How to identify and treat common cannabis pests and diseases

For most home gardeners and cultivators, growing cannabis is a joy. And while these generally hardy plants have evolved in a variety of climates and conditions over millennia, they can fall prey to different pests, diseases, and nutrient deficiencies.

Below is a collection of the most commonly faced problems in cannabis gardens and commercial cultivation settings. Here, we include tips for identification, along with methods of treatment and prevention so that your cannabis can grow healthy from start to finish.

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Aphids

The term "aphid" is used to refer to a group of small sap-sucking insects that are common pests in most gardens and commercial cultivation settings. A new, cannabis-specific form of aphid (Phorodon cannabis) was first spotted in Colorado in 2017, and can be quite destructive if left unchecked. All aphids suck sap from leaves, which is harmful to the plant, and excrete a thick sticky substance called “honeydew" that can promote mold growth.

Identification:

• Aphids live under leaves, away from direct sunlight.
• Look for small, pear-shaped six-legged insects that are light beige to light green when young, but may also be light pink or brown.
• You might also see their cast-off exoskeletons (“cast skins") left on the underside of leaves.

Treatment:

• Prune infected leaves and securely dispose of them.
• Use organic insecticides, essential oils, or insecticidal soaps.

Prevention:

• Introduce aphid predators such as lady beetles, hoverfly larvae, parasitic wasps, aphid midge larvae, crab spiders, and lacewings in spring and late summer.
• Outdoor gardens are usually more at risk of an aphid infestation. If you can control environmental variables, ensure good airflow, along with moderate temperatures and humidity.

Barnacle scales

Barnacle scales are unmoving and strange-looking insects that could be mistaken for growths rather than parasites, though some do look like insects. They produce a similar sticky “honeydew" as aphids as they suck the sap and moisture from plants. They reproduce quickly and should be addressed immediately but are typically relatively easy to treat.

Identification:

• Typically on stems and the underside of leaves.
• Look for unmoving clusters that may be fuzzy or moldy looking — they can also look like globs of grey or beige wax.
• They come in a variety of shapes, colors, and patterns.

Treatment:

• Spray down plants with a small pressure sprayer to remove as many scale barnacles as you can.
• Treat plants with organic insecticides, essential oils, or insecticidal soaps.

Prevention:

• These insects tend to attack unhealthy plants.
• Introduce predators such as lady beetles, ladybugs, and lacewings periodically.

Boron deficiency

Boron deficiencies are relatively rare, but they can have big impacts on the growth and vigor of a plant. When using a quality growing medium or cannabis-friendly nutrients you probably won't run into this deficiency, but it can develop when plants aren't getting enough water, are in very dry environments, or when the water's pH levels are off. These deficiencies can mimic calcium deficiencies since the plants need boron to produce calcium.

Identification:

• The uppermost leaves of a plant will grow abnormally or slowly, tips may look twisted or dead, and the leaves may look wrinkled or curled. Stems may become rough or hollow.

Treatment:

• Water plants with a solution of one teaspoon of boric acid for every gallon of water. While affected leaves may not recover, watch for continued symptoms in new leaves. Treatment should yield results within a day or two.
• When plants show recovery, flush the growing medium and recommence a normal feeding schedule.
• Make sure water pH is 6.0 - 6.5 pH in soil, 5.5 - 6.2 in hydro to ensure boron absorption.

Prevention:

• Keep humidity levels above 25% and ensure your growing medium doesn't completely dry out too often.
• Over-fertilizing can cause nutrient lockout, so don't overdo it with nutrients.
• Since boron is in most tap water, growers using filtered water or reverse osmosis water are generally at a higher risk for this deficiency.

Broad mites

Like aphids and barnacle scales, mites suck nutrients and moisture from new and young cannabis leaves that can hamper the plant's growth. They also inject a growth regulator into the plants, which often causes nutrient deficiencies. They are common garden pests that aren't unique to cannabis. Unlike aphids and scales, however, broad mites are extremely small and almost impossible to see with the naked eye.

Identification:

• Use a high-powered (60x loop or stronger) magnifying glass or microscope to look for six-legged teardrop-shaped bugs that are often pale yellow or clear.
• New growth on the plant is often yellow, drooping, twisted, or blistered, with a wet appearance.
• Feeding sites will have a yellow-speckled appearance on leaves and will be concentrated in certain spots of the plant.

Treatment:

• Quarantine infected plants, remove infected plant matter and safely dispose of it.
• Treat plants with organic insecticides, essential oils, or insecticidal soaps daily until new growth is healthy and unaffected.
• If using chemical miticides, do not use them during the flowering stage.

