If you grow cannabis, chances are you'll encounter cannabis pests and/or disease at some point during your cultivation career.
Cannabis is susceptible to pests like spider mites, aphids, and fungus gnats. It's also at risk for common cannabis diseases, like root rot, bud rot, and the dreaded white powdery mildew.
Whether you grow weed indoors or out, understanding these risks (and how to battle them) is essential to ongoing success. The following guide covers all the most common marijuana pests and diseases you'll likely encounter in your grow, including how to identify, prevent and treat.
What we are going to cover in the following cannabis pest and disease guide — feel free to jump ahead:
- Options for Controlling Cannabis Pests and Diseases in Cannabis
- Integrated Pest Management for Cannabis
- White Powdery Mildew
- Spider Mites
- Fungus Gnats
- Aphids (Hemiptera) 5
- Bud Rot 6
- Yellow and Brown Leaf Spot (Septoria)
- Root Rot (Fusarium, Pythium, and Rhizoctonia)
- Russet Mites (Aculops cannibicola)
- Thrips
- Whiteflies
- Prevention Key to Getting Marijuana Pests and Diseases Under Control
- Summary for Controlling Pests and Disease
Options for Controlling Cannabis Pests and Diseases in Cannabis
When it comes to cannabis, spraying chemical pesticides and fungicides usually isn't an option. Many chemicals pose a serious risk if ingested.
What's more, when it comes to cannabis, the final product is usually inhaled. Many pesticides approved for application on vegetables have never been tested on cannabis.
What happens if there is a residue on your flowers. When burned in a joint or bowl and inhaled, there could be severe risks to your health and respiratory tract.
Cannabis cultivators who care about the final product and the health of those that consume it need to pay close attention to what goes into their plants, especially in bloom. Organic, biological approaches are best. It's why we suggest the Integrated Pest Management approach.
Integrated Pest Management for Cannabis
Integrated Pest Management (IPM) for cannabis is "an ecosystem-based strategy that focuses on long-term prevention of pests or their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant varieties."
Under this mandate, pesticides are only applied as a last ditch effort and only according to approved guidelines. Prevention is first, management second, and then pesticide as a last resort.
Because pesticide application in cannabis poses so many risks, many of which are unknown and untested, IPM is an ideal approach and one highlighted throughout this guide on marijuana pests and disease.
White Powdery Mildew
The bane of any gardener, cannabis or otherwise, is white powdery mildew (also called White Powdery Mold). Technically, this broad name applies to many different species of fungus, including Golovinomyces and Erysiphales species.
This plague on indoor and outdoor growers comes from environmental factors. White powdery mildew thrives in hot, humid environments, just like those perfect for growing cannabis. Microscopic spores, carried on the wind or a contaminated surface, can quickly reproduce to damage and destroy your plants. Greenhouses are extremely susceptible.
White powdery mildew proliferates quickly. In the right environment, it can move from one plant to an entire room within a matter of days. Identifying white mold is relatively easy, but it is often frustrating to get it under control before the spores have settled elsewhere within the grow room.
Signs of Powdery Mildew
- White or grey powder and spots on the upper and/or lower surface of fan leaves
- As mold spreads, new growth, stems, and even buds become infected
- Plant growth noticeably slows down.
- Discoloration of leaves in most serious cases
Prevention of Powdery Mildew
- Reduce relative humidity below 50 percent (as per Ed Rosenthal and others)
- Increase airflow through and underneath the canopy
- Reduce moisture on leaves (i.e., watering techniques and foliar sprays)
- Constantly monitor leaves for signs of infection
- Cultivate mold resistant strains (see MRS resource)
Treatment for Powdery Mildew
- Immediately prune areas of infection (if localized)
- Destroy infected plant material
- Remove infected plants, if the issue is widespread
- Address environmental issues (humidity, airflow)
- Natural treatments include spraying solutions of garlic, chamomile, horsetail, or propolis.
- If necessary, chemical options include Climoxanilo, chlorothalonil, Imazalil, metalaxyl, Fosetil-Al, Triadimenol, Tebuconazole, myclobutanil, and Bitertanol
Spider Mites (Tetranychus urticae or cinnabarinus)
Spider mites (Tetranychus urticae or cinnabarinus) are a common arthropod species that attacks many different crops, from corn to cannabis.
In one recent survey of California cannabis cultivators, 70 percent of growers reported dealing with mites, including spider mites.
Spider mites often infiltrate the grow room on contaminated clothing, soil (or growing medium), tools, containers, or even through unfiltered HVAC inflow.
For indoor growers, grow room sanitation is a key factor in reducing the risk. Outdoor growers typically encounter spider mites during hot, dry periods (July, August).
