Grow Guide: Bugs and Mold


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Bugs and mold are two of the biggest obstacles facing growers today. They can turn a perfect room into a disaster in the blink of an eye. Knowing the first signs of pests and mold will put you a step ahead and help you prevent infestations before they get out of control.

Leaf discoloration or abnormality is the first sign of a bug infestation. This is also the first sign of a nutrient deficiency so if you suspect pests are causing stress in your plants begin searching for the culprits.

Many pests are visible with the naked eye but in recent years we’ve seen a flux of new-age mites such as Broad mites and Russet mites. These nasty and hard-to-treat mites were developed and released by the CIA to control “noxious weeds” on the sides of highways but are seemingly a match made in heaven for cannabis since they are attracted to the fragrant, sticky resin. These new mites are so small that they cannot be identified with the naked eye. Finding these destructive bugs requires a light scope, like this KindScope.

Leaf contortion (twisting) is one of the first signs of Broad or Russet mites. If you notice your leaves acting funky, grab a scope and check out the underside of the leaves. Another more obvious and severe sign of broad and/or russet mites is when a single finger of a fan leaf breaks in half. These mites like to burrow into the vein of the leaf causing it to curl or bend in half. Predatory mites, such as Western mites (Typhlodromus occidentalis or Mesoseiulus occidentalis) can help remedy a russet and/or broad mite infestation.

Spider mites like to hang out on the underside of leaves. Photo courtesy of Allie Beckett.

Spider mites have arguably caused the biggest problems for growers in the last decade (before Russets and Broads came along).

White spots on fan leaves are the first sign of spider mites. Photo courtesy of Allie Beckett.

White spots on fan leaves are the first sign of spider mites. Photo courtesy of Allie Beckett.

They are small, quick to replicate, and difficult to eliminate without harmful chemicals. The first sign of spider mites is white spots and discoloration on the fan leaves. Spider mites are very small but if you flip the leaf over you will be able to see small dots scurrying around underneath.

If left unnoticed and untreated, spider mites can completely devastate a crop. They will begin spinning webs around the fan leaves and eventually the buds – as shown in the horrifying image below. It should never get this bad if you can spot the signs early, but you will be surprised at how fast these mites can take over a room. If spotted during the white leaf phase, immediately remove and discard the infected leaves. Spider mites can be warded off by incorporating predatory mites or ladybugs into your grow who will kill and eat the spider mites. More traditional remedies include spraying neem oil or commercial sprays such as AzaMax.


Spider mite infestation on flowering plants. Unfortunately, these plants are long past repair. Photo courtesy of Allie Beckett.

Thrips are small winged insects that poke holes in the fan leaves to feed of the juices inside. The first sign of thrips will be shiny bronze or silver spots on your leaves, followed by brown leaf discoloration and a brittle, dry texture. Thrips are tiny but you will be able to see them moving around the leaves. Unlike mites, thrips can be shaken off your plant by literally grabbing a branch and shaking it around. But for a more permanent solution I recommend introducing beneficial insects such as parasitic wasps and predatory mites. And if all else fails you can always turn to dousing your plants with a neem oil spray.


Sign of thrips on fan leaves. Photo courtesy of

Fungus gnats are another common pest for cannabis growers. They are tiny flies that thrive in damp soil. Fungus gnats don’t attack cannabis leaves but the damage they cause in the soil will prompt the fan leaves to wilt and turn yellow with brown discolorations. While these pests don’t directly feed on roots, the larvae chomp on the delicate root hairs which can cause serious damage. The adult gnats can also spread diseases that cause root rot. You will know you have gnats if you see any low-flying bugs in your soil. Fungus gnats typically appear in grows that are over-watered, so a simple remedy is to let the first inch of top soil dry out between each watering. Placing yellow sticky traps nearby will also help eradicate them. Fungus gnats can climb up your plants or fly around and get stuck in your buds but this is easy to prevent by rubbing tanglefoot around the base of the stalk – the gnats get stuck in the stickiness and cannot climb any further.


Fan leaf damage caused by Fungus Gnats. Photo courtesy of

Russet mites, Broad mites, spider mites, thrips and fungus gnats are the five primary pests to look out for as a cannabis grower. Typically, these pests can be spotted and treated early on, preventing reparable damage. However, it’s time to move on to a more stealthy cannabis killer: mold.

Powdery Mildew infestation on fan leaves. Photo courtesy of Allie Beckett.

Powdery Mildew infestation on fan leaves. Photo courtesy of Allie Beckett.

Mold can be difficult to detect and is often spotted too late. Powdery mildew is a fungal disease that is quite easy to spot; it looks like powdered sugar or flour sprinkled on the fan leaves. Powdery mildew thrives in high humidity climates so a simple way to prevent this fast-spreading mildew is by keeping the humidity under 55%. If spotted while the plants are in their vegetation stage, powdery mildew can be treated by burning sulfur in the room or spraying sulfur directly on the vegging plants. If spotted in flower, powdery mildew can be more difficult to treat because it’s advised not to spray anything on plants developing flowers. However, desperate times call for desperate measures and there are organic sprays that can treat low-level powdery mildew infestations such as hydrogen peroxide, potassium bicarbonate, or a blend of diluted essential oils like peppermint and rosemary.

Advanced stage of botrytis (bud rot). Photo courtesy of Allie Beckett.

Advanced stage of botrytis (bud rot). Photo courtesy of Allie Beckett.

Botrytis or gray mold is one of the most devastating cannabis diseases. It is extremely hard to detect and spreads in the blink of an eye. Botrytis is a pathogenic fungus that is usually caused by a humidity issue, so making sure your room is always less than 55% humidity will keep it at bay. It’s best to avoid big temperature differentials from day to night as a change in temperature will create beads of moisture that get trapped inside your buds. This will turn the inside of your flowers into a breeding ground for this type of mold. Good airflow and low humidity are key for avoiding mold contamination. Botrytis requires a food source before it invades, so it often begins by attacking wounded leaves or dying plant tissue. Throughout the growing cycle, carefully look out for any damaged stalks, leaves, or flowers that could be vulnerable to botrytis. Botrytis first appears as white growth but will quickly turn into a gray mold, and the spores are easily spread by the wind or in water. There are no quick fixes for botrytis, so it is crucial to take the necessary preventative steps.

Botrytis or Gray Mold on dried cannabis flowers. Photo courtesy of Allie Beckett.

Botrytis or Gray Mold on dried cannabis flowers. Photo courtesy of Allie Beckett.

Preventing bugs and mold is all about being observant and in tune with your plants. Cannabis should always have dark green leaves that point upwards towards their source of light – if they are doing otherwise then something is wrong, and since they can’t help themselves it’s up to you to bring them back to health. It may seem like common sense but weak, deficient plants are more prone to pests disease while strong, healthy plants with high brix are more resistant to pests and disease. Dr. Carey Reams taught that “All disease is the result of a mineral deficiency,” so keeping your plants healthy and happy is the number one step for preventing devastating infestations. Good luck and most importantly, have fun (the plants feel your energy)!  

I’d love to hear your experiences in the comment sections below!

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