What is Cannabis Leaf Septoria and How to Treat and Prevent It?

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Added 10 November 2022

When growing cannabis, you need to be prepared for anything. For a few growers, it is the risk of a SWAT raid at their house, but for most growers, it is simply common diseases and infections that may affect their plants. 

One such infection that can latch onto your cannabis plants is cannabis leaf septoria — a notorious fungal infection that can be a pain to deal with. And if left unchecked, it can even kill your plant. 

So, what is leaf septoria, how can you treat it and prevent it? Learn all about leaf septoria, its life cycle, how it affects cannabis, and its treatment in this article.  

What is Leaf Septoria in Cannabis?

leaf septoria in cannabis

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Cannabis leaf septoria is a problem that plagues many cannabis gardens around the world. It is a plant pathogen — a type of fungus — that is identifiable by yellow spots on the leaves of cannabis plants. This is caused due to Septoria lycopersici — a fungus that tends to stay dormant in old plant foliage of cannabis plants. 

Usually, this infection is spread via splashing water and wind, most commonly during rainfall, which is why you will hear about most leaf septoria cases during the first rains of summer. Such instances trigger spore movement, making the infection worse. But it can even spread due to pests or human carriers. 

During the early stages of the infection, the leaves of your cannabis plant will start developing yellow spots. Shortly after, the leaf blades will start turning brown, too. And while this infection does not affect the buds directly, it can still make your plant weak, leading to lower yields.

But this is not where it stops. Leaf septoria in cannabis starts slow but spreads quickly, and in severe cases, it can affect the plant entirely. It can affect the plant’s growth, quality of yield, and a lot more, and if unchecked in time, it can even kill your plant. More on this later.

How Does Leaf Septoria Affect Cannabis Plants?

Leaf septoria on marijuana plants

Leaf septoria is a fungal infection that begins on the lower tiers of the plant first, where the leaves start developing yellow spots. This usually occurs during the early stages of the flowering stage. 

Once it takes hold of the lower leaves, it starts moving upwards quickly, leading to various light-colored patches on the higher leaves, too. The affected leaves soon turn yellow and, if not treated in time, turn brown and wither completely.

Leaf septoria in cannabis goes beyond just yellow leaves — it damages the rest of the plant in many ways. For instance, when this infection destroys the foliage, it stresses the plant and stunts its growth. It also affects the photosynthetic processes of the plant, which weakens it further and makes the plant structurally frail.

But leaf septoria is not necessarily fatal to cannabis plants unless you ignore it or fail to treat it in time. In such cases, if it is left to spread, it can eventually kill your plant. But there are many ways to save your plant, as you will learn in the later sections of the article.

What is Cannabis Leaf Septoria’s Life Cycle?

spots on cannabis

Before getting into treating leaf septoria, it is essential to learn the infection’s life cycle to gain a better insight into how it affects the plant. 

Leaf septoria in cannabis is a polycyclic infection, which produces perithecium when it infects the plant. It is an asexual infection but can continually infect new plants in the same garden during a growing season.

Generally, leaf septoria in cannabis has a life cycle of around 15 to 18 days, but it usually tends to stay dormant for up to 28 days before showing any signs of infection on the plant, like the yellowing of the leaves. 

When the infection starts acting up, it produces ascospores in the perithecium, which allows the infectious fungus to hibernate in the old foliage of the plant. When this fungus receives the right amount of moisture, it gets triggered.

Once the first rainfall occurs in early summer, the spores are activated and carried by air to other plants nearby. And when it sets in in cannabis plants, it starts spreading quickly via conidia production in a second cycle. 

This process occurs simultaneously with ascospores production and pycnidium formation on the lesions on the leaves. This process causes the yellow spots on the leaves to turn brown and darken. 

For many growers, getting rid of leaf septoria is harder because of the same reason — the infectious spores hide in the epidermal tissues of old leaves, and get transferred to new plants in the following season.

Leaf septoria has a predictable and short life cycle, but it is usually difficult to deal with for cannabis growers because it keeps spreading every time a spore finds a new host plant. The only effective way to stop the life cycle is to discard all the infected crops and soil. Next, you need to remove all the spores in the plants and the entire garden (as much as you can see), using various methods as listed below.

How to Identify Leaf Septoria in Cannabis?

leaf spots on cannabis

As mentioned above, the first sign of leaf septoria is the presence of yellow spots on the cannabis leaves which turn brown after some time. These spots tend to be 1.5 to 6.5 mm wide and start with a light shade of yellow but get darker as the infection progresses. 

Each spot is a location where a diseased spore landed, so the more spots you notice, the more spores have infected your plant. 

Additionally, some crops’ leaves may also develop brown borders — a sign that the plant tissue is dying. 

But identifying leaf septoria in cannabis can be tricky as it is quite similar to other nutrient deficiencies, especially calcium deficiencies. 

