Cannabis growers hate gray mold — and for good reason. Also known as botrytis cinerea, bud rot or bud mold, this malicious destroyer can overwhelm a grow room and quickly ruin the buds within.
Gray mold will also attack harvested buds, even long after they’ve been dried and cured. I’ve read a lot of articles about how to prevent gray mold and used that advice in my grow rooms, but gray mold has evolved — as have cannabis laws, cultivation standards and consumer expectations. In this article, we’ll look at the latest methods and research data most likely to give you victory over the dreaded gray mold.
Gray Mold Has Evolution On Its Side
Botrytis cinerea is actually an amazing pathogen. Present in all but the driest areas of the world, it can even hibernate in totally frozen landscapes and spring into action once temperatures warm up. It attacks hundreds of plant species including fruits, vegetables, ornamentals and, of course, cannabis. B. cinerea is one of at least 20 different species in the genus botrytis, and it’s the most pervasive and pernicious one, having plagued indoor and outdoor cannabis plants for decades.
Gray mold never sleeps. Rather, it evolves rapidly to develop a resistance to fungicides and other chemicals that growers use to stop it. Farmers are beginning to notice that expensive, complex fungicides they’ve often relied on in the past to kill gray mold no longer work well, if at all. Indeed, scientists have documented that gray mold enacts genetic adaptations in real time during crop cycles — that is, it evolves so quickly that the effects of any fungicide are diminished or eliminated.
Many growers first try to keep gray mold from their grow ops with traditional methods, such as using carbon filters or micro-filters in an attempt to block spores from getting into their space and onto plants, or by ensuring their grow ops are completely sealed from the outside world. The problem is, mold spores are microscopic and travel on the wind and via vector contact. Spores can live in all grow mediums, on clones, clothing, domestic pets and grow-room equipment, in air conditioning ducts and vents, air conditioning filters and in carpets.
If you live in a humid region where gray mold flourishes outdoors, it’s virtually impossible to create a grow op that’s sealed and filtered enough to block all spores. As soon as new air enters a grow op, spores are likely to come with it. Thus, a key to preventing catastrophic gray mold is to keep spore density levels as low as possible, and create grow-room conditions as inhospitable to gray mold as possible. Having tried all the typical grower tactics to get rid of gray mold, eventually I discovered a more effective battle plan against it.
Measuring Grow-Room Humidity Accurately
Gray mold has a difficult time getting established on your plants when relative humidity is 45 percent or below, so a fundamental part of eliminating gray mold is monitoring humidity levels. If you have defective monitoring equipment and don’t know the humidity level of your grow room, you’re already losing the battle.
There are several companies that make gear and apps for wireless monitoring and transmission of real-time grow-room climate data. Investing in a psychrometer or an accurate wireless digital system will help you avoid problems caused by inaccurate hygrometer measurements. Some high-tech hygrometers even allow you to set alerts for text or email warnings if humidity falls out of optimal range.
Growers need an extremely accurate hygrometer to measure humidity, but most hygrometers sold for less than $100 are untrustworthy. For that reason, it’s recommended investing in a Sling Psychrometer from Carolina Biological Supply, which gives the most precise humidity readings presently on the market. We also suggest you use a professional Hioki LR5001-20 Temperature/Humidity Data Logger, like this one from TEquipment.
Monitoring Your Grow Room For Gray Mold
Gray mold often begins out of sight, hidden away on the main stem of buds. By the time you see active spores, which can appear as fuzzy, white clouds, it’s too late to save that bud. Constant scrutiny of your marijuana plants — especially during peak- and late-bloom phases — is of prime importance. And there are two measures of scrutiny you can utilize: visual and olfactory.
Firstly, the visual. You’ll want to look closely at your plants from top to bottom — with the naked eye and with a magnification loupe. Botrytis tends to start on leaves and stems just above the root zone. Root-zone moisture creates a high humidity area extending from the root zone a foot or more up into the plant, and gray mold often thrives in soil and root-zone media.
If you see areas of buds — including individual leaves or leaf blades — that are grayish, brown or auburn, or are curling, dead or detaching from the main bud, it’s possible that gray mold is festering at that site from inside the bud. In advanced stages of infection, you’ll see the gray mold spore cloud as a white mass.
Some growers will excise only the affected portion of the bud and leave the rest on the plant. This is a big mistake. The entire bud — including the whole branch or stalk it’s on, or even the entire plant — should be immediately removed from the grow op and quarantined or destroyed. If there are enough spore masses on a plant to be visible, you can bet the gray mold has traveled to other parts of the same plant, and possibly to adjacent plants. The spores may also have spread to equipment, filters, air conditioning ducts and other structures that are extremely hard to disinfect.