Prevention:

• Introduce predatory mites, such as the Neosiulus types.
• It can be hard to prevent these mites outdoors since they can arrive via the wind, infected plant material, or be deposited by animals.
• Indoors, they most often come from infected clones.

Bud rot (botrytis)

Bud rot (also known as botrytis cinerea, botrytis, or gray mold) can quickly and efficiently destroy cannabis flowers. It's a fungal infection that develops deep in the heart of cannabis flowers, typically infecting the flower while it's young and remaining dormant until later stages of development.

Identification:

• Bud rot initially appears as pale, powdery mildew on buds, but gradually becomes darker in color and the bud will develop a slimy consistency.
• When the mold has fully infiltrated the bud, it will easily break apart showing a dark and dusty interior which is the mold spores.

Treatment:

• Unfortunately, buds that are infected must be discarded as they are completely unfit for consumption.

Prevention:

• Bud rot is most common outdoors in humid, wet conditions. In these climates, keep plants as dry as possible with tarps or in greenhouses.
• Choose cultivars that are known to be more resistant to mold and powdery mildew.

Calcium deficiency

As an important secondary nutrient for cannabis plants, calcium is needed for stress resistance and cell wall strength while playing a role in regulating water uptake and metabolism. A deficiency in calcium is a fairly common problem in cannabis plants that can be present for weeks before symptoms show up, but it can be difficult to diagnose since calcium deficiencies are often accompanied by magnesium, iron, and/or other cannabis deficiencies.

Identification:

• Look for yellowing leaves with brown spots concentrated on new growth at the top of the plant, burned leaf tips and edges, stunted growth
• Weak plant structure and drooping, hollow, or rotting stems.
• Can also look like a fungal infection called "leaf Septoria".

Treatment:

• Check the pH of your growing medium, as this is often the culprit. The pH for optimal calcium absorption is between 6.2 and 7.
• Outdoors, you can modify soil with garden lime, dolomite, powdered bones, crushed eggshells, and fish meal.
• Indoors, flush your plant with a neutral pH water solution containing dolomitic lime or a commercial calcium-magnesium solution.

Prevention:

• Regularly check the pH levels of your growing medium, with coco being a common culprit for this deficiency.

Caterpillars

Caterpillars are more common in outdoor cannabis grows, and can wreak havoc on plants with their voracious appetites. The most common in cannabis plants are the vampire moths (Spodoptera exigua), silver Y moths (Autographa gamma), or cotton bollworms (Helicoverpa armigera) caterpillars, along with the Eurasian hemp borer (Grapholita delineana) that tunnels into stems and buds and is widespread in eastern Colorado.

Identification:

• Caterpillars are long, slender insects with soft, multi-segmented bodies that can take on a variety of colors and patterns
• The tell-tale sign of their presence is bite marks and holes on leaves.
• Since they are more active at night, inspect plants after dark to find and remove them.

Treatment:

• Use natural bacterial sprays containing Bacillus thuringiensis, but avoid using them too close to harvest time.
• Garlic, potassium soap, or tobacco sprays may also work as organic pesticides.

Prevention:

• Look out for butterflies and chase them away if you see them resting on your plants since they could be laying eggs.
• Check for yellow or translucent eggs on the underside of the leaves.
• Introduce parasitic wasps and praying mantises, or install nest boxes for caterpillar-eating birds such as goldfinches, sparrows, or robins.
• Pheromone traps can attract and contain male caterpillars, rendering them unable to attack or reproduce.

Copper deficiency

Copper is an important nutrient that helps cannabis plants with photosynthesis and respiration, metabolizing sugar and proteins, along with maintaining general health and normal development. Copper deficiencies are not all that common, though they can crop up at any point in the life cycle of the plant, particularly in some cultivars during flowering.

Identification:

• Leaves near the top of the plant will become dark with blue or purple undertones, while the tips and edges of the leaves will turn a bright yellow or white.
• The leaves may take on a shiny or metallic appearance and feel stiff to the touch.
• Nutrient burns can also cause yellow tips. You can distinguish nutrient burn from a copper deficiency when the leaf tips turn brown and dry out. With a copper deficiency, the tips will remain yellow.

Treatment:

• Flush the plant with pH-balanced water with the proper nutrients added. Symptoms should resolve in a week.
• While affected leaves may not recover, new growth should be healthy and unaffected.

