Common Cannabis Diseases & Pests

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Like any plant, cannabis is susceptible to pests and diseases. In some cases, the symptoms of these irritants and infections are not visible until it is too late to save the affected plant. That’s why it is so important that the grower be able to identify cannabis pests and diseases as soon it is visible – that ability gives him or her best chance at saving the plant, or at least the rest of the crop.


Bud Rot

Bud rot, also known as Grey Mold, acts quickly and can develop, spread, and ruin an entire cannabis crop in just a few days. Bud rot begins when the humidity is too high during lower temperatures (such as the rainy season of late summer). This is one reason the humidity should be lowered, if possible, during the cannabis plant’s flowering stage.

Poor ventilation in the grow room, or close growth that prevents air circulation, can encourage bud rot. The small leaves that protrude from the bud site, or cola, of the cannabis plant may begin to wither. The clump of mold that develops around the dead leaf quickly spreads to the rest of the cannabis flower, and then the other flowers on the plant.

If you notice bud rot on your cannabis plant, carefully remove the area that is infected, and up to one inch below it. Be gentle with the moldy section, as the lightweight spores can easily take to the air and infect surrounding flowers and plants. Once the infected section has been removed, the flowers should be checked daily for contaminated areas.

Root Rot

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Root rot leaves your plants droopy and malnourished.

Root rot can occur when the temperature in the grow room or garden is very high, and the roots of the cannabis plant are not receiving enough oxygen. This infection is most common to cannabis plants grown using hydroponics, especially Deep Water Culture, a method in which roots are permanently submerged in water. Root rot can also affect roots planted in soil that is too heavy to allow enough air circulation.

Root rot can be identified by droopy cannabis tops, and you may notice the plants have ceased to grow because the roots are no longer delivering adequate nutrients. The cannabis leaves can begin to yellow, a common sign of nutrient deficiency. The roots themselves will be stinky, slimy, and brown in colour.

There are store-bought remedies and home solutions that may help combat root rot in cannabis plants, such as hydrogen peroxide. The success rates of these remedies vary. A plant root supplement may also help, at least as a preventative measure. Most importantly, cannabis gardeners should ensure the soil in which their plants are planted is light and loose, with good drainage, and that hydroponic water baths have sufficient air rocks or other equipment that introduces oxygen to the water.

Read More: Learn About The Parts of The Cannabis Plant

Tobacco Mosaic Virus

The Tobacco Mosaic Virus affects, not surprisingly, tobacco plants as well as tomatoes, petunias, and cannabis. Mottled dark and light green sections to develop on the leaves, and less-hardy plants may become stunted, with curled and discoloured leaves.

If the virus attacks your cannabis plants, they will not die, but their yield will be greatly compromised. Once a plant is infected, it cannot be saved and should be removed immediately to avoid spreading the virus.

To prevent tobacco mosaic virus, don’t use tobacco products while working around the cannabis plants in your garden or grow room, as any residue can transfer to the growing plants. Also be sure to wash your hands well between handling tobacco and the cannabis plants.

White Powdery Mildew/Mold

White powdery mildew or mold can easily develop on cannabis that is exposed to high humidity and too little circulation, much like bud rot. When leafy plants grow so close together that their leaves touch and overlap, the mildew can develop at these intersections.

Mildew on cannabis leaves can be carefully wiped away with plain water. If found once, it is important the leaves be checked regularly to make sure the mold does not spread or begin elsewhere in the grow room.

To prevent the growth of mildew, prune away any leaves that do not serve a purpose in the flowering of the cannabis plants, allowing more space for air flow. Placing oscillating fans around the room can also help circulate the air through the plants. If the humidity in the room is too high, a dehumidifier may come in handy.


Spider Mites

These tiny spiders can quickly infest your cannabis plants, leaving behind damaged leaves with brown and yellow dots and a burnt appearance. If the infestation grows large enough, you may see webbing encasing the leaves as well.

In the case of outdoor cannabis gardens, a few spider mites will likely be taken care of by natural protectors like ladybugs and beetles, but an organic insecticide is often required for larger infestations, and can also be used as a preventative measure. Diluted detergent is a non-toxic way to keep these pests off your cannabis plants. The mixture can be poured into a spray bottle and applied to the both sides of the leaves.

Fungus Gnats

It isn’t uncommon to see little black flies around the soil of your cannabis plants. They may not be harmful in small quantities, but their larvae can lie unseen in the soil and eat away at the roots, causing nutrient deficiencies and wilted leaves.

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An infestation of aphids isn’t a pretty thing

An infestation of aphids isn’t a pretty thing

Gnats appear when there is too much stagnant water in the grow room or garden, and poor air circulation. By letting the top inch of soil dry completely between waterings, you can prevent these pests and even get rid of an existing infestation. If the soil is not drying quickly enough, placing fans around the room to blow across the soil can speed up the process.


Aphids can sometimes be found on the undersides of the leaves and stem of cannabis plants, feeding on the sap within. Leaves may wilt or become yellowed, and a fungus that grows on the aphid waste can turn the plant black.

The presence of aphids should be caught early, and a serious infestation can be prevented by careful examination of the plants at regular intervals. If a few aphids are discovered, an insecticide soap should take care of them and be no detriment to the cannabis plants.

Fungicides can help prevent some diseases in cannabis plants, but should not be used once the flowers have developed – the residue can be very harmful if inhaled when smoking your medical cannabis.

Last Tip: Looking for a natural way to keep pests away from your cannabis plants? A spray made from pureed and strained onion and garlic spritzed lightly on the plant leaves can deter some bugs.

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