Common Cannabis Fungus Problems


So you started growing indoors and are trying to identify a problem that you might think is some sort of nutrient deficiency. Well the chances are that your problem is being caused by fungus, likely Downy Mildew or Powdery Mildew shown below. They will appear on leaves at the top, middle, or bottom of the plant. Downy Mildew on Cannabis looks like yellow and then necrotic patches on the leaves, leaf tips and along their edges. Powdery Mildew on Cannabis looks like white powder on the upper sides of leaf surfaces. If you don’t identify these very common problems early, they will run through your garden and leave you with nothing. So what should you do now?

Downy Mildew


Powdery Mildew


Fungus and Disease Management

  1. Use a lamp in the 4k spectrum during vegetative growth. Using a lamp that is very red during vegetative growth will lead to leaf tissue weakness allowing fungus to easily penetrate cell walls and take over. Watch our grow videos by Harley Smith for a full explanation.

  3. Many fungus problems start in the soil. To be successful you have to ensure the best environment for the plant roots to thrive. Monitoring the soil Ph throughout the life cycle is critical as it will drift over time causing nutrient uptake issues. The second thing every grower should utilize in both organic and inert growing mediums is to utilize Mycorrhizae, preferably in a product that also contains helpful bacteria like Bacillus. This beneficial fungus and bacteria combo will attack unwanted pathogens in the soil preventing disease in your plants while also forming a symbiotic relationship with plant roots that will allow for better uptake and utilization of nutrients.

  5. Only use inert growing mediums! Use a mixture of Peat, Perlite, and powdered Lime. This mixture plus Mycorrhizae will start you off in the right direction and will save you lots of cash. Using soils will lead to failure indoors as they are filled with unwanted organisms/pathogens & fungus gnats.

  7. Only use RO (reverse osmosis) filtered water.

  9. Buy a HEPA air filter/purifier for the grow space. Every grow space should utilize a HEPA air purifier. It would be wise to buy a unit that is rated for at least twice the sqft of your space.

  11. Cleanliness is critical in an indoor grow room. Treat your space like a surgical room, don’t handle plants without gloves and don’t be tracking things inside on your shoes!

  13. Inspect your plants daily and take immediate action if you see something. Buy one of these cheap handheld magnifiers (shown on right), it will help shed some light on what you are dealing with, I got one at a dollar store a few years back…score. You can see the mycelium and fruiting bodies of the fungus that your eyes can not.

  15. Your humidity might be too high in your grow space and/or your air is stagnant. Even if you have fans blowing air in every direction, high humidity can bite you. Try to keep humidity around 30% if you are having a fungus issue. Grow room humidity must be managed and maintained at all times, letting it slip for an hour or 2 is enough of a nitch for fungus to invade. Buy a dehumidifier if you have to but keep in mind that they will generate some extra heat that you may have to adjust for. Air is an important variable when growing indoors, if you have it wrong then you will fail.

  17. Make sure the air in your grow space is continually exhausted and replenished. If you can exhaust/replenish the entire volume of air in your grow room in about 1 minute you are doing good. This isn’t an option, if you are not managing the air quality in your grow space you will have a chain reaction of problems that you will not be able to recover from.

  19. Make sure you are using oscillating fans in your grow space and that there are no dead zones.

  21. Do not spray water on the plants leaves (unless you are using a fungicide to address an existing issue). There are a few different kinds of Downy and Powdery Mildew and they can be living in your water, so when you think you are giving your plants a nice relaxing foliar spray you are actually introducing contaminants.

  23. If you are reading this article chances are that you already have a problem. Please do not use chemicals in an effort to control your situation. Instead I have found this foliar spray recipe far better than any harsh chemicals. Baking Soda – Mix 2 tablespoons baking soda into a gallon of water, add 1 tablespoon of vegetable oil and 1 teaspoon of dish detergent. The vegetable oil will help adhere the solution to the leaf surfaces and the dish detergent will help the solution spread out. For established plants make sure you spray both the top and bottom surfaces of all leaves once every 2 weeks, more than that can stress your plants out. Powdery Mildew is usually on the top and Downy Mildew on the bottom. I would not use this on seedlings. If you have fungus issues on seedlings then you need to fix your air problem and start over.

  25. And finally I would suggest that after this growing season is over that you bleach out your growing space. Then for your next crop look for a strain that is a bit more resilient to these 2 common Cannabis Fungi.
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