yellow spots on weed leaves

Leaf Septoria – An In-Depth Guide To Yellow Spots On Cannabis Leaves

There’s nothing worse than taking a routine look at your plants and spotting anything but beautiful, healthy green leaves. Discovering yellow spots or brown and black spots on leaves can be discouraging, to say the least. Luckily, it’s just a sign from the plant showing you that changes need to be made. If leaves could talk, this is how they’d do it. Leaf septoria shows up initially as yellowing leaves with brown spots. This plant disease is a fungal pathogen that can affect the growth of your marijuana plants. It’s therefore important to know how to prevent as well as effectively treat it if necessary. Here, we’ve put together an in-depth guide into leaf septoria and everything you need to know about this cannabis condition.

What Is Yellow Leaf Spot?

Yellow leaf spot, or leaf septoria, is a common disease that affects many types of foliage and plants including cannabis. When left untreated, leaf septoria can stunt the growth of the plant and negatively affect the final size and quality of the harvest.

Leaf septoria is caused by the fungus Septoria lycopersici. This fungus is known for originating on dead foliage and is often found in garden weeds, meaning the spores can be transferred quite easily to common garden equipment or found abundantly in outdoor environments. When transferred to cannabis, leaf septoria causes unnecessary stress and hinders the overall growth of the plant.

Tips For Identifying Leaf Septoria

If you’re looking to avoid disappointing harvests, you’ll want to keep an eye out for the growth-deterring leaf septoria. Fortunately, it’s pretty simple to identify. If your plant is infected, you’ll find yellow leaves with brown spots on the lower areas first. The spots tend to vary in color but usually begin as uniform yellow circles. Inside the circle, there may be a raised bump, which is actually the spore of the fungus. The affected areas will transform from yellow to brown before drying out and withering to death. Once you’ve identified and confirmed that your marijuana leaves are turning yellow with brown spots, a sure sign of leaf septoria, there are a few ways to treat the disease. Let’s cover these treatment options next.

How To Treat Leaf Septoria

To effectively treat leaf septoria, remember: the condition originates from spores, which makes it easy for them to travel through water, direct contact or wind. So, in addition to treating the disease with one of the methods below, you’ll want to deep clean the entire area along with any tools or materials you’ve been using. The spores are capable of hiding out in winter and re-emerging with the arrival of warmer weather, so it’s important to completely dispose of any of the plant material that’s been affected, too.

Here are a few methods you can use to treat yellow spots on weed leaves, aka leaf septoria.

Remove Infected Leaves

As we’ve mentioned, any infected leaf is carrying a spore of the fungus. The first action you need to take when discovering yellow spots on cannabis leaves is therefore to completely remove every one you find. However, do so carefully. You won’t want the spores traveling to other plants if they’re not infected yet. At the same time, you might want to de-leaf or prune any stems to improve nutrient circulation.

Small Plants

Improve Air Flow And Temperature And Eliminate Moisture

Because leaf septoria thrives in hot and wet conditions, you’ll want to immediately improve your air circulation. In addition, adjust or monitor the temperature and humidity to remain at 20-25C (68-77F) and 50% respectively. Improving your airflow could be as simple as adding a fan or adding ventilation in heat-retaining rooms. De-leafing will also help, as this allows more air to circulate throughout the plant. These measures are obviously more important to implement in indoors rather than outdoor grows.

Avoid Moisture

What adds to the spread of spores? Excess moisture. Therefore, you’ll want to take a few precautions to avoid any additional moisture pooling up or affecting plants that have leaf septoria. This includes being careful not to spray the plant leaves with water. You may even consider watering less to allow your soil to completely dry out. When you do water, be sure to do so in the morning so that the water can dry out throughout the day in full light or sun.

Mediate Your Soil And Remove Weeds

If you’re growing outdoors, you’ll want to mediate your soil to ensure you’ve eradicated leaf septoria spores completely. To do so, rake the area and remove all vegetation or leaves that may have fallen from the plants and weeds in the ground. If you can, let the area dry out as much as possible. Once dry, add a layer of mulch, which will help protect the emergence of the spores again. After your harvest is complete, you may want to move your growing area completely or else replace all the soil.

Apply Anti-Fungal Sprays

Copper-based fungicides and broad-spectrum fungicides can be effective at eliminating the spread of the leaf septoria fungus. These options can also be used as preventative measures, too. A more natural option that could help treat the spread of leaf septoria is neem oil. Be careful with the application of neem oil, however, as you won’t want it to get on your buds. It can leave an unpleasant taste and smell.

How Do I Prevent Leaf Septoria?

To avoid finding yellow spots on weed leaves or preventing the spread of leaf septoria there are a few measures you can implement. First, be sure you monitor your environment for proper air circulation, temperature and humidity levels. This includes avoiding extra moisture, watering in the morning when possible, and allowing for proper drainage. Of course, keeping your plants healthy is just as important. Leaf septoria is less likely to infect healthy plants or strong genetics that are more immune to detrimental diseases. Purchasing your seeds from trusted breeders will also help you from the get-go. Lastly, begin with fresh soil that has plenty of nitrogen to get off on the right foot with the proceeding crop. Adding helpful bacterias or fungi like Bacillus Pumilis and Trichoderma to your soil is an extra supplement that can promote health and prevent disease.

Finally Armed Against Yellow Leaf Spots

Coming across yellow spots on leaves isn’t a good sign for any cannabis plant, but fortunately, leaf septoria is both treatable and preventable. As always, it’s important to take preventive measures and monitor your plants for proper nutrient uptake and normalcy in growth. If you’re not already doing so, make sure you’re taking a good look at your plant’s leaves regularly. Doing so can help you discover leaf septoria early and treat the condition before your crop’s end product is affected.

