Alternaria and Cannabis Plants

Learn How to Spot This Common Fungi

If you’re a cannabis grower, then the word fungi should send a chill through your spine.

Fungi are primary pathogens that prey on cannabis plants, and once your plants contract it — it’s game over. One of the most common fungi to affect cannabis crops is Alternaria, a fungus that’s responsible for the dreaded Alternaria leaf spot disease.

Read along to understand what Alternaria is, how it affects cannabis plants, and how you can prevent it from taking over your garden.

Alternaria and Cannabis Plants
Chapter 1

What is Alternaria?

Alternaria is a genus of fungus that’s well-known to cause the Alternaria leaf spot disease on plants.

Alternaria leaf spot affects over 350 species of plants, which includes Cannabis sativa.

What is Alternaria?

Alternaria flourishes in specific conditions, which enhance its ability to spread and infect plants. Typically, moist environments are the primary condition that Alternaria enjoys.

The Alternaria fungus resides in multiple places, whether it’s actively infecting a plant or overwintering for the upcoming season. In cannabis plants, Alternaria is found within stems, seeds, and seedlings. Outside of cannabis plants, the Alternaria fungus is found in the soil and on dead plant material.

It’s essential that you understand that Alternaria is not spread by insects, but instead, via infected soil. As the fungus matures, it releases spores, which live on to continue its infectious lifecycle.

Not only is Alternaria dangerous for your cannabis plants, but it’s also harmful to humans. Alternaria is known to cause respiratory issues, such as asthma.

Why Is Alternaria Dangerous For Cannabis Plants?

Think of cannabis leaves as the engine to the entire plant. Cannabis leaves are the primary source of energy via photosynthesis. Without leaves, the cannabis plant won’t flourish, let alone flower.

Alternaria leaf spot goes after the leaves of cannabis plants and quickly turns the lush green color into yellow, brown, and black spots.

As the infection spreads, the leaves undergo necrosis. As leaf necrosis continues, the leaves from the affected cannabis plant will defoliate, and plant growth will completely stop.

As growth stops, flowering will not occur. In other words, all of your labor is for nothing. Alternaria and Alternaria leaf spot disease is every cannabis cultivators’ nightmare. Sadly, the Alternaria leaf spot is systemic, which means that it’s inside the plant.

Chapter 2

How to Spot & Prevent Alternaria Leaf Spot Disease

As you inspect your cannabis crops, you may stumble upon a leaf that looks as though it’s undergoing a mild nutrient deficiency, such as magnesium or calcium.

The telltale sign of Alternaria leaf spot is necrotic spots on leaves, slow growth, nutrient deficiencies.

If you notice any of these three symptoms, you must consider an Alternaria leaf spot infection.

How to Spot & Prevent Alternaria Leaf Spot Disease

How to Prevent Alternaria Leaf Spot Disease

The best way to combat the Alternaria leaf spot on your cannabis plants is by not getting it in the first place. Prevention is always a better alternative to last-minute efforts to fight a full-on outbreak.

Here’s a list of the best preventative measures to prevent Alternaria fungus and Alternaria leaf spot disease.

Maintain Low Humidity Levels

Since Alternaria flourishes in moist environments, you must keep your cannabis garden dry. The best methods to accomplish this is by:

  • Incorporate multiple oscillating fans in the grow room
  • Utilize inline fans to bring in the fresh air and exhaust hot air
  • Remove low growth near the soil line (lollipop method)
  • Never overwater
  • Utilize a dehumidifier
  • Clean leaf litter and clean all water droplets on the ground
  • Space your plants adequately

Use New Soil After Each Harvest

Although it’s more economical and sustainable to re-use soil, it may be best to use new soil to prevent Alternaria leaf spot disease.

If you purchase potting soil, always make sure that the soil is coming from a reputable dealer that guarantees that the soil has been tested for harmful pathogens.

Use Mulch

Since Alternaria spores are found primarily in the soil, adding a layer of mulch is helpful to keep the spores tucked away from your cannabis plants. Not only does this help prevent Alternaria leaf spot disease, but it also decreases your cannabis crops’ water needs.

How About Using Fungicides on Alternaria?

Organic fungicides, such as copper spray and Captan, are nearly ineffective when dealing with Alternaria leaf spot disease. You should never apply conventional fungicides to your cannabis plants because you (or someone else) will eventually consume them.

If all of your preventative techniques fail, then you must destroy your cannabis crop and restart. If you restart, everything must be sanitized to ensure that no spore is left standing in your grow room.


If you want to have the best cannabis garden imaginable — you need to prevent all pathogens, especially fungi. Alternaria is a lethal fungus that will not stop until your cannabis crop is destroyed. By following these helpful tips, you can cultivate an Alternaria-free cannabis garden.

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