Prevention:

• Like other nutrient deficiencies, the problem is not caused by a lack of nutrients in the medium or water so much as a pH imbalance around the roots, so be sure to monitor the pH levels regularly.
• Ideally, the pH will be between 6.0 and 7.0 in soil, and between 5.5 and 6.0 in hydroponic grows.
• Some varieties are more prone to copper deficiencies, especially during flowering, so monitor these plants more carefully.
• Don't overwater as this can cause copper deficiency, too.

Fungus gnats

Fungus gnats are a fairly common cannabis pest that causes damage when their larvae feed on your plants' root systems beneath the soil. While most mature plants with large root systems can survive these pests, seedlings and young plants are most susceptible to fungus gnats. Luckily, they are fairly easy to treat and prevent.

Identification:

• Since fungus gnats attack roots, the symptoms can resemble a variety of diseases or deficiencies. The tell-tale sign will be small flies buzzing and jumping around the surface of your soil.
• You may also see maggots in the topsoil.
• Look for deceased plant signs such as yellow leaves, wilting, spotting, and drooping.

Treatment:

• Allow soil to dry out completely between waterings.
• Use sticky traps at the soil surface.
• Sprinkle Diatomaceous earth powder on the surface of the soil.
• Use a natural bacterial spray containing Bacillus thuringiensis israelensis.

Prevention:

• These insects thrive in moist conditions, so don't overwater your plants and allow the soil to dry out completely before watering.
• Buy properly sterilized soil.
• Avoid introducing gnats via clones by purchasing from trusted and high-quality sources.

Fusarium

As a particularly sneaky and petulant problem, fusarium is a fungus that hides beneath the soil where it spreads to the root system of your plant, causing the dreaded “fusarium wilt". It is such an effective cannabis pathogen that it was even used as a weapon against crops in the War on Drugs. Once it attacks the root system, the plant becomes unable to absorb nutrients and will eventually wilt and die.

Identification:

• Fusarium symptoms can present like many other deficiencies — yellowing leaves and stunted growth, for example.
• You might also see red-colored roots just under the topsoil, orange, pink, or white fungal growth on the stems, or a spread of decay up the stem

Treatment:

• Unfortunately, there is no way to treat these infections. Once infected, a plant must be destroyed.

Prevention:

• After infection the fungus will remain in the soil for years to come, so don't use the infected soil again. Using pots can contain outbreaks and prevent a crop-wide infection.
• You can introduce beneficial fungi such as mycorrhizal fungi, which will help roots absorb nutrients and ward off pathogens.
• Keep pH in check, as fusarium prefers acidic soil.

Heat stress

Cannabis plants generally favor warm conditions, but high heat can stress them out. When facing heat stress, the plants struggle to maintain their physiological homeostasis (balance) which can ultimately impact their overall health and your overall yield. If your plant looks tired and worse for wear despite getting adequate nutrients, water, and light, then you might have heat-stressed plants on your hands.

Identification:

• Plants will also look generally dry and withered. Look for the tell-tale sign of fan leaves developing tips that curl inwards.
• You might see large brown spots or blotches on the leaves.
• During the flowering stage, you might see the plant “foxtail" and grow a new bud out of the top of an existing bud.

Treatment:

• Indoors, adjust environmental controls to lower temperatures, introduce fans and an exhaust system if needed, and ensure plants aren't too close to light sources.
• Outdoors, move pots to cooler locations or create temporary shelters to protect plants from the peak heat and sun of the day.
• Some growers use kelp extract to help plants recover from heat stress.

Prevention:

• Keep plants properly watered.
• Keep indoor grow temperatures between 70 - 85°F (20 - 30°C).
• If you're growing outdoors in a hot climate choose cultivars that are known to be more heat resistant.

Hemp russet mites

Hemp russet mites (Aculops cannabicola) are specific to hemp and cannabis plants and are often sneaky nuisances that are hard to detect until it's too late. They cause symptoms similar to spider mites, thrips, mold, fusarium, pollen, tobacco mosaic virus, overwatering, or heat damage, so be sure to visually identify them before diagnosing.

Identification:

• These tiny pests measure between 0.05 mm (eggs) and 0.2 mm (adults) long so you'll need to use a strong hand lens to see them, but they are best identified under high magnification using a dissecting microscope. They are pale-yellow or pale-pink cigar-shaped insects with soft-looking bodies.
• They are typically found on stems and petioles, but when populations are large you'll also find them on the underside of leaves.
• Look for stunted leaf and/or bud growth on the plant, dull, greyish, or bronzed leaves (which are coated with the actual bugs), and upward curling of leaf edges.

