Leaf septoria (also known as septoria leaf spot or septoria blight) is a general term for the damage caused by a range of fungi that can grow on plants; on weed specifically, the culprit is Septoria lycