You may have invested in the best cannabis seeds, a high-end LED grow light and a sophisticated grow room. But all that can be decimated if pests and diseases get a grip on your crop. Many people don't know one pest from another, or the best way to eliminate them. But a quick read of this guide will explain all you need to know.
Identifying the most common cannabis pests
When caught in time, cannabis pests can be dealt with. The more serious problems tend to arise when pests are allowed time to establish themselves in your grow room. Some of the more difficult pests, and what to do about them, are discussed below.
It’s worth mentioning that many indoor growers (and greenhouse growers) now routinely add UVB supplemental lights to their grow location. The UVB emitted often has a positive effect at reducing (perhaps even eliminating) pests before they get chance to become established and is a good tool to control pests.
if you ever find and remove pest-damaged tissue/leaves from your plant, one useful tip is to ensure that you never keep this waste material in your grow room. Cannabis cuttings/clones from friends are another frequent source of unintended pests/disease. Leaving your indoor plants outside for an afternoon in the outdoor sun is also an excellent way to inadvertently introduce pests into your grow room.
Cannabis aphids
Cannabis aphids can be a serious nuisance to both indoor and outdoor growers. They are often found on the underside of leaves and come in an array of sizes according to their age and maturity.
Soap-based sprays can be useful to reduce aphid activity, so too can aphid predators such as ladybugs, which can be bought online. Leaving an aphid infestation unchecked will allow rapid population expansion and a sticky mess on your plants which reduces harvest quality/quantity.
Barnacles / scale insects
These white or red creatures look similar to seaside barnacles and attach themselves to your plant stem, branches and leaves. They are also called wax scales. Like aphids they produce a sticky waste material known as honeydew all over your plants. Again, a soap spray can help remove them. So too can a powerful water spray, neem oil or predator insects such as lacewing and ladybugs.
Cannabis broad mites
Broad mites are a serious nuisance once established. They are so tiny you may not notice them, even with a magnifying glass. The tortured leaves look twisted, droopy, glossy, blistered, and unhealthy. Growers often mistake the infestation for pH, heat stress or nutrient problems, compounding the difficulties.
Specialist chemical sprays (known as Miticide sprays e.g. ‘Forbid’, ‘Avid’ & ‘IC3’) can be used but are often not favored by cannabis growers. Instead, neem oil, soapy sprays. Mite predators such as Neoseiulus are a good help. Diatomaceous earth can also help.
Caterpillars & inchworms
These can munch through your plant leaves, often leaving black droppings. Removing them keeps your foliage intact and helps plant growth. Anti-Caterpillar sprays do exist, but with organic buds and a healthy smoke at the top of your priority list, it would be easier to pick the pests off by hand.
Crickets
Crickets will often feast on cannabis leaves but are easy enough to manually remove. Generally speaking, crickets are not major pests; however, “mole crickets” can burrow into the cannabis root system and are a nuisance even though they are infrequently encountered. Plant netting can be useful to keep such insects away from your plants.
Fungus Gnats
Fungus gnats often emerge from your soil after hatching from their eggs. Soils containing compost made from wood chips can be especially prone to fungus gnats. Some indoor growers purposely buy wood-free soil mixes to minimize the chances of fungus gnats, which can cause significant damage to root systems and stunt growth.
UVB supplemental lights have a satisfyingly destructive effect on gnats - along with other insect pests. Rolls of sticky fly tape are also effective. Diatomaceous earth creates a barrier that prevents many larvae from emerging from the soil. Many growers encounter fungus gnats during a grow; a low-level presence doesn’t mean automatic disaster for your crop, but it can reduce plant health and growth rates.
Grasshoppers
Grasshoppers will happily eat the foliage on your plants, but they’re easy to remove by hand. A layer of netting around your plants can prevent grasshoppers from landing on them.
Leafhoppers
Leafhoppers come in a surprisingly broad array of colors. Their main problem-causing activity is sucking sap from your plants, leaving behind spots that may be white, brown, or yellow. They have wings, six legs, and can jump.
They are a particular problem in hot, dry weather, when their need for moisture and therefore sap is greatest. Specialist insecticidal sprays are available, though few growers want to spray their buds with chemicals they will later vape. Many prefer neem oil, as well as beneficial predators such as the cannabis grower’s friends: ladybugs, lacewings, and parasitic wasps.
Leaf Miners
Leaf miners are actually larvae that live inside the tissues of your cannabis leaves. Their damage is visible as white, wavy trails on the leaves. Parasitic wasps known as Diglyphus isaea are a good solution, as are neem oil or spinosad.
