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Cannabis Leaf Septoria: Everything You Need To Know About this Fungus

October 7, 2022

It's very rewarding to cultivate marijuana from seed and see it bloom into a bountiful trichome-rich harvest. However, growing weed comes with its fair share of trials too. One such challenge is a possible infection by the dreaded cannabis leaf septoria.

Just the sight of yellow leaf spots is enough to put any grower into panic mode. The fungus can lie in wait for a long time, and when it hits, it spreads like wildfire. 

Carried by water and wind, it's almost impossible to get on top of the plant-destroying pathogen. Fortunately, there are methods and tactics you can use to identify, treat, and get rid of the disease. Read on to find out everything you need to know about the yellow leaf spot affliction and what to do about it. 

leaf septoria

What is cannabis leaf septoria (yellow leaf spot)

Cannabis leaf septoria or yellow leaf spot is a plant pathogen that attacks the leaves of marijuana plants. The cause is a fungus called Septoria lycopersici, which likes to stay dormant in old foliage and weed plants.

The infection spreads by wind and splashing water, such as rainfall. Usually, the first summer rains trigger the movement of spores. The first thing you notice is the yellow spots on cannabis leaves. Shortly after that, the blades turn brown on the surface and bottom

Even though it doesn't attack the actual flowers, the infection of the leaves and stems make the plant weaker.   

It's a quick-spreading disease and, in extreme cases, attacks the entire plant. This plague severely hampers vegetative growth, affecting the quality and quantity of the yield. If not treated, it's one of those cannabis pests that'll ultimately kill the crop. 

Leaf septoria life cycle

Septoria cannabis is a polycyclic disease that produces perithecium during the infection process. The disease develops asexually and continually infects new plants during the growing season

Cannabis leaf septoria has a standard life cycle of 15–18 days. However, it can remain dormant for up to 28 days with no yellow spots on marijuana leaves showing.

The disease starts with the production of ascospores in the perithecium. These allow the fungus to hibernate in dead foliage until triggered by moisture. Many growers experience recurring infections in their crops since the infectious spores lie hidden in the epidermal tissue layers of the previous season's leaves. 

When the summer rains start, the spores are awoken and carried by the wind to the nearest plant host. Once leaf septoria cannabis sets in, it spreads through the production of conidia in a second cycle. 

This process is accompanied by ascospores' production and pycnidium formation on the leaf lesions. This process is visible through the color change of the infection from yellow leaf spots to brown.

Even though septoria leaf spot cannabis has a set life cycle, the disease continues to spread each time the spores find a healthy plant. The only way to stop this cycle is to remove the infected crops and soil.

Symptoms of cannabis leaf septoria

The most prominent sign your crops have cannabis leaf septoria is the onset of yellow and brown spots. They're most visible on the leaves and are 1.5–6.5mm wide. The yellow leaf spots start light in color but get darker as the disease progresses. 

Some of the affected areas develop a reddish-brown border, which shows the plant tissue has died. Each spot indicates where a diseased spore landed and infected the crop. 

Fungal infections in marijuana plantations can mimic the symptoms of cannabis deficiencies. The best way to be sure your plants have cannabis leaf septoria is to keep an eye on the progression of the coloration from yellow to brown. 

To learn how to differentiate nutrient deficiencies and leaf septoria, check out our article about "calcium deficiency vs leaf septoria".

leaf septoria cannabis

Leaf septoria damage in weed plants

The damage weed mold can inflict on cannabis plants goes beyond yellow leaf spots. The disease attacks the lower leaves first, usually after the crops enter the flowering phase. 

It spreads quickly, moving upwards and causing light-colored patches to appear. The leaves turn yellow and then darken to brown before withering completely

Even though cannabis leaf septoria doesn't affect the flowers, it does plenty of damage to the rest of the plant. The destruction of large amounts of foliage creates stress for the bushes and stunts their growth

It also impacts the size of the yield and the quality of the harvest. This destructive fungus interferes with the photosynthesis process and ultimately weakens the structure of the plant. 

Cannabis leaf septoria isn't actually a deadly plant disease. However, if left to spread, it can degrade the plants to such an extent that it eventually kills them.

