Cannabis pests and diseases

Cannabis pests and diseases

Growing cannabis is generally very easy and straightforward. However from time to time growers may see their plants suffering from ill health. Sometimes these problems can be solved, but first you have to know what caused the problem.

In this weeks Dutch Passion blog we look at some of the more common cannabis pests and diseases facing the average home grower. We show you what they look like and what can be done to prevent the problem recurring.


Bud Rot. How to deal with cannabis plant mold

There are several different types of mold which can be a problem for cannabis. Bud rot (Botrytis) and powdery mildew are two of the most notorious. Bud rot is difficult to spot, usually starting in the centre of the cannabis bud on or near the stem. A damaged and infected area of the plant is especially prone to bud rot. Bud rot spores are all around us, literally. That’s why it is such a common problem. By the time you see bud rot it is already established. Once bud rot takes hold it spreads producing it’s own spores which will wait for the next opportunity to cause damage. The rot may look white and fluffy initially but turns grey, black and slimy in time.



Bud rot on the buds, now what?

Most growers chop off the affected branch and carefully remove it from the grow room. It’s not safe to smoke/vape rotten buds. The plant will continue to grow, but be extra observant in case another outbreak is seen. Some growers carefully peel open the occasional bud to inspect for rot.


How to prevent bud rot

Bud rot thrives in high humidity environment with mild temperatures and poor ventilation. Lots of leafy plants blooming in the same grow room will increase humidity. Any plants with a weak or damaged immune system will be pre-disposed to rot. Indica varieties with dense hard buds can be most susceptible to bud rot. These indica varieties which originate in dry mountain Asian climates are less well adapted to humid tropical conditions. Sativa varieties with lighter, less dense buds allow better air flow through the blooms and can be less prone to bud rot. Reducing humidity levels, with increased air extraction is a help. A lower plant/foliage density in the grow room is also a help.



Powdery mildew is another common cannabis mold. It usually appears as a white powdery mould which coats the leaves, inhibiting photosynthesis. It spreads quickly and damages plant health rapidly resulting in early plant death unless treated. Prevention is definitely better than cure. Non-powdery mildew produces yellow or translucent stains on the leaves. Mildew and white powdery mildew have similar causes to bud rot and are difficult to deal with once established. Reducing humidity, increasing ventilation and reducing plant density will all help prevent mildew. Traditional gardeners remedies for mildew including spraying with dilute 1% cider vinegar or a 50% milk solution. However, nether method is guaranteed to work. Many growers remove infected plant areas and continue growing with extra vigilance. Professional growers can go to great extremes to prevent mildew. Some growers treat incoming air with Ozone and UV light to kill all traces of spores which would otherwise affect their plants.




Whitefly usually colonise the underside of cannabis leaves, feeding on the plant sap. They spread and multiply quickly. Each white fly can lay 200 eggs. Most cannabis growers don’t like to use pesticides or chemicals on their plants, but there are other ways to get rid of white fly. Some growers try to use a water spray to wash/blast them off the underside of the leaves. Lady birds are a natural predator and are used by some growers. Use of sticky fly paper isn’t a cure, but many growers feel it is good practice to have a roll of sticky fly paper in their grow room. The sticky fly paper won’t eliminate a pest infestation, but it can give you advance warning of the presence of pests. Black fly and greenfly are aphid pests and are often treated in similar ways to whitefly.



Spider Mite

Spider Mites on cannabis plants

Spider mites can be one of the most damaging of cannabis pests. They are spiders rather than insect pests, with 8 legs. They are almost too small to see, but the wispy webs they build to protect their colonies and eggs are the easiest way to spot them. Each generation takes less than a week to hatch and start reproducing, so a rapid response is vital. As with white fly, a violent water spraying will dislodge them from their beloved shelter underneath the leaves. Lady birds are a natural predator. Spider mites dislike the cold, so a temperature reduction to nearer 20ºC may slow them down. And increased extraction/ventilation may make life more difficult for them. Some growers find that the low energy hand held ‘baby’ vacuum cleaners can suck off the predators. Though this technique requires real skill and is time consuming even for one plant. A 10% alcohol (ethanol) spray is an effective option. Grow shops do sell chemical remedies, though not all growers feel comfortable using chemicals on their crops. Again, prevention is better than cure


Rust Cannabis


Several fungal infections can produce brown ‘rust’ colored patches on leaves. Chemicals are great at controlling fungal infections but are rarely used by the responsible cannabis grower. Rust can find it easier to infect wet leaves, so ensure that ventilation is good and plants have space to grow without growing into each other. Most growers will remove the damaged leaves and try to complete the harvest. If the rust is not severe that should be possible. All old leaf material should be always removed from the grow room.


Prevention is better than cure

Maintaining a clean grow room is vital. Always clean your grow room between each grow. Remove waste/old foliage and traces of old soil or growing medium. Wipe down the surfaces of the grow room or tent. Many growers uses a dilute solution of disinfectant, some use diluted Hydrogen Peroxide (often used by hydroponic growers as a nutrient additive). Ensure your extraction fan is adequate for the size of your grow room. If necessary, get a bigger fan. Inspect your plants regularly looking for the first hints of disease or pests. Remember that many pests and diseases are introduced accidentally when plants/cuttings from other growers are brought into your grow room.

Typical reasons for pests and disease

Getting cuttings/plants from other people can often be the accidental cause of new pests and disease in your grow room. Be careful about accepting plants from others unless you are really confident about the grower. Sometimes people will take plants from their grow rooms and put them outside on a sunny day for some real sun. But this can allow pests like greenfly or whitefly to hijack a ride back to your grow room.

Benefits of growing from cannabis seed rather than cuttings

One of the best ways to ensure a clean grow room is to grow from feminized cannabis seeds rather than continually importing plants/cuttings from an external source. This eliminates the chance of introducing plants that are already carrying pests or diseases. It’s easy to buy cannabis seeds online. And you have a full choice of autoflower seeds or photoperiod feminized seeds to select from. Outdoor cannabis seeds are a great way to grow your own. And CBD rich seeds are a popular choice with medical growers. Good housekeeping is essential for the serious cannabis home grower. Keeping your grow room spotlessly clean, free of water spills, nutrients spills, plant waste and rubbish is a good way to make life difficult for pests and disease. If you have had a bad experience with pests (e.g. fungus gnats) from a particular brand of soil or coco fibre then try looking for a better brand next time. Cleaning your grow room carefully after each crop, and especially after pests/disease is a hassle, but well worth it.

Cannabis pests and diseases
March 13th 2019
Categories : Cannabis Plant

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