Because spider mites are only 0.5 millimeters big, it's difficult to catch them with the naked eye. Once the problem is visibly noticeable, it's often too late to save the plant. Prevention is critical to avoiding an infestation of mites.
For a full breakdown of the spider mite dilemma (and solutions), check out our in-depth spider mite guide here.
Signs of Spider Mites
- Small white pinpoints appear on leaves, typically following veins
- Tiny spider webs appear underneath leaves, and spots spread
- Leaves discolour
- Patches of eggs (a magnifying glass may be required) visible on the underside of leaves
- Spider webs proliferate, and adult spider mites are visible, protecting eggs and on webs
Prevention of Spider Mites
- Sterilize all tools and equipment before bringing them into an indoor grow room
- Invest in a filtered intake fan (read more in our complete guide on grow room HVAC)
- Quarantine all new clones and seedlings before bringing them into the grow room
- Invest in a LED Loupe for close up inspection of leaves
Treatment of Spider Mites
- Removal and destruction of localized infestations
- Removal and destruction of plants with severe infestations
- Improve air circulation in and around the canopy
- Increase relative humidity (slightly)
- Introduce natural mite predators (Amblyseius californicus, Feltiella acarisuga, and Macrolophus caliginosus)
- Apply natural pesticides, such as diluted essential oils, neem oil, dish soap, and alcohols
Fungus Gnats
Although not an overly common pest for cannabis cultivators, fungus gnats are still one to note.
These tiny flies come from two families: Mycetophilidae and Sciaridae. They begin life as larvae on the soil's surface, munching away on organic material - like cannabis roots and beneficial organisms.
Fungus gnats love shady, humid environments, which is why they hang out underneath the canopy on the surface of the soil. They typically infiltrate a grow room or greenhouse in contaminated soil or hydroponics equipment. Hydroponic growers should be on the lookout for fungus gnats because of the humid environment.
The short life cycle of a fungus gnat is only a week. But this means that an infestation can quickly reproduce until there are small swarms of flies hovering around every plant. Thankfully, fungus gnats are easy to identify and control without harsh chemical pesticides.
Signs of Fungus Gnats
- The most notable sign is a swarm of small flies around the base of the cannabis plant
- The appearance of small white worms with blackheads in and around the surface of the soil
- The appearance of tiny white cocoons in and around the surface of the soil
- As gnats damage roots, young plants may appear wilted or droopy from a lack of nutrients
Prevention of Fungus Gnats
- Cover substrate with a layer of mulch or vermiculite so adult flies cannot lay eggs
- Do not overwater your plants
- Install yellow sticky traps near the infestation
- Apply neem powder to the surface of the soil or water with a mild neem solution
Treatment of Fungus Gnats
- Inject biological controls with natural predators like nematodes or predatory mites like Hypoapsis Miles
- Apply neem powder to the surface of the soil or water with a mild neem solution
- Spray the surface of the soil, lower regions of your plants with a solution of clove essential oil and insecticidal soap
Aphids (Hemiptera)
Roughly 17 percent of California cannabis cultivators battled aphids, according to a 2018 survey. Unlike other types of bugs on weed, a robust aphid colony can be challenging to get under control.
Aphids are small oval insects, usually under 3 mm long. They may appear in many colours, including yellow, green, or black. Babies are translucent.
Aphids usually don't have wings, although if using a magnifying glass, you may see a small pair of clear wings on some varieties.
Adults and immature aphids on cannabis cut into the leaf or stem to suck the plant's sap (nutrients and water). If left to its own devices, an aphid infestation will quickly cripple a plant by literally sucking it dry.
As with many cannabis pests, aphids appear on cannabis when it's hot and humid. For outdoor growers, that's usually July and August. Indoor growers must pay attention to relative humidity levels at all points.
Signs of Aphids
- Typically found on the underside of leaves, on new growth, and stems
- The appearance of honeydew drops (excretion of aphids) around new growth and buds
- Honey dewdrops are followed by black mold if left unchecked
- The appearance of ants on the plants, which drink the honeydew
- Plants look stressed, wilted, and yellowed.
Prevention of Aphids
- Constant monitoring, including undersides of leaves
- Invest in a filtered intake fan
- Implement a quarantine period for new clones and seedlings before entering the main grow room
- Sterilize all equipment and reused soil between crops (or introduction into the grow room)
Treatment of Aphids
- Release of biological controls such as parasitoids (Aphelinus abdominalis and Aphidius) or the aphid midge
- Introduction of natural predators such as lady beetles (bugs) and others
- Treatment with natural pesticides like diluted potassium soap, neem oil, or pyrethrins
- For localized infestations, physical removal of insects
- Removal and destruction of infected plants
Bud Rot
One of the most common diseases in commercial cannabis cultivation is bud rot. Also called bud mold or gray mold, scientifically, it's a species of fungus called Botrytis. It thrives in those same hot and humid environments that other cannabis diseases do.