Treatment:

• Often, these pests are not detected until the mites have fully infested your plant(s), which will require removing and destroying infected plants. Treat remaining plants with organic insecticides, essential oils, or insecticidal soaps.
• Sulfur is a miticide that can also be effective against the hemp russet mite.
• Entomopathogenic fungal-based pesticides like Beauveria bassiana and Isaria fumosorosea are also effective.
• Since phytotoxicity with sulfur and fungal-based pesticides is possible, make sure to water your plants before treatment. Treat regularly for several weeks to ensure the mites are eradicated, but beware as this can be very hard on the plants.

Prevention:

• Quarantine new plant material before introducing it to your garden or grow room.
• Examine your plants regularly using a 16X hand lens and occasionally check leaves under a dissecting microscope.
• Workers should wear rubber gloves and change them routinely throughout the day when handling plants to avoid spreading the mites.
• Introduce predatory mites (Neoseiulus californicus, Galendromus occidentalis, and Amblyseius andersoni) periodically throughout the grow season.

Hop latent viroid

Hop latent viroid, or HLVd as it is often abbreviated, is one of the newer menaces to crop up in cannabis cultivation and it's quietly wreaking havoc across North America. It's a plant-specific pathogenic RNA that is easily spread, and infected plants may not show any symptoms. The viroid can lay dormant in the plant for extended periods of time, making HLVd hard to detect and really only confirmable with laboratory testing. Once it rears its head, however, it can drastically reduce the quality and yield in your crops.

Identification:

• Often called “dudding disease," one of the main symptoms of this viroid is the overall stunting of plant growth. Look for shorter plants with smaller leaves along with smaller and less dense buds producing fewer trichomes.
• Abnormal branching, brittle stems, and yellowing leaves might also be present.
• Confirm its presence with laboratory testing.

Treatment:

• Once plants have the viroid, there is not much you can do to treat them. But you can preserve and restore proprietary or high-value genetics through tissue culture remediation.

Prevention:

• Test crops and mothers for this virus regularly.
• It can be easily spread through tools and touch, so maintain tight sanitation protocols, bleach seems to be best for decontamination.
• Maintain tight quarantine protocols for any new plants entering your grow and have them tested.
• Research is ongoing, but it's suspected that insects may also be able to transmit the viroid through bites, so robust pest management may also help mitigate the spread.

Iron deficiency

Iron is vital to the plant's overall health, and yet it can be difficult to diagnose since it's often mistaken for calcium and magnesium deficiencies.

Identification:

• Since iron is essential for the production of chlorophyll, the easiest sign to spot is the yellowing of leaves. Yellowing is a symptom of many plant problems, so be sure to also look out for a bright, almost white yellowing on new/upper leaves, which is a tell-tale sign of iron deficiency.
• Iron deficiency is often caused by a pH issue, and cannot be absorbed in levels higher than 7.0 so check the pH of your medium.
• It is more common in coco and soil than in hydro grows.
• It is also commonly caused by an excess of phosphorus or by environmental stress.

Treatment:

• Flush your plant with pH neutral water. If your pH is normal at your next watering (between 6.0-6.5) and you still suspect iron deficiency, you can water with a supplement.
• Tap water typically contains enough iron for the plants, but those using reverse osmosis or highly filtered water will need to supplement.
• Unlike most other deficiencies, leaves affected by an iron deficiency may recover and return to a healthy green.
• Symptoms should improve within a week, so be patient as iron moves slowly through the plant.

Prevention:

• Check pH levels regularly.
• Keep the environment consistent.
• Use high-quality growing mediums and nutrients.
• Water regularly.

Leaf miners

“Leaf miner" is a term used to refer to the larval stage of many moths, sawflies, wasps, and beetles. These larvae can emerge at any time of the growing season, and their predecessors (pupae) can even survive through the winter.

Identification:

• Regardless of the species, these pests are referred to as “leaf miners" thanks to their signature eating style: look for pale yellow squiggles on the surface of leaves.
• Since they live inside the leaves, you probably won't see the insects themselves.

Treatment:

• Remove and dispose of affected leaves.
• Some organic pesticides — such as spinosad — may be more effective on some species than others.
• Growers often use neem oil for treatment.

Prevention:

• Parasitic wasps such as Diglyphus isaea work well for both treatment and prevention.
• Row covers and sticky traps are helpful in preventing adult insects from laying eggs and spreading the problem.

Leafhoppers

One simple way to think of leafhoppers is as smaller versions of grasshoppers. There are over 20,000 species of leafhoppers out there, but they all share some commonalities that should make them easy to identify and deal with. Like aphids and barnacle scales, leafhoppers suck the sap out of your plants' leaves. They often appear when the weather is dry and they are thirsty.