Here are the four main differences between cannabis leaf septoria and calcium deficiency:

  • Look at the Color of the Leaves

In leaf septoria, the leaves develop yellow spots that turn brown over time. But during calcium deficiency, the base of the leaves will develop a yellowish-green tint between the veins that become reddish-brown over time and the leaves also curl up.

  • Check the Location of the Spots

If the yellow spots are on the bottom leaves, it is most likely leaf septoria since this infection starts at the bottom of the plant. Calcium deficiency symptoms can be seen across the canopy from day one.

  • Check the Branch Strength

If the branches are weak along with spots on the leaves, it is likely due to calcium deficiency and not leaf septoria. In the latter condition, branches weaken when the disease has severed. 

  • Check the pH Levels

Check the pH of the water for your cannabis plants. If the pH levels are normal, it is likely leaf septoria, but if the pH is lower than recommended, it is calcium deficiency. 

How to Treat Leaf Septoria in Cannabis?

how to treat leaf septoria in cannabis

Once you spot any yellow spots on your cannabis leaves, you can be sure it is leaf septoria, unless there are any other signs of nutrient deficiencies. And now, time is of the essence because you need to act quickly before it starts affecting your plant enough to reduce the yield. 

Plus, it is easier to treat the infection if you act immediately — time is crucial here. The longer it takes to treat leaf septoria in cannabis, the more difficult it will be.

Here are the steps you need to take to treat leaf septoria in cannabis.

1. Remove All Infected Leaves from the Plant

The first step to treat leaf septoria in cannabis is to remove all infected leaves from the plant — any leaf that has yellow spots on it. This process is straightforward. Go through the leaves on your plant and prune the leaves that appear infected. 

When doing so, it is recommended that you use sharp, disinfected pruning scissors to protect your plant from excess damage. 

Once you have cut off all the infected leaves of cannabis, you need to dispose of them properly. Do not use it in your compost as the spores can travel to other plants. Ideally, consider burning the leaves. 

However, you can skip this step if the infection has reached the buds as cutting off leaves near the buds can weaken the plant and affect the bud quality.

2. Improve Airflow in Your Garden

The next step is to improve the airflow in your garden or grow room — airflow is crucial for cannabis plants and it also helps manage other pests and diseases. 

If you are growing cannabis indoors, you can improve air circulation within the room by simply adding an extra fan and spacing out your plants. When adding a fan, ensure you don’t point it directly at the plant as it will dry out the plant. Instead, use an oscillating fan so it can circulate the air properly.

Additionally, check the vents in your grow room — ensure they are clean and free of any obstruction. 

On the other hand, if you are growing cannabis outdoors, improving airflow can be a bit difficult. In that case, you can consider pruning your plant to improve airflow. Trim down dense parts of the plant and try to not have any leaves touching each other.

If there are any tall obstructions around your plant, consider removing them as they may be stopping the flow of air. You can also elevate your plant if you are using a container, so they can catch more air. 

Lastly, you can always try to keep a fan outdoors, too. However, ensure the fan is placed properly and there is no forecast of rain — electric shocks are no fun to deal with.

3. Regulate Moisture

Similar to the previous point, you should also try to regulate moisture since it is one of the leading causes of fungal infections in cannabis plants, and leaf septoria is no exception. By regulating moisture, you can limit the spread of this infection to a great extent.

The first step in regulating moisture is to prevent overwatering the plant. Only water as required. Ideally, water the plant when the top inch of the soil is dry. In addition, make sure there's adequate runoff so the plant doesn't suffer from underwatering. 

Also, avoid overhead watering your cannabis plant as it can dampen the leaves and create various issues. Instead, use a basic watering can to water the soil early in the morning. 

4. Maintain the Temperature and Relative Humidity

Many growers make the mistake of keeping their grow rooms slightly warm and wet to mimic the outdoor conditions. They don't realize that a tropical climate is the breeding ground for fungi like cannabis leaf septoria and various other pathogens.

If you are doing this, you need to change the temperature and relative humidity in the grow room.

For this, you need to factor in the strain you are using, your local climate, and light conditions, and set the temperature and relative humidity accordingly. Ideally, the temperature and relative humidity for cannabis plants must be:

  • Seedling stage: 68°F to 77°F (20°C to 25°C) with 65% to 70% RH
  • Vegetative stage: 72°F to 82°F (22°C to 28°C) with 40% to 70% RH
  • Flowering stage: around 73°F or 22°C with 35% to 40% RH

5. Mulch the Growing Medium

Many growers have also found mulch to be quite effective at stopping the spread of leaf septoria. This is because usually, the fungus stays dormant in the soil until it starts raining. 

While you can’t change your medium halfway through your plant’s growth in most cases, you can use mulch to prevent the spread of fungal spores. 

To mulch your soil, rake up all the dead and old plant matter and let the soil dry out for some time. Then, place a layer of mulch on top and resume watering your plant as usual. This will prevent the spores of leaf septoria from traveling upwards and infecting your plant. 