How to treat & get rid of leaf septoria in cannabis plants

The moment you notice any yellow spots on pot leaves, you need to act. There's a better chance of curing the disease if you treat the symptoms of leaf septoria early

Follow the steps below to combat the disease once you've confirmed that your crops have cannabis leaf septoria.

Remove infected leaves

The first step of leaf septoria treatment is to remove any infected foliage. This process should be relatively simple as you can easily see the yellow spots on cannabis leaves.

Once you've done that, don't throw the leaves onto a compost heap. It's better to dispose of them or, better yet, burn the whole lot

If the disease has reached your flowers, then skip this step. Removing leaves from the flowering areas weakens the plant and reduces bud quality.  

Pruning of fungal-diseased cannabis plants

Improve airflow 

Increasing the air circulation reduces the humidity levels and stops the cannabis leaf septoria from spreading. Improving the airflow is as simple as adding an extra fan if your crops are indoors. 

For outdoor crops, it's not as easy. Pruning is another way to help get more air around your plants. Fewer leaves mean more air between branches and smaller moisture pockets. 

If your outdoor plants are in pots, elevating them exposes them to more wind, keeping them dry. If all else fails, run an extension cord outside and get a fan to blow on the crops

Regulate moisture

Moisture is a mitigating factor in the appearance and spread of cannabis leaf septoria. Avoid overwatering your plants to prevent the spread of the fungus. 

Don’t use overhead watering systems as they wet the leaves and don't give the soil a chance to dry out. Use a watering can instead and dampen the crops early in the day, so they get a chance to air. 

Ideal Temp and Humidity for Grow Tents


Another useful leaf septoria treatment is to use a mulch to prevent the spread. The fungus can hide in the soil, where it's dormant until the rains start in summer. 

You can't do much to change your grow medium halfway through the season, but you can take steps to avoid further spread.  

Start by raking up dead plant matter and soil containing possibly infected foliage. The next step is to dry out the soil. Finally, place a layer of mulch on top and water the plants. This process helps to stop the spread of cannabis leaf septoria.  


Keep an eye on the weeds amongst your cannabis plants. Nightshade is an example of a leaf septoria weed. Another culprit to look out for is horsenettle. Perform a thorough check regularly and get rid of pesky parasite plants before they infect your entire crop

Temperature and humidity

If you have your crops growing indoors, keep an eye on the temperature and humidity levels. It's a common mistake to keep indoor setups warm and wet

If the environment becomes too tropical, it becomes a breeding ground for cannabis leaf septoria and other water-loving pathogens. 

how to treat leaf septoria

How to prevent leaf septoria in cannabis

As with most diseases and infections, prevention is better than cure. Use the following tips to keep the dangerous cannabis leaf septoria from bringing down your entire plantation.

  • Make sure your plants have ample air circulation. The better the airflow, the less the chance for high humidity levels, promoting fungal infections. Optimum ventilation can reduce moisture in the air by 50%. 
  • Water your plants in the early morning to give the soil a chance to dry during the day. Cannabis leaf septoria uses water molecules to spread, so keep the H2O minimum. 
  • Keep the temperature changes in your marijuana garden to a maximum difference of 5℃. Plants recover faster when there are smaller temperature fluctuations.
  • Make a homemade fungicide for plants as a natural leaf septoria treatment. Several recipes are available online, but we love a simple solution of 20% horsetail tea and neem oil. Spray the solution onto the bushes every fifteen days.

Cannabis leaf septoria: the colorful fungus

Fungal infections are a pain for marijuana cultivators, and cannabis leaf septoria is a sneaky one. 

The pathogen lies dormant in old foliage, and with the summer rains, it sends out its spores. Once they latch onto a healthy host plant, the disease spreads upwards.

Noticeable by its yellow and brown spots, you can treat it if caught early enough. Several methods exist to combat the infection, but the most important is keeping the humidity levels under control. Cannabis leaf septoria loves moisture and thrives in humid conditions. The best way to protect your plants is to be vigilant and deal with the infection before it spreads

It's not easy being a marijuana cultivator. Share this article with your weed-loving friends, so they can also find out how to deal with the dreaded yellow leaf spot. Be sure to keep an eye on our YouTube channel. We’re constantly adding new videos to maximize your cannabis growing efforts. 

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