Mealybugs
Mealybugs look like tiny, hairy white woodlice. Their honeydew residues can attract black mold, and they do nothing for plant health. Spraying with neem oil, soapy solutions, or even alcohol has been tried as a remedy with some success. Ladybug predators may also be useful.
Planthoppers
Planthoppers are small, winged insects that produce a white, mold-like excretion. They suck sap from your plants and reduce overall plant health and vigor. The nymphs resemble tiny crabs with red eyes. The usual soapy sprays or beneficial predator insects are the best approach.
Russet Mites
Hemp russet mites may appear as a yellow-colored coating on branches, causing limp foliage. Drooping or curling leaves are common, and russet mite infestations are often misdiagnosed as other issues. The individual mites are almost too small to see. Symptoms are often most severe near the tops of the plants. In the worst cases, your whole plant may need to be discarded. In mild cases, try the usual sprays. Insecticidal sprays can be effective, though they are difficult to justify on buds you plan to vape. Predatory mites such as Amblyseius andersoni may also be an option.
Slugs / Snails
Slugs and snails will eat and damage foliage and need to be removed promptly. They are usually easy to spot and remove by hand, so infestations should be rare.
Spider Mites
Spider mites can be a real nuisance. Often found as small specks on the undersides of leaves, their main sign of damage is the numerous small white bite marks visible on the upper side of leaves. In severe cases, webbing may be present.
Treat spider mites by increasing air circulation and depriving them of the low-humidity, high-temperature conditions in which they thrive. Soapy sprays are useful, as are dedicated anti-mite sprays. Predators and diatomaceous earth may also help, as can lowering temperatures and raising humidity. Repeated treatments are essential to kill newly hatched mites. Predatory mites work especially well against spider mites.
Thrips
Thrips are small, dark-colored, winged insects with long antennae that can be a real nuisance if they become established in your grow room. You may also encounter yellow or gold thrips, and even colorless ones. As with other pests, preventing them is far easier than eliminating them. They can damage plant foliage, reducing health, yield, and potency.
Light-colored patches often remain on the leaves after thrip damage. The best treatments include spraying with soapy water or neem oil, applying diatomaceous earth, and performing regular inspections. Natural predators work well against thrips, with predatory mites - especially Orius and Amblyseius cucumeris - being among the most effective options.
Whiteflies / White fly
Whiteflies look like miniature white moths that live on the undersides of leaves. They cause damage and leave a sticky residue that can attract mold. When infested, you can shake a branch and watch them take off in flight. Spraying with soapy water or treating with neem oil are two effective approaches. Predator insects such as ladybugs are also a good option.
Identifying the most common cannabis diseases
There are numerous cannabis diseases. However, if you grow from cannabis seed rather than clones, which always carry a risk of imported disease, and you maintain a clean grow room, you are unlikely to encounter them.
Bud rot or mold
Also known as botrytis, bud rot is a devastating disease that can turn harvest-ready buds into mold. Dying leaves and rot at the base of the leaves are common signs. Avoid bud rot by maintaining low humidity levels during bloom. UVA and UVB supplemental light can help reduce mold spores and may be worth considering. Improving airflow, removing excess foliage, promptly removing infected plant areas, and closely monitoring plants are also effective measures. In severe cases, you may need to harvest early.
Root Rot
Brown, slimy roots, unlike healthy white or cream-colored roots, are a sign of root rot. Your plant will droop as it fails to receive the nutrition it normally absorbs. Root rot can be caused by various pathogens, fungi, algae, and parasites, but it often produces the same symptoms. Hydroponic growers are especially prone to it.
Excessively hot water tanks, low oxygen levels in nutrient solutions, or light leaks in the root zone of hydroponic systems will make the problem worse. Yield and potency will suffer unless root rot is corrected. Beneficial bacteria can help, as can eliminating light leaks. Some growers also find that specialist silver-based antibacterial nutrient additives or hydrogen peroxide can help restore root health.
Tobacco Mosaic Virus (TMV)
First discovered in tobacco plants, TMV can affect more than 100 plant varieties. It causes unusual pale mosaic or mottled markings on leaves, along with twisted growth. Infected plants may appear sickly and grow slowly. TMV cannot be cured and is often confused with other diseases or issues, making it a difficult - and possibly quite rare - problem for cannabis growers.
White Powdery Mold
If you have ever seen a white, powdery substance on your stems or leaves, you may have encountered white powdery mildew or mold, often called “WPM.” It is reversible and treatable if caught early enough; if not, your plant may die. WPM thrives in conditions with poor air circulation and high humidity. Treatments include spraying with a 33% milk solution or a dilute potassium bicarbonate spray (1 tablespoon per gallon of water).