However, note that this is not a final treatment because you are not solving the problem at its core — this is a makeshift fix that helps manage the issue until the end of the season. 

6. De-weed Your Garden

No, we don’t mean cut down your cannabis plants. Essentially, you need to remove any parasitic weed plants from your cannabis garden. Focus especially on weeds like nightshade and horsenettle as they are notorious for harboring fungi like leaf septoria. 

7. For Severe Leaf Septoria: Apply Natural Oils or Fungicides

In minor cases of cannabis leaf septoria, the steps listed above would be enough to treat the infection, but if the infection is severe and the above steps do not help much, you can use fungicides or natural oils. 

You can choose broad-spectrum fungicide sprays available in your local gardening store — they work really well. For extra effect, you can also use copper-based fungicides. 

When using these products, always refer to the manufacturer’s recommendations on using them as each product varies. Additionally, do not get these products on your cannabis buds — they can leave a nasty aftertaste that will hamper your quality of buds.

Alternatively, if you want a more natural approach, you can use horticultural oils. Here, neem oil is the most popular for dealing with all kinds of infections and pest problems. Use a mister to apply it lightly to the canopy. Alternatives to neem oil include rosemary, lemon, eucalyptus, and cinnamon oils. 

Again, do not get these oils on the buds — they have a strong scent and will affect your buds’ flavor and aroma profiles.

How to Prevent Cannabis Leaf Septoria?

Whether you are dealing with leaf septoria in your cannabis or simply learning about the possible problems you may face while growing cannabis, you should know how to avoid such problems from occurring (again). And it is relatively straightforward to do if you know what to do. We will outline a few steps below that may help. 

  • Keep Your Cannabis Garden Clean

Fungi generally grow where there are a lot of dead plant material and other contaminants present. So, it is crucial that you regularly clean the growing space and remove any dead or old plant material and other contaminants. This step especially applies to indoor plants. 

  • Ensure Enough Air Circulation

As mentioned above, air circulation can treat leaf septoria, but if you manage air circulation properly from the moment you plant the seedling, it can also help prevent leaf septoria from ever occurring. With better airflow, the humidity levels stay in check, preventing fungal infections from breeding out of control. 

  • Water Your Plants Properly

To prevent cannabis leaf septoria, ensure you water your plants at the right time: early in the morning. Doing so will give the soil a chance to dry properly throughout the day. It is best to only water your cannabis plant once the top inch of the soil is dry. 

You should also ensure that your cannabis container drains the water properly, otherwise the water will pool within the root zone, leading to not only leaf septoria but also other serious problems like root rot. 

This step is crucial because septoria spores use water to spread. 

  • Keep the Temperatures in Check

Additionally, use the above-recommended temperatures and relative humidity in your grow room even if you are not treating leaf septoria. The recommended figures are optimal for cannabis plant growth. 

Also, ensure the temperature fluctuations in your grow room are never more than 5 degrees. Narrow fluctuations don’t affect the plant as much, but higher fluctuations can stress out the plant and make it difficult for them to recover well.

  • Rotate Your Plants

Leaf septoria spores, like other fungi spores, are notoriously good at surviving and coming back with a bang to attack plants. So, it is recommended to rotate your plants during each season and avoid growing plants in the same place as the previous batch. 

  • Use Homemade Fungicide as a Preventative Measure

Don’t wait until your plant suffers from leaf septoria. Instead, use mild fungicides to prevent them entirely. Here, use homemade, natural fungicides like 20% horsetail tea and neem oil on the canopy every two weeks. 

  • Prune Your Plants Regularly

Pruning is a terrific way to boost your plant’s yield but also to prevent fungal infections like leaf septoria. So, consider pruning your cannabis plants regularly. 

Summary: What is Cannabis Leaf Septoria and How to Treat and Prevent It?

When you spot yellow spots, pun intended, on your cannabis plants, panic is usually the first reaction, but that need not be the case. Cannabis leaf septoria is potentially dangerous, but if you take the right measures, it is completely treatable.

Use the tactics listed above to treat cannabis leaf septoria and prevent it from coming onto your plants. These methods are easy to incorporate and double up as a necessary investment for a better yield. 

Although, there are a few things you must keep in mind, such as the following:

  • Leaf septoria is a fungal infection, not caused by pests, and usually occurs in humid weather 
  • It begins at the lower tiers of the plant and moves upwards — a feature that distinguishes it from calcium deficiency 
  • What makes this disease such a nuisance is that it can easily spread to other plants by insects, wind, or water, so you need to take utmost precautions in discarding the infected plant materials and prevent it from spreading to other plants
  • Fortunately, leaf septoria can be completely cured, so don’t toss your plant the moment you see yellow spots on the leaves 

Due to their nature, fungal infections on cannabis are difficult to deal with and scary for many new growers. But with timely action and tact, most fungal infections can be dealt with before they affect your plant (and its yield) in any major way. 



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