Identification:

• Leafhoppers can come in a variety of colors and patterns, but they will all look like mini-grasshoppers. They are generally about ¼" large and cannot jump more than an inch or two at a time. They are winged insects with six legs (two often hidden under the wings), two eyes, and a pair of antennae.
• Some species will cause the tips of leaves to die, or leave white specks on the surface of leaves.
• They hide under leaves and leave small puncture marks on leaves.

Treatment:

• Treat plants with organic insecticides like spinosad, essential oils like neem, or insecticidal soaps.

Prevention:

• Introduce predators like lady beetles, parasitic wasps, ladybugs, and lacewings.
• You can also use floating row covers and sticky traps to protect plants.

Magnesium deficiency

After the "big three" essential nutrients — nitrogen, potassium, and phosphorous — magnesium is the most crucial secondary nutrient for cannabis. A magnesium deficiency can stunt growth and even kill your plants. It can also be easily mistaken for other deficiencies, so be sure to give plants a thorough inspection if you suspect it.

Identification:

• Magnesium deficiencies are referred to as “bottom-up" since you'll first notice symptoms on the lower leaves.
• Leaves will yellow and leaf tips will be crunchy and brown and you may notice reddening in the stems.

Treatment:

• pH imbalance is often the cause of this deficiency, so check your medium's pH, flush plants with pH-neutral water, and check the pH of the runoff.
• Next, water with a solution containing cannabis-specific nutrients or a magnesium-specific supplement.
• New growth should return to healthy green within a few days.

Prevention:

• Most common in coco or in plants getting reverse osmosis water, so pay special attention when using these as you may need to supplement with extra magnesium.
• Overwatering can also cause this deficiency, so be sure to water plants properly.
• Plants tend to need more micronutrients during later stages of growth, so be sure to provide proper and balanced nutrition via a high-quality growing medium and proper cannabis-specific nutrient supplementation.

Manganese deficiency

Manganese is essential to chlorophyll production and photosynthesis. It also helps the plant absorb nitrogen and strengthens the root system during its development. Like all deficiencies, it can be tricky to distinguish a lack of manganese from other deficiencies, but there are a few specific things to look out for.

Identification:

• Look for yellowing that starts at the base of the leaves and moves its way out, along with mottled brown spots on the leaves.
• You'll see these signs in new growth at the top of the plant.
• Leaves may curl under.

Treatment:

• Often caused by a pH that is too high, flush plants with pH-neutral water and test the runoff.
• When you next water, include a light cannabis-specific feeding of nutrients.
• New growth should return to a healthy green within a week, but affected leaves will probably not recover and should be trimmed.

Prevention:

• Use a high-quality growing medium and cannabis-specific nutrients in a water solution that hits the proper pH (6.0-7.0 in soil, 5.5 to 6.5 in soilless).

Mealybugs

Mealybugs are mostly found in outdoor crops and can be a serious nuisance. Not only can they be mistaken for mold, but they also produce a sticky sweet substance called “honeydew" that can facilitate the growth of mold. You'll often find mealybugs in the presence of ants, who like their honeydew and protect them against predators.

Identification:

• Mealybugs most often attach to stems but can also be found on leaves. They leave bright sticky stains (the honeydew).
• Look for white oval and fluffy or moldy-looking clumps. You might also see a coating of white mold (sooty mold) that grows on the honeydew they excrete.

Treatment:

• Pressure spray plants to remove bugs, then treat them with insecticidal soaps, neem oil, or a spray made from nine parts water to one part rubbing alcohol.

Prevention:

• Introduce predatory wasps, lady beetles, ladybugs, and lacewings.
• Inspect plants regularly, especially in hot climates where they thrive.

Molybdenum deficiency

This is another nutrient deficiency that can be difficult to diagnose since it can present symptoms similar to more common deficiencies such as nitrogen, calcium, and magnesium deficiencies. Molybdenum is an important nutrient for converting nitrate to ammonium, which is important for the production of amino acids. A deficiency can also impact nitrogen use, which can have wide-ranging ripple effects on plant health.

Identification:

• While you may see common symptoms like yellow leaves that twist and curl, with this particular deficiency, you'll see yellowing start mid-plant and move upwards.
• Some plants will also show an orange, pink, or reddish discoloration at the edges of the leaves, which is a tell-tale sign of this deficiency.

Treatment:

• Often caused by a pH that is too low (acidic), so flush plants with a pH neutral 6.0 pH water and test the runoff.
• When you next water, include a light cannabis-specific feeding of nutrients.
• New growth should return to a healthy green within a week, though affected leaves will probably not recover and should be trimmed.