Hop Latent Viroid (HLV)
HLV is an infectious pathogen known as a “viroid” that causes only minor damage to hops but leads to the more serious dudding disease in hemp and cannabis. The effects in cannabis can be particularly dramatic.
HLV can spread from contaminated blades used when making cuttings. Plants infected with HLV may show stunted growth, brittle stems, smaller leaves, short internode distances, patchy or inconsistent trichome production, and an aroma similar to decaying leaves (such as leaves removed a day or two earlier).
Another telltale sign of HLV is lighter leaf color (chlorosis) caused by loss of chlorophyll. HLV-infected plants cannot easily be identified without lab testing. However, once HLV is established, it becomes easy to spot the sick, pale, short plants. Destroy them immediately.
Tips: Few home growers will have encountered HLV, especially those who grow from cannabis seeds. Those who trade cuttings should be extra vigilant. Professional growers always keep new or purchased cuttings in a separate area until they have completed a full, healthy grow cycle with them. So far, HLV has mainly affected commercial growers who focus heavily on clone production or purchase clones from specialist companies.
Damping Off
Damping-off is a seedling disease that causes seedlings to collapse, sometimes surrounded by white fungal growth. It is caused by several different fungi and fungus-like organisms, including Pythium, Phytophthora, Rhizoctonia, and Fusarium.
There are no particularly suitable fungicides to prevent damping-off disease. Even if there were, many cannabis growers would not want to use them. This means that prevention is the only realistic approach.
- Quality professional soil mixes are usually free from damping-off fungi. Homemade compost may benefit from steam sterilization.
- If reusing plant containers or seedling trays, give them a thorough cleaning. It may be safest to discard any containers that held diseased plants and completely clean and sterilize your grow environment.
- Avoid overwatering; excessively moist conditions encourage the fungi associated with damping-off.
- Ensure good air extraction and air movement with a fan.
Fusarium
Fusarium refers to a genus of filament-like, soil-borne fungi. The infections it causes can block the cannabis xylem, which carries water. This leads to stunted growth and wilting, often killing the plants. There is no real cure; avoidance is the only approach.
Thorough cleaning of your grow area is essential. Professional growers sterilize grow areas and surfaces between crops with solutions such as 3% hydrogen peroxide. This safely decomposes into oxygen and water and is highly effective for disinfection.
As with any diseased plant, remove infected plants from your grow room as soon as possible. Take the opportunity to thoroughly clean and disinfect the area before growing again.
Verticillium Wilt
Verticillium wilt is caused by the fungi Verticillium dahliae and Verticillium albo-atrum. Both are found in soil and can infect cannabis as well as many other plants.
Infection begins in the roots and then spreads upward through the xylem, the plant’s water-conducting tissues. This causes wilting of the upper parts due to water deficiency. Lower leaves may turn yellow and shrivel.
Plants affected by verticillium wilt may show black or brown discoloration in the stems or branches when cut with clippers. As with many plant diseases, avoidance is the best cure. Good housekeeping, grow room cleanliness, and regular sterilization are the most effective approaches.
Leaf Septoria
Cannabis leaf septoria is a fungal disease that often strikes in early bloom. It typically manifests as yellow or brown spots on the leaves. It can be treated by removing and destroying infected leaves. Reducing grow room moisture and humidity also helps. Using an air circulation fan can further reduce pockets of stale, humid air.
A layer of mulch above the soil can help prevent the fungal spores from spreading. Spraying with neem oil may also be effective. Cannabis leaf septoria can show some confusing similarities to calcium deficiency.
The following article is useful reading for anyone wanting to know more about the subtle visual differences between the two.
Rust Fungus
Rust fungi are a group of specialized plant pathogens that cause disease in foliage and leaves. Occasionally, rust (a brownish leaf discoloration) can also affect stems and blooms.
With rust fungus, a pale spot on the leaf eventually grows and darkens, producing “pustules” that release fungal spores. Pustules are often found on lower cannabis leaves and can vary in color from orange, brown, yellow, black, or white.
The disease often has a rusty brown appearance that causes leaves to die. Heavy infections reduce plant vigor, stunt growth, and lower harvest quality.
Good grow room hygiene is the best defense. Removing and destroying affected leaves can slow the spread of rust fungus. Although fungicides are available, they are never recommended for cannabis cultivation.
Good to know:
- Using leaves to diagnose an underlying disease or nutrient issue is far from foolproof. Even experienced growers can sometimes reach the wrong conclusion or apply the wrong treatment.
- Good grow room hygiene (regular, thorough cleaning and removal of all old foliage) is one of the best ways to avoid pests and diseases. Preventing problems is easier than curing them.