Secondly, the olfactory scrutiny. If you have a sensitive nose, you can often smell gray mold before you see it. Although people often use the word “dank” to describe cannabis, a dank smell means bud is too moist and possibly molding. A small percentage of cannabis strains have terpenoids that smell like rotten fruit, but that’s different from the smell of mold. If fat buds smell rotten, it’s likely because botrytis is living within. If you suspect gray mold is in a bud, immediately remove said bud from the grow room and away from any air intake, then pry it apart. The mold will most often be seen a fraction of an inch inside the bud, originating from the bud’s stalk.
As you’re drying and curing your buds, always be on the lookout for gray mold. The fungi are bad for lungs, and if you see even a hint of mold, make sure you do not inhale them via vaporizer or combustion.
Creating An Anti-Mold Grow Op Environment
Along with keeping the humidity of your grow op below 45 percent, there are other strategies for ensuring your plants and grow op are less susceptible to botrytis. You don’t want to add moisture to your buds when you’re trying to fight botrytis, and overwatering can cause many problems in your cannabis grow room, including overhydration of stalks, stems and leaves, which creates humid conditions favorable to gray mold.
One such tactic is to water as infrequently and as little as possible, without causing plants to wilt. Growers are advised to reduce watering to the lowest safe levels starting in peak bloom — especially when dense, fat indica or Kush buds are present.
You can also try watering your crop early in the day so moisture has time to evaporate from the root zone and transpire out of leaves before lights off. Lights-off cycle is often a time of lower temps and stagnant air. If you water the roots or foliar spray just before turning the lights off, you run the risk of gray mold. Foliar spraying buds starting in peak bloom phase — even thin sativa buds — is also asking for botrytis-themed trouble.
If you’re cloning, know that when you mist the cuttings and use a humidity dome, you may be creating an environment that’s also ripe for gray mold. However, you might not see the mold until bloom phase. The mold spores are evolved enough to lie dormant from clone phase onward, springing into action when they have moist buds to attack. You already know that bringing in clones from outside your grow room is a major vector for pests and diseases, so it’s best to create your own clones in house.
Some bloom-phase supplements, especially organic supplements, may contain nitrogen. Your bloom-phase plants need nitrogen, but not very much. Reducing the amount of nitrogen present in your plant nutrients is another tactic used to deter gray mold, which is drawn to nitrogen-laden plant tissues. Most hydroponic base nutrients that are designed for bloom phase contain less nitrogen in relation to phosphorus and potassium when compared to grow-phase nutrients, but even that lower amount of nitrogen can be too much for bloom phase, thus creating conditions favorable to gray mold.
Adding any nitrogen beyond what’s in the base nutrients is often too much, so ensure you examine your bloom-phase supplements for nitrogen, using them sparingly if at all.
Additionally, any standing water on plants or in the grow room creates micro-zones of humidity favorable to gray mold. Also, be aware of any debris in the grow room, especially old leaves or pet hair; the cleaner the grow room, the less likely pathogens can infect it. You should never let pets inside the grow room, especially if they’ve been outdoors, and limit who comes in the grow room and for how long. During the humid months of the year, if I’ve been outdoors, I always change my clothes before I enter my grow room.
Botrytis is especially likely to flourish where there are wounds on your cannabis plants. If you’re pruning, removing leaves, lollipopping (removing the bottom branches near the root zone that will never get direct light penetration) or performing any other type of work that is likely to create open, raw tissue on your plant, those wounds will attract pathogenic diseases and insect pests. I seal my plants’ wounds with Tanglefoot Tree Wound Pruning Sealer & Grafting Compound, which is also useful for sealing open flesh on mother plants after cuttings are taken.
Lack of air circulation, filtration and exchange can also present grow-room problems. For this reason, I use oscillating fans set at different heights to push air from the floor of my grow room all the way to the ceiling and back again. I also use cloth pots instead of plastic ones, because they breathe better and lower the humidity in and around the root zone. Stagnant pockets of humid air are beloved by botrytis, which makes using the highest grade of filter material on air intakes of primary importance, as well as a freestanding HEPA filtration unit and carbon filtration units to cleanse the grow-room air.
Fungicides For Stopping Gray Mold
When it comes to cannabis plants, using fungicides as foliar sprays is fraught with problems. One of the primary issues is that fungicides are toxic chemicals that can cause harm to cannabis plants, growers and consumers. In Canada and US states where cannabis growing and consumption is legislated and buds are consequently lab tested for contamination and compliance, the presence of fungicides and pesticides means those compromised buds will be blocked from distribution. This is also true of buds used for producing cannabis concentrates.
There are many state-specific restrictions on which fungicides can be used on cannabis, so the best option is to consult regulators in your region to find out which fungicides are allowed in your grow op. As stated, botrytis has been seen to quickly evolve and make itself immune to fungicide effects within days of contact. For that reason, it’s recommended you use a rotating variety of fungicides with active ingredients and different modes of action so you’re not helping botrytis develop fungicide resistance. According to The University of Tennessee’s Institute of Agriculture, these modes of action are classified as FRAC (fungicide resistance action committee) codes. FRAC is a scientific panel set up to manage the use of agricultural fungicides and advise growers on how to use them without creating super fungi that are increasingly fungicide resistant.