Coming across yellow spots on leaves isn’t a good sign for any cannabis plant, but fortunately, leaf septoria is both treatable and preventable. Here, we’ve put together an in-depth guide into leaf septoria and everything you need to know about this cannabis condition

Leaf Septoria (Yellow Leaf Spot) On Cannabis Plants

Cannabis plants, unfortunately, are susceptible to a wide variety of diseases and pests. Leaf septoria is one such disease, and can greatly damage the foliage, development, and yield of your cannabis plant. Learn how to properly treat and prevent leaf septoria from devastating your crop.

Leaf septoria is a harsh plant disease that regularly affects the foliage of a variety of plants, including cannabis.

If not handled properly, leaf septoria can be devastating to plants, destroying foliage, stunting their growth, and ultimately affecting the size and quality of their yields.

In this article we take a close look at cannabis leaf septoria, what it is, and how to treat/prevent this disease from affecting your cannabis crops.


Leaf septoria, also known as septoria leaf spot or yellow leaf spot, is a plant disease caused by a specific kind of fungus known as Septoria lycopersici.

This fungus usually overwinters on dead foliage or common garden weeds. Fungus spores can also spread onto equipment like garden stakes and netting, before germinating when conditions are right.

Leaf septoria can be an extremely damaging disease, greatly affecting the foliage and growth of a variety of plants, including tomatoes, parsley, and obviously cannabis.

As the name suggests, leaf septoria is characterized by yellow and brown spots forming on both the upper and lower sides of leaves. The spots tend to be circular with dark brown margins and tan or greyish centers. They usually measure between 1.5 to 6.5mm.

Leaf septoria usually affects plants just after they enter the flowering stage and usually forms on lower leaves first. As the disease develops it spreads its way upwards, quickly affecting multiple leaves all across the plant.

Affected leaves will usually turn slightly yellow, then brown, and eventually wither completely. Leaf septoria rarely spreads on to fruit, so it generally won’t affect cannabis flowers.

If left uncontrolled, the disease can destroy a lot of foliage. This ultimately creates a lot of stress for plants and stunts their growth as well as the size of their harvests. Leaf septoria is particularly prevalent in areas affected by extended periods of wet, humid conditions.


As with most garden pests and disease, early detection is extremely important when dealing with leaf septoria. Make sure to pay close attention to your plants during extended periods of hot and humid weather, as well as during the early stages of the flowering cycle.

Once you’ve identified the disease, make sure you follow the following steps to control it and stop it from spreading:


The first step to effectively dealing with leaf septoria is removing infected leaves. If caught early, you can usually prevent the spread of the disease by simply removing all infected lower leaves and burning/destroying them.

However, if the disease has spread to the height of your flowers, you’ll generally want to skip this step. Removing foliage from flowering areas will greatly weaken a plant and reduce the quality of its buds.


Proper air circulation is extremely important for cannabis plants and plays a big role in the management of pests/diseases.

If you’re growing indoors, improving air circulation can be as simple as adding an extra fan into your room and creating some space between your plants. If you’re working outdoors, however, this might be a bit more difficult.

Pruning is also a great way to help create airflow in and among plants. Try trimming down extremely bushy areas of your plants and avoid having leaves touching or laying on top of each other.

If you’re working outdoors, you may want to try elevating your plants slightly so that they catch a bit more wind. Alternatively, also consider running an electric fan on outdoor plants if possible.


Moisture is another major player in the spread and germination of fungal spores. Hence, you’ll want to avoid moisture as much as possible.

Avoid overhead watering as this will wet the leaves of your plants and consider watering slightly less regularly in order to give the soil a chance to really dry out. Also water early in the day to allow the soil to dry out during daylight hours.


Fungal spores often spread into soils where they hang out over the winter until conditions are right for germination.

While you won’t be able to change your growing medium mid-grow, there are some steps you can take to avoid any spores from the ground spreading onto your plants further.

Start by removing any dead foliage and raking the soil to remove any possibly infected vegetation. Next, dry out your soil properly. Finally, apply a thick layer of mulch to the top of your soil then water your plants.

This will help stop the spread of fungal spores from the soil up onto your plants.


Night shade and horsenettle are common hosts of Septoria lycopersici spores. Hence, make sure you run through your garden and remove any weeds that could possibly host the fungus.


Remember that leaf septoria naturally strikes in hot, wet conditions. Hence, if you’re dealing with an infection, try driving down the humidity and temperature levels in your grow space (where possible).


If you’re dealing with a minor case of leaf septoria, steps 1-6 might be enough to kill the disease and stop it from spreading any further. However, if you’re dealing with a more serious infection you may need to rely on some heavy-handed fungicides.

Broad spectrum fungicides and disease control sprays will usually do the trick. For extra protection, try opting for a copper-based fungicide. Either way, remember to carefully follow the package instructions when using any kind of disease control agent, and avoid getting any of it on your buds.

If you’re after a more natural alternative, we suggest turning to essential or horticultural oils like neem. Neem oil is commonly used to treat all kinds of garden pests and diseases and can easily be applied to your plants using a mister.

Alternatively, consider trying eucalyptus, rosemary, lemon, and cinnamon oils. Either way, remember that these oils have strong aromas and should never go near your buds to avoid contaminating their aroma/flavour.

Neem oil serves as a completely natural way to protect your cannabis plants against pests.

Leaf septoria, also known as yellow leaf spot, is a fungal disease that can greatly damage cannabis crops. Here's how to deal with leaf septoria on weed plants.