- Choosing a wide-latitude grow method is a great way to avoid nutrient deficiencies.
- Growing in large (30–50 liter / 6–10 gallon) aerated containers (felt sacks, air pots, etc.) with slow-release organic nutrients (BioTabs are highly recommended) allows you to grow with the water-only method for much of the cycle, reducing the risk of nutrient deficiencies or excesses.
Some growers report that supplemental UVA/UVB lighting (often used during bloom) helps reduce pest infestations and may lower mildew severity.
Anthracnose
Anthracnose is a fungal disease that can cause dark lesions on cannabis leaves. Many plants, not just cannabis, are susceptible to anthracnose. The disease is worse in cool, wet weather and when irrigation water splashes onto the foliage.
The spread of anthracnose can be slowed by removing and destroying affected foliage. Fungicides can help but should not be used on cannabis.
Downy Mildew
Downy mildew is a disease that affects cannabis foliage, producing discolored leaves. It is caused by a fungus-like organism known as Oomycetes. The mildew spreads from plant to plant by airborne fungal spores.
It spreads most effectively in wet weather or in humid indoor grow rooms, as infection is favored by prolonged leaf dampness. Downy mildew is less obvious than powdery mildew. Look for blotchy, discolored areas on the upper leaf surface.
The best approach is to remove and destroy affected leaves. Avoid densely packed grow rooms and keep humidity under control. Good air circulation can eliminate pockets of stale, moist air. No fungicides are available for downy mildew control.
Additional diseases some cannabis growers may encounter
Algae Overgrowth
Algae is a collective term for a large and diverse group of photosynthetic eukaryotes. It can be a nuisance for hydroponic cannabis growers, and if not controlled, a green slime can appear in the root zone.
Algae can soak up all available oxygen, causing typical oxygen deprivation symptoms such as stunted growth, wilting, nutrient deficiencies, and leaf spots. It can also affect pH—critical for hydroponic growers—and lead to pest infestations and possible fungal diseases.
Algae often appears as a light green discoloration in nutrient reservoirs, pipes, surfaces, and other equipment. Since algae needs light to photosynthesize and survive, keeping your equipment “blacked out” will prevent it from growing where it should not.
Maintaining low grow room temperatures, especially in nutrient solutions, helps discourage algae. For severe outbreaks, a dilute 3% hydrogen peroxide solution is excellent for cleaning pipes, containers, and surfaces. Hydrogen peroxide can also be added in low concentrations to nutrient solutions to help deter algae formation.
Sooty Mold
One of the easier diseases to identify, sooty mold gives cannabis leaves a darkened appearance, as if they were lightly sprinkled with a black, sooty deposit. Sooty mold refers to a collection of fungi that can colonize cannabis plants and affect their leaves.
It can form on aphid waste (known as honeydew) as well as on the sugary materials produced by cannabis. The name comes from the distinctive black, soot-like appearance of the mold. Treat by removing any aphid infestation with a soapy water spray and by lightly cleaning or rubbing affected leaves.
Sooty mold affects only the leaf surfaces and is not as deadly as other fungal infections, but it still requires prompt attention from the grower.
Cankers
Caused by the fungus Sclerotinia sclerotiorum, this disease can lead to stem cankers and bud rot. The initial symptom is often water-soaked lesions visible on the leaves and buds. The canker fungus can infect the stem near affected buds, causing the death of the upper parts of the cannabis plant. Cankers can also develop lower down on cannabis or hemp plant stems.
Once infected, the stems and buds may appear whitish due to the production of fluffy, white mycelium. Infected buds frequently develop sclerotia on both internal and external plant surfaces. Newly formed sclerotia are white and turn black when fully developed. Specialist chemical treatments are available but are not recommended for cannabis cultivation.
Bacterial Blight
Bacterial blight is a disease caused by Pseudomonas cannabina. It affects cannabis plants, leading to rot on the stems and leaves. Affected areas, or “lesions,” are often pale (chlorotic). Bacterial blight can be encouraged by overwatering and plant stress.
Yellowing of leaves is usually the first sign, followed by the formation of small (2–5 mm), angular, water-soaked lesions. These areas may develop chlorotic halos and tan or brown centers. They are often mistaken for the effects of overwatering, nitrogen deficiency, phosphorus deficiency, or septoria. Eventually, these lesions turn black.
As with other diseases, prevention is the best strategy. Treating bacterial blight is difficult, and many growers use an outbreak as an opportunity to reset grow room hygiene and carry out a thorough cleaning.