Prevention:

• Since Molybdenum deficiency is commonly caused by low pH levels (overly acidic soil), maintain a pH of 6.0 - 6.5 in soil and 5.5 - 6.0 in soilless mediums like coco and hydroponic grows.
• The amount of molybdenum in the soil can also decrease significantly in cold temperatures, so maintain moderate temperatures if growing indoors.
• If growing outdoors in cold climates, keep a lookout for this deficiency and supplement as needed.

Nitrogen toxicity or deficiency

Nitrogen is one of the “big three" essential nutrients for cannabis plants, crucial for photosynthesis, the growth of cells and tissues within the plant, and for chlorophyll formation. But there is a fine balance where too little nitrogen can impact growth and too much and cause a whole host of problems.

Identification:

• Too little nitrogen will cause bottom leaves to turn a pale yellow, wither, and eventually die. Too much nitrogen will cause leaves to become a darker green with tips turned downwards like claws.
• While it's possible to have a deficiency, toxicity is more commonly a problem among new growers who give their plants too much nitrogen. Plants need less nitrogen during the flowering stage than during vegetative growth.
• Yellowing leaves at the bottom of the plant during flowering is normal and doesn't need supplementation.

Treatment:

• Since nitrogen deficiency could be caused by low pH, flush plants with a neutral (6.0) pH water, and on the next watering, feed with a cannabis-specific nutrient and water solution.
• With toxicity, flush plants and cut down on the nutrients given.

Prevention:

• Use a high-quality growing medium and cannabis-specific nutrients in a water solution that hits the proper pH (6.0 - 7.0 in soil, 5.5 - 6.5 in soilless) as needed.

Phosphorus deficiency

Alongside nitrogen and potassium, phosphorus is another of the "big three" nutrients essential for cannabis health. Phosphorus is crucial for plant growth, photosynthesis, and energy transfer. This deficiency can be confused with copper, nitrogen, or sulfur deficiencies, so be diligent when diagnosing this and any other nutrient deficiency.

Identification:

• Look for leaves on the lower/older parts of the plant that have turned dark green or yellow with brown/bronze or slightly blue splotches.
• Leaves may also become stiff, thicker, and curl.
• The stems of the plant may turn bright red or purple.
• This deficiency is often seen alongside a calcium deficiency.

Treatment:

• This deficiency can arise when the pH is too low (acidic), so flush plants with a neutral (6.0) pH water.
• When you next water, feed with a cannabis-specific nutrient and water solution.
• It can also be caused by excessive rainfall or cold temperatures, so in this case, supplement with nutrients as needed.
• Symptoms should resolve within a week, though affected leaves will not recover and should be trimmed.

Prevention:

• Use a high-quality growing medium and cannabis-specific nutrients in a water solution that hits the proper pH (6.0 - 7.0 in soil, 5.5 - 6.5 in soilless) as needed.
• Keep temperatures above 60°F when possible.
• Compacted soil and overwatering can also trigger this deficiency, so use well-aerated soil and water only when the medium is dried out.

Potassium deficiency

One of the “big three" nutrients plants need to thrive, potassium plays many roles in the cannabis plant. It is needed for photosynthesis and plays a role in the synthesis of carbohydrates, proteins, and amino acids. It supports resiliency during droughts and strengthens plant tissue.

Identification:

• While you will likely start to see symptoms on lower/older leaves, they can also appear at the top of the plant. Leaves with a potassium deficiency can appear to have nutrient or light burn with brown or burnt edges and tips, but unlike a nutrient or light burn, you will also see yellow on the inside margins of the affected areas.

Treatment:

• Since this deficiency is often caused by a pH that is too high (alkaline), flush plants with a neutral pH (6.0) water. Follow up with a cannabis-friendly nutrient mix in a water solution.
• Symptoms should resolve within a few days, though affected leaves will not recover and should be trimmed.

Prevention:

• Potassium deficiency is more likely to happen when using heavily filtered or reverse osmosis (RO) water, so be sure to feed plants with cannabis nutrients appropriate for the growth phase.

Powdery mildew

Powdery mildew — also known as white powdery mildew/mold — is a parasitic fungus that is very common among cannabis growers and can usually be effectively treated when caught early, but without swift treatment, it can take a toll on your crop since it spreads quickly and easily.

Identification:

• Look for bright white and circular patches of flour-like dust on your cannabis leaves and stems.
• Typically, you'll find it on younger/upper leaves, which will eventually turn yellow and die.
• As the plant becomes covered in the fungus it will become increasingly disfigured.

