your watering schedule and make sure you are not over watering your plants. Extra water in the room or pooling up in your garden will evaporate causing the humidity to increase. Warm air holds more water than cool air, so try to water only after the sun is up, or lights have come on and your room has reached its normal daytime temperature. This way the majority of the water evaporation takes place during the warm period which will not increase the humidity of the grow room nearly as much.
In addition to over watering, another common mistake for indoor gardens and greenhouses is using larger containers and more growing medium then necessary. While you certainly don’t want your plants to be root-bound, too much soil means extra expense on nutrients, extra water and extra humidity from evaporation.
CO2 generators produce more than just CO2, they also produce heat and water vapor. The heat will help keep the room above 70 degrees, but the water vapor increases humidity that must be dealt with. Make sure you are only running your CO2 generator during the daylight hours since this is the only time the plants benefit from CO2 anyway.
If you are running a sealed grow room or the air outside is above 50% humidity a dehumidifier can quickly and inexpensively remove extra moisture from the air while helping to heat the room. A 30 to 65 pint per day dehumidifier is capable of maintaining a grow room the size of a normal bedroom and is available for under $200. These dehumidifiers are easy to use but must be emptied daily, create some heat and generally draw between 400 and 700 watts.
If your grow room uses ventilation to keep it cool, make sure your fan is strong enough to quickly remove moist air sufficiently. If the air outside is above 50% humidity, you may need to stop drawing in outside air and use sealed light hoods and a small air conditioning unit to keep the rooms humidity low and the air within the correct temperature range. Make sure any outside air that is drawn into the room is filtered to remove as many outside contaminants as possible.
In addition to the airflow required to keep the room at the proper temperature and humidity levels, you must also maintain airflow through the plants themselves. Oscillating circulation fans will prevent pockets of moisture and help move air through the plants foliage and across the surface of the growing medium. An added benefit of circulation fans is that this airflow will also strengthen the plants stems so you won’t require as much staking towards harvest. Use selective pruning to remove any unnecessary foliage from the lower parts of the plant and avoid over crowding the plants.
As a preventative measure plants can be treated with foliar sprays. These can be either a non organic chemical based fungicide, an organic fungicide containing copper or sulfur, or they can be inoculated with foliar sprays containing beneficial fungi.
Chemical and copper or sulfur based fungicides can’t be used along with biological sprays because they kill fungi indiscriminately and will effectively wipe out your beneficial fungi along with the harmful parasite. While often effective, the drawback is that chemical or copper and sulfur bases sprays is they can only be used during a vegetative period. These organic and non-organic chemical sprays can be extremely harmful if smoked, especially for medical marijuana patients, and should never be used after your plants begin to flower.
Biological based foliar sprays containing Gliocladium and Trichoderma, or products like Serenade that contain Bacillus subtilis can be used to both prevent and treat outbreaks of botrytis all the way up until harvest. These species are also fungi’s but colonize plants in a symbiotic relationship protecting them against fugal attacks.
Once an outbreak of Botrytis is discovered it is imperative that you take action immediately. If the conditions are right, an outbreak can spread and wipe out an entire garden of buds in a matter of days. Sterilize your pruning shears in alcohol and cut the bud at least one inch below the affected area. You must be very gentle while removing the bud to prevent spreading the spores. Make sure not to let the bud, your hands or the shears touch any other buds until they have been sterilized.
Once every kola has been inspected, and the damaged areas are removed, drop the humidity in the room as low as possible and make sure the temperature does not fall below 70 degrees. If the humidity can be maintained below 50%, continue to treat with biological foliar sprays all the way until harvest. If the outbreak is already widespread, consider cutting your losses and harvesting the garden early.
Botrytis cinerea may cause “winegrower’s lung”, which is a rare form of hypersensitivity pneumonitis. To a layperson this means an allergic reaction in the lung and airways that affects some who are predisposed. We can’t avoid mold spores, they are literally everywhere, and we smoke some in every joint without concern. Some medical marijuana patients however have compromised immune systems or respiratory problems and can become quite sick from smoking marijuana with high spore content. If your garden has had a severe outbreak of botrytis or any other fungus, it is imperative that you remove all infected bud and have your marijuana lab tested before offering it to patients.
Preventing and Treating Bud Rot – Botrytis,