Black Root Rot (Thielaviopsis)
Cannabis black root rot, also known as thielaviopsis root rot, is a fungal disease caused by Thielaviopsis basicola. It infects plant roots, and infected cannabis roots may develop dark brown or black bands.
Root tips may turn black, and areas of stem below ground can develop black cracks. Leaves can turn yellow, wither, and die. Plant growth becomes stunted, and in severe cases, the plant will die.
Good grow room hygiene is essential. Fungicides can help but are not recommended for cannabis cultivation. Maintaining healthy soil with beneficial bacteria and fungi (such as Pseudomonas fluorescens and mycorrhizal fungi) can help deter black root rot.
Charcoal Rot
Charcoal rot symptoms often appear during hot weather or in overheated grow rooms, when foliage wilts and turns yellow. Plant growth is stunted, and the plants may die. These symptoms can be mistaken for verticillium wilt or bacterial blight.
Charcoal rot is often caused by Macrophomina, a fungal plant pathogen, and usually develops with alarming speed, in contrast to verticillium wilt. A dusty, charcoal-like discoloration is often present, alerting the grower to infection. There is no safe, effective treatment, but cooler grow conditions can help deter charcoal rot.
Pink Mold
Mold usually has a white, black or grey appearance. Occasionally is has been reported as pink. In any mold situation you don’t want to be consuming the affected buds. Remove and destroy affected plant areas immediately.
How to control cannabis bugs and pest organically
Prevention is always better than cure, and early detection is always better than dealing with a full infestation. Monitor your plants regularly and carefully. Most home growers keep only small plant numbers, so extra checks will not take much time. Avoid the temptation to use clones or cuttings, since even trustworthy clone sources can suffer serious pest or disease problems. Growing from cannabis seed in a regularly cleaned grow room is good practice.
Preventing pests on outdoor cannabis
Often, pests have natural predators that help keep infestations under control on outdoor plants. If that is not the case, you may need to take action yourself. Buying and releasing natural predators onto your plants is always a good idea. In severe infestations, you may need to consider spraying your plants with a soapy detergent to remove many plant-borne pests.
Preventing pests on indoor cannabis
Preventing pests on indoor cannabis can be more challenging since, by definition, there are usually no natural predators to control pest populations. This means your main options are to introduce natural predators, such as ladybugs, lacewings, and predatory mites, or to be vigilant and proactive with plant treatments.
Mold-resistant cannabis strains
It would be wonderful to have cannabis strains that are resistant to all pests and diseases, but unfortunately that is not the case. However, there are some mold-resistant cannabis strains, and outdoor strains with mold resistance are particularly valuable. As always, the best results come when plants are grown from cannabis seed to harvest under the best conditions you can provide.
Frisian Dew - tough & strong with XL yields, outstanding all-round outdoor cannabis strain
Frisian Dew is one of Dutch Passion’s top-performing and best-selling outdoor cannabis strains. She has thousands of repeat growers and is one of the most reliable outdoor strains, coping particularly well with the cold, cloudy summers of northern Europe and the more temperate northern regions of the United States. Frisian Dew is usually ready to harvest around the end of September or early October in the Northern Hemisphere, and in March or April in the Southern Hemisphere.
Passion #1 – Classic outdoor favorite with reliable yields
Passion #1 seeds are especially popular with outdoor growers in central and southern European latitudes. The mild climate and long growing season in these regions are ideal for outdoor cannabis cultivation. Passion #1 delivers genuinely heavy yields, and you can often expect buds even stronger than those from Frisian Dew.
Passion #1 is an original member of the Dutch Passion High-THC seed collection. Expect premium-strength outdoor cannabis with great taste and highly enjoyable effects.
Durban Poison - resilient sativa powerhouse for indoor and outdoor growing
Durban Poison is one of South Africa’s best sativa strains. She is equally popular with indoor and outdoor growers, with genetics that are tough, durable, and easy to grow in a wide range of conditions. Outdoors, she has a well-deserved reputation for XL yields, even in cooler temperate climates. The buds pack a seriously enjoyable punch. Highly recommended for growers of all types!
Prevention is always better than cure!
Monitor your plants carefully and try to ensure that if you do have to deal with a pest/disease outbreak that it is a minor outbreak. Growing from cannabis seeds rather than clones/cuttings is another way to avoid imported infestations. Those that use supplemental UVA/UVB lights may have noticed how they help suppress pests. Good grow room design, eliminating pockets of hot/stale air will also help keep your grow room conditions optimized.
But it’s also worth adding that most growers that follow the basic advice rarely see serious issues or infestations. For most growers, your harvest will arrive successfully with few worries! Choose good quality cannabis genetics from a seedbank you trust and enjoy your grow!