Treatment:

• Discard any buds with powdery mildew as it is likely infiltrated deep into the bud and is unsafe for consumption.
• You can spray plants lightly but evenly with a milk/water solution — one part milk to three parts water — or with a solution of 1 tbsp of potassium bicarbonate, 1 tbsp vegetable oil, and 3 drops of liquid dish soap per 1 gallon of water.
• Because the spores spread so easily, it's also a good idea to disinfect the rest of grow room surfaces with a water and bicarbonate solution.

Prevention:

• High humidity and poor and unfiltered airflow are the main causes of powdery mildew, so be sure to maintain optimal humidity levels (about 40 - 60% depending on the growth phase), and have fans and proper ventilation for indoor grows.
• Introducing infected clones/plants is another common cause, so quarantine any clones or new plants for two weeks before introduction.

Rice root aphids

These pests recently became an increasing problem for cannabis growers and they are commonly found in other crops like cotton, foliage houseplants, and grass. Root aphids can be found in cannabis crops throughout the US and Canada and provide a significant threat to crops. They can be found in indoor and outdoor growing environments in addition to hydroponic and aeroponic systems.

Identification:

• Unlike general aphids and even the cannabis-specific Phorodon cannabis, root aphids live beneath the soil and feast on root systems.
• They are green and round like other common aphids but tend to be a darker green with shorter legs and antennae.
• An infected plant's leaves will yellow and growth will slow.
• If you have ruled out nutrient deficiencies and problems with pH you'll want to scout out the roots.
• You may also see winged adults in and around the soil in very large populations.

Treatment:

• Entomopathogenic nematodes are microscopic predators that kill root aphids, along with the fungal species Beauveria bassiana.
• Neem oil can be applied as a root drench.

Prevention:

• Introduce nematodes and Beauveria bassiana to the soil.
• Maintain clean growing conditions and check roots regularly for this common pest.

Slugs and snails

Slugs and snails are another common garden and commercial pest that can make an appearance in your cannabis grow, chomping away at your leaves and buds to their hearts' content.

Identification:

• A trail of silvery slime on leaves or on surfaces near your plants.
• Bite marks, which could be confused with caterpillars, but these bites will have distinguishable scalloped edges.
• They are long, soft-bodied creatures that have two large antennae.
• Slugs do not have a shell, whereas snails do.

Treatment:

• You can hunt them down, remove, and relocate them by hand in the early morning or evening when they are most active.
• Put beer traps into the soil around plants to attract and kill them, but be sure to clean these traps regularly.

Prevention:

• Encourage or introduce predators like frogs and birds.
• Create physical barriers around the plants with lime, eggshells, Diatomaceous Earth, or sawdust — all textures and surfaces that slugs and snails will not want to cross.

Spider mites

Spider mites are one of the most troublesome pests a cannabis grower might face, earning the nickname “the Borg". They are not insects but arachnids like spiders and are capable of spinning protective webs that can cover plants. They can strike indoor and outdoor grows, and reproduce prolifically, meaning they can quickly wreak havoc on your plants and severely impact your whole crop. They are resistant to chemical insecticides and often seem to be gone only to reappear later with a vengeance.

Identification:

• Their webbing is often the first sign of their presence.
• They are tiny and hard to see with the naked eye, so use a high-powered hand lens to identify them on leaves.
• They can be tan, green, or red and often have two dark spots on either side of their bodies.
• They also leave tiny white marks behind on leaves after feeding that may look like dusting.

Treatment:

• For small outbreaks, prune affected plants or completely discard heavily infested plants.
• Spray the remaining plants down with a mix of water and alcohol (9:1 ratio).
• Introduce natural predators such as ladybugs, lacewings, and predatory mites.
• You can also use organic insecticides and insecticidal soaps until the signs are gone.

Prevention:

• They like hot and dry climates so keep temperatures and humidity moderate.
• Fans and airflow will also help.
• Quarantine any new plants before introducing them to your grow and periodically bring in beneficial predators.

Thrips

It is estimated that there are more than 4,500 different species of thrips, which can be found across many different crops in home and commercial gardens alike. They are common pests, with the Frankliniella occidentalis thrip being the most common in cannabis. They are not considered to be the most dangerous cannabis pest, but they can be tough to get rid of once established, so prevention is key.

Identification:

• Young thrips are slow-moving worm-like insects. Adults are fast-moving winged insects, darker than their younger counterparts and slender, measuring about 1-1.5 mm in size.
• They are hard to see with the naked eye, so use a magnifying glass for identification.
• They are typically found in the mid and lower sections of the plants, often on the underside of leaves.
• They suck sap and moisture from leaves and will leave behind signature shiny silver or bronze spots.