Many growers use Eagle 20 against botrytis and powdery mildew, but those growers usually find out the hard way that this fungicide stays in their buds, and as a consequence cannabis regulators often order the quarantine or destruction of these contaminated consumables. Many fungicides are systemics, and not just surface application materials, so can still be present in buds at harvest. These fungicides disqualify buds from being sold to dispensaries in legal states with bud-testing requirements. Because fungicides inherently create safety risks for the growers applying them, as well as for the people consuming the buds, it’s advised you never use these chemicals past grow phase, if at all.
Cannabis Strains Resistant To Botrytis?
Many growers believe that some terpenoids (and perhaps cannabinoids) in cannabis resins naturally repel or even kill botrytis. Conversely, some strains appear to have terpenoid and cannabinoid profiles that are favorable to gray mold. My experience with botrytis cinerea suggests that strains in the sativa, haze and chem families, with scents and tastes like lemon, pine, diesel, pepper and turpentine might contain anti-botrytis compounds.
On the other hand, some strains seem to attract botrytis. My anecdotal grow-room data shows that these strains have tastes and scents like mango, berries and skunk and are correlated with thick indica and Kush buds that have high density and a large diameter. With these strains, moisture can get trapped in the heart of the bud, creating a perfectly humid microclimate for botrytis — regardless of what the grow-room humidity is. Or, it could simply be that the sweet tastes and scents of these strains accelerate the development of bud rot.
Using UV-C Light Against Bud Rot
Some growers have had success using ultraviolet light to kill gray mold, or at least control its growth. Ultraviolet light fixtures can be used as freestanding units or placed in the air conditioning and ventilation systems. Effective UV-C units produce UV-C light that’s been shown to disable gray mold and provide other germicidal effects.
When using freestanding UV-C that’ll directly radiate plants, the optimal way to kill gray mold is with five to 10 minutes of UV-C every few days during bloom phase, followed by at least four hours of no lights. According to the International Journal of Molecular Sciences, longer UV-C exposure damages plants by interfering with photosynthesis, while gray mold that was treated with UV-C and then subjected to four hours of darkness was unable to reproduce and re-infect. However, gray mold treated with UV-C and then placed under light was only partially hobbled by the UV-C, and was still able to reproduce and re-infect.
I recommend applying UV-C light to harvested buds, starting at time of yield all the way through drying and curing. Application can be constant during the drying and curing process because there’s no photosynthesis to interfere with. In fact, UV-C apparently degrades chlorophyll (which is why you should use it sparingly around live plants), and the less chlorophyll in buds, the better the buds taste and smell. I use freestanding UV-C units in my drying chamber.
The good news is there’s a sterilizing product that can be installed in-line in your HVAC system that uses various modes of action to safely kill gray mold spores and other biological pathogens, as well as deodorize your air.
The Reme Halo from RGF is used in hospitals, schools, grow ops, food prep sites, and any place where sterilization and sanitization of air is of prime importance. This product is a professional device that has the added benefit of keeping your air handler coils clean and fully operational, but must be provided and installed by an HVAC contractor. Reme Halo units sold on Amazon are often gray-market knockoffs or refurbished units that carry no warranty and may do more harm than good.
Using Dehumidifiers To Combat Gray Mold
There are two basic types of dehumidifiers. One is the common freestanding unit that costs between $200 and $500, yet isn’t meant for indoor agriculture. To that end, the cannabis grow-op industry has embraced industrial-strength Quest Dehumidifiers.
Quest units are quiet, emit minimal heat, and are way more energy-efficient than regular dehumidifiers. Made in the USA and tested to perform flawlessly, even in large-scale commercial grows, these units are rock solid. Before I used Quest units, my regular dehumidifiers were always cycling on and off, making loud noises, flooding, leaking and shorting out my electrical grid, all while doing a lousy job of keeping my grow-room humidity below 50 percent.
As soon as I installed my Quest units, I was relieved to see they sucked more water out of the air in a few minutes than the cheap dehumidifiers removed in several hours, while generating less heat and using less electricity. Whenever I enter large-scale commercial grow ops or connoisseur home cannabis gardens run by growers who want the best for their plants and customers, I see Quest dehumidifiers.
After losing many pounds of buds to gray mold, I now use these new tactics and haven’t lost any buds to botrytis since. The $1,000 I spent to have a Reme Halo installed is perhaps the most useful investment I have ever spent on my grow op. I’ve known growers whose indoor growing structures were so infested with gray mold that nothing could get rid of it. They chose to do a total tear-down, tent the building and dose it for a week with fungicides — or even relocate altogether. Hopefully, you won’t have to go through that chaos, and you’ll have victory over botrytis like I finally did.