Because bud rot on weed is one of the most prevalent diseases, growers need to understand the signs and conditions that promote infestation. High humidity and big changes in temperature from day to night are notable triggers.
Outdoor growers must remain especially vigilant. Strains with long flower periods or incredibly dense buds are highly susceptible.
Signs of Bud Rot
- Buds dry, from the inside out
- Soft and cotton-like consistency in the interior of the flowers
- The Resinous leaves around the bud (and sometimes the sugar leaves) turn brown and dry out
- Flowers lose color, turn gray
Prevention of Bud Rot
- Constant surveillance during flower, especially indicas and long flowering varieties
- Keep relative humidity under 50 percent
- Increase airflow throughout the canopy and lower areas of the plants
- Sterilize equipment between use and before introduction into the grow room
- A dedicated set of cleanroom clothing or coveralls
- Water plants in the morning, and always at the container level (never over the canopy)
- Prune fans leaves and lower branches to promote airflow
- Choose mold resistant strains of cannabis
Treatment of Bud Rot
- Immediate removal of infected areas or plants
- Improve environmental conditions (relative humidity, airflow)
- Careful inspection of surrounding plants, removal if necessary
- Application of beneficial bacteria like Trichoderma Harzianum or Gliocladium Roseum
- In serious cases, application of Botryprot (EM) chemical pesticide (follow instructions for cannabis)
Yellow and Brown Leaf Spot (Septoria)
Yellow leaf spot and brown leaf spot are rarely deadly to cannabis plants, but these marijuana diseases will reduce yields and likely THC level.
Leaf spot of any color is caused by several fungi species, most notably the Phoma and Ascochyta species. There is also some indication that plants experiencing a nitrogen deficiency will be more susceptible to leaf spot.
Yellow and brown leaf spots are issues for both indoor and outdoor growers. For outdoor cultivation, it's likely to appear after heavy summer rains. It often occurs in the period leading up to and into flower.
Signs of Yellow and Brown Leaf Spot
- Yellow spots initially appear on lower leaves of the plants
- Yellow spots are generally uniform but may have a brown or dark border
- Brown or yellow spots could have a raised bump in the middle.
- Eventually, if unchecked, plants experience stunted and slow growth
Prevention of Yellow and Brown Leaf Spot
- Adopt pruning techniques like lollipopping, to remove undergrowth
- Improve airflow with circulating grow room fans
- Reduce relative humidity
- Never water from the top-down, always direct water over the surface of the substrate
Treatment of Yellow and Brown leaf Spot
- Immediately remove and destroy all infected areas
- Remove and destroy plants with serious infections
Root Rot (Fusarium, Pythium, and Rhizoctonia)
Root rot is caused by one of several fungi, including Fusarium, Fusarium, Pythium, and Rhizoctonia. It's most common within overly wet environments, like hydroponic setups and in cases of overwatering.
While relatively easy to identify, it can be very challenging to correct. It can be incredibly frustrating because spores may linger for long periods in soil, substrate, and hydroponic reservoirs.
Root Rot thrives in warm temperatures (68 to 93ºF, or 20 to 34ºC). Hot reservoirs, a dysfunctional/inadequate aeration stone, or supersaturated soil are ideal environments for root rot in cannabis.
Signs of Root Rot
- Roots are coated in a thick, brown slime
- A rotten smell emanating from the container and roots
- Leaves begin to turn yellow and brown, eventually dying off
- Leaves may begin to curl
Prevention of Root Rot
- Ensure the aerator stone is functioning properly
- Adoption of a correct watering schedule
- Complete sterilization of hydroponic systems between crops
- Sterilization of substrates between crops
- In soil-based systems, use soil with good drainage
Treatment of Root Rot
- Removal and destruction of the plant at first signs of root rot
- Removal of all associated soil, substrate, containers, and other touching pieces of equipment
- Sterilize entire system
Russet Mites
Mites, in general, are perhaps the most problematic pest for cannabis cultivators. Russet mites (Aculops cannibicola) the biggest culprits among all growers but are often mistaken for spider mites.
Russet mites usually fester in the lower branches of a plant, on the undersides of leaves. As the infestation grows, it creeps upwards. Because of their microscopic size ( 0.1mm to 0.2mm), infestations often go unchecked until well established.
Unfortunately, because mites are so small, it's difficult to identify an issue without a magnifying glass (which is why it's always essential to have a LED loupe in the grow room).
If plants indicate signs of nutrient deficiency or pH imbalance, also assess for possible russet mite infestation. The indications for one are often confused for the other.
Signs of Russet Mites
- Physical stress, which resembles those of nutrient deficiencies
- Slow, deformed, and stunted growth
- The appearance of drooping and wilted leaves
- Yellowing and curled leaves
- Buds dry out and die
Prevention of Russet Mites
- Sterilize all tools and equipment before bringing them into an indoor grow room
- Invest in a filtered intake fan
- Quarantine all new clones and seedlings before bringing them into the grow room
- Invest in a LED Loupe for close up inspection of leaves.
Treatment of Russet Mites
- Removal and destruction of localized infestations
- Removal and destruction of plants in severe infestations
- Improve air circulation in and around the canopy
- Apply natural pesticides, such as diluted essential oils, neem oil, dish soap, and alcohols
- Introduce natural mite predators (Neoseiulus sp. and Phytoseiulus sp)
Thrips
Out of the 300 arthropod species known to infest cannabis, thrips are most problematic (especially within greenhouses and indoor spaces).
These super tiny flying insects are under two millimeters long as adults, with the eggs and larvae much smaller.
Unlike other marijuana pests and diseases, thrips flies generally don't kill cannabis. Instead, it weakens the plant to reduce yields and possibly THC levels, as indicated by a recent controlled study. This 2019 study reported a 30 percent reduction in yields for highly infested plants.
Signs of Thrips
- Eggs appear as small grey (or silver) dots on the surface of leaves.
- Thrips defecation appears as tiny black dots on the surface of the leaves
- The appearance of flying insects around and below the canopy
Prevention of Thrips
- Constant surveillance of plants for flying insects
- Installation and examination of yellow sticky fly traps
- Reduction in relative humidity
- Increase airflow through the canopy
Treatment of Thrips
- Install yellow sticky fly traps in and around cannabis plants to trap adults
- Ongoing treatment with potassium soap or natural pyrethrins
- Introduction of predatory insects, like Lacewings and predatory mites.
Whiteflies
Out of all the flying insects that will attack your cannabis plants, whiteflies (Aleyrodidae sp) are easy to identify because of their white coloring. These small flies, with delicate long white wings, are only two millimeters long.
They suck the sap of cannabis plants, much like aphids. Whiteflies have a longer reproductive cycle than other flying pests, with roughly four cycles possible every year. Still, one fly can lay upwards of 200 eggs, which is a big risk throughout a full grow.
Because they fly, whiteflies quickly move from one plant to the next. They may also take with them other diseases, like molds. It is crucial to get a whitefly cannabis infestation under control before it damages and reduces the final harvest.
Signs of Whiteflies
- Small white flies in and around the canopy and lower branches
- White powdery looking spots and solid dots on the undersides of leaves
- Clear, white, or yellow nymphs (young whiteflies) on the undersides of leaves.
- Wilting, drooping, and indications of plant stress
Prevention of Whiteflies
- Grow beneficial companion plants that repel whiteflies, like basil and marigold
- Constant monitoring of undersides of leaves for nymphs and eggs
- Installation and examination of yellow sticky fly traps
Treatment of Whiteflies
- Install yellow sticky fly traps in and around cannabis plants to trap adults
- Ongoing treatment with potassium soap or neem oil
- Treatment with a solution made with essential oils like clove, cinnamon, and/or rosemary
- Introduction of predatory insects like lacewings and lady beetles
Prevention Key to Getting Marijuana Pests and Diseases Under Control
With hundreds, if not thousands, of insects and fungus species eager to infest a cannabis plant, preventative measures are key.
First, it all begins with strain selection. Choose strains resistant to mold and suitable for your climate.
Second, consider the environment. Reigning in unchecked relative humidity levels, increasing airflow, and otherwise adjusting the local environment will make it inhospitable to nuisance guests.
Finally, consider IPM strategies. Because cannabis is both inhaled and ingested, growers need to reduce the risks posed by conventional pesticides. It's why IPM practices, organic solutions, and biological controls come first.
Only when you've exhausted all other options should you even begin to consider chemical pesticides. Check with local regulations on approved, cannabis-specific pesticides and fungicides should you choose to go that route.
Summary of Controlling Cannabis Pests and Disease
If you remember nothing else from this in-depth guide on marijuana pests and diseases, take away these two points:
- Monitor your plants daily, especially during flower for signs of mold, mildew, and insect infestation.
- Maintain a strict clean grow room by sterilizing all equipment and substrates before use.
Together, these two points will reduce the risk of pests or spores infiltrating your canopy.