Treatment:

• Treat plants with insecticidal soaps, neam oil, spinosad products, or pyrethrin-based insecticides every two to three days.
• Sprinkle topsoil with diatomaceous earth.
• Introduce predatory insects such as green lacewings, predatory mites, minute pirate bugs, and specific parasitic wasps (Thripobis simulteus, Ceranisus volumes, or Gotheana Shakesoearei).

Prevention:

• They are most common in indoor and greenhouse grows, so monitor closely in these environments.
• Sticky traps at the base of plants can help with detection.
• Introduce predatory insects periodically throughout the growing season.
• Remove unwanted weeds growing near cannabis plants.
• Row covers can protect plants from thrips.

Tobacco mosaic virus

While common in tobacco plants, this virus has been found in cannabis and other crops such as tomatoes, peppers, eggplant, spinach, and marigolds. It cannot spread to humans, but it can be spread by humans when handling infected plants or using contaminated tools. The virus stunts plant growth and reduces yields.

Identification:

• Leaves become twisted and curved and develop a tell-tale mottled and mosaic yellow pattern, hence the name.
• Leaves may also turn brown with burnt edges or develop yellow stripes.
• Stunted growth and small buds are other symptoms.

Treatment:

• There is no cure, and infected plants should be removed from the grow. It can survive on equipment, carpets, soil, and dead plant matter, making it hard to eliminate from a grow room.
• Plants may also be carriers without ever developing symptoms — the virus can also be present in pollen and seeds.
• Fully sanitize all grow room surfaces and tools before your next growth cycle.
• Wear disposable gloves or thoroughly wash your hands after handling infected plants.

Prevention:

• Since tobacco and cannabis products can be infected, do not use them around plants or wash your hands thoroughly after use.
• Be scrupulous about where you source clones.
• Grow from seed.

Whiteflies

Whiteflies are a common garden and agricultural pest around the world. In cannabis, you will most commonly find the greenhouse whitefly (Trialeurodes vaporariorum) in your grow, though there are over 1,500 different species. Whiteflies can be quite difficult to deal with since they are prolific breeders, which makes regular monitoring and prevention key for fighting them.

Identification:

• Very small (1 – 1.5mm long) white flies that may have two or four wings — the greenhouse species has four. Two eyes, two antennae, six legs, and a dusty appearance.
• Their feeding leaves have yellow or translucent marks on the surface.
• Like aphids and barnacles, they leave behind a thick and sticky substance called honeydew, which can encourage the development of sooty mold.
• Honeydew attracts ants, which you will see crawling on the plants. Ants also act as protection against whitefly predators.
• White eggs are generally laid on the underside of leaves.

Treatment:

• Prune and discard infected leaves, or completely discard heavily infected plants.
• Introduce predators such as ladybugs, lacewings, and predatory mites.
• Use a spinosad product, insecticidal soap, or neem oil to spray down plants. It might take several applications to eradicate them.

Prevention:

• Introduce predatory bugs frequently throughout the grow season.
• Whiteflies like a warm environment, so keep indoor temperatures between 68 – 77ºF (20 – 25ºC).
• They also like stagnant environments, so ensure proper ventilation and airflow in indoor spaces.
• Place sticky traps around the grow area for detection.

Yellow leaf spot

Also known as leaf Septoria, yellow leaf spot is a disease caused by a fungus called Septoria lycopersici. It often shows up as plants enter the flowering phase, and while it doesn't typically infect buds it can drastically impact yields if left untreated.

Identification:

• Uniform yellow and brown circular spots on leaves, starting at the bottom of the plant and moving upwards. The yellow spots turn brown over time.

Treatment:

• Prune and properly discard infected leaves and disinfect hands and any tools that may have come in contact with infected foliage. Consider removing some of the surrounding foliage as well.
• Add a layer of mulch to prevent the spread of spores from the topsoil to plants.
• When growing indoors, lower the humidity and temperature of the grow room.
• Broad-spectrum fungicides (especially copper-based) and disease-control sprays are typically effective. You can also use a spray made with neem oil and other essential oils such as eucalyptus, rosemary, lemon, and cinnamon.

Prevention:

• Particularly prevalent in areas affected by extended periods of wet, humid, and hot conditions.
• Maintain proper humidity in indoor spaces according to the growth phase.
• Avoid overwatering and allow the medium to dry out between waterings.
• Ensure proper ventilation and air circulation.
• Keep topsoil clean and free of debris.