Symptoms
Leaf Colour:
Edges Appear Brown or Burnt
Pale Colour Leaves
Yellow Leaves - New Growth
Yellowing Between Veins
Black or Gray Patches on Leaves
Brown or Dark Spots
Leaf Symptoms:
Upper Leaves / Newer Growth Affected
Small Inner Leaves Affected
All Leaves Seem Affected
Leaf Edges Appear Burnt
Yellowing Between Veins
Spots
Slow Growth
Twisted Growth
Abnormal Growth
Leaves Curl Under
Wilting / Drooping
Plant Symptoms:
Slow Growth
Twisted Growth
Leaves Curl Under
Plant Wilting / Drooping
Root Symptoms:
Slow Growing
Other Symptoms:
Bugs
Buds Not Fattening
Fungus gnats look like tiny flies buzzing around cannabis plants, especially around the soil. Their larva grows in wet soil, and they often appear when the topsoil stays wet for too long between waterings.
How to Get Rid of Fungus Gnats:
If you let the top inch or so of soil dry out before watering each time, they will go away on their own, but as long as you still have a fungus gnat problem, let the plant dry out even more between waterings to make it really tough for them to reproduce.
Need to get rid of gnats fast or have a really bad infestation? Take a multi-step approach:
1.) Blow air over soil to help the topsoil dry out and make it harder for gnats to fly around and reproduce.
2.) Treat top layer of soil with Neem oil (follow the directions and use a pressure sprayer/mister so it gets distributed evenly). Neem oil is safe to use up until a few days before harvest, though you want to avoid getting it on buds as it has an odd smell that can linger for a few days. Spray Neem oil anywhere you see fungus gnats, it should kill them almost instantly.
Note: Neem oil is great to have around the cannabis garden because it kills tons of different types of bugs, including Aphids, Barnacles, Broad mites & Russet mites, Caterpillars, cabbage loopers & inchworms, Crickets, Fungus gnats, Fungus/Mould (White Powdery Mildew, Bud Rot, Leaf Septoria etc), Grasshoppers, Leafhoppers, Leaf Miners, Mealybugs, Scales, Spider mites, Thrips and Whiteflies!
3.) Apply diatomaceous earth. Use a Powder Duster to apply an even layer of diatomaceous earth over the top of the soil and around the edges. You can also apply it around your room wherever you see bugs crawling around. This will kill both adults and larvae that touch it because it is sharp on a microscopic level, but it is completely safe for humans to touch or eat.
4.) Put out yellow sticky traps wherever you see tons of gnats, to both bring down their numbers and help you know when you've beaten them for good.
5.) Check daily. If you have an actual infestation you will likely need to re-apply the treatments for a few days until the bugs are completely gone.
6.) If you have a really bad infestation, and you're still noticing some fungus gnats after treating them for a few days, try switching to one of the other fungus gnat cures because they'll get more resistant to your original method over time, and a new insecticide will hit them harder.
More Cannabis-Safe Insecticides that Work on Fungus Gnats:
SM-90 - I like this option because it is also relatively good for your roots, so you can water your plants with it. Next time your plants get thirsty, water your plants with 1 part SM-90 to 5 parts water. I know the bottle doesn't say anything about bugs on it, but it will kill a whole lot of larvae directly. Make sure to still pH your water right before giving it to your plants. Don't do this more than once every week or two, though you can use a mister and spray the SM-90 mixture on the topsoil like you would with neem oil. SM-90 is natural and safe for humans (you can even eat it!), and like neem oil can be used basically right up until harvest. It has sort of a citrus scent that I personally find nice.
Essentria IC3 Insecticide is a mix of various horticultural oils that is organic and safe for humans. It is often marketed as a "bed bug killer" but is also very effective against fungus gnats when the plants are treated regularly. You want to apply this every other day or possibly combine with other options. You will need a mister (also called a "One-Hand Pressure Sprayer") to spray the soil evenly, and make sure to follow the directions! Like all the options, it's safe to use until a few days before harvest.
BT Bacteria - Another thing you can water your plants with is something that contains "BT" bacteria (specifically, Bacillus thuringiensis var israelensis - there is another type of BT that's meant for caterpillars), for example Mosquito Bits or Mosquito Dunks. However, they need to be crushed up into a powder so it will mix in the water for the best results when watering your plants. Just like any other time, always pH your water right before giving it to your plants!
Hydrogen Peroxide (food grade if possible) - If you don't have access to any of the products, you can start getting creative at home. Usually these types of remedies aren't as effective as the "real" stuff but they will definitely still kill a lot of bugs! Use 1 part hydrogen peroxide to 4 parts water (for 3% pure H2O2). This is just another type of brute force attack to kill larvae directly. I've also heard of growers doing this with 100% rubbing alcohol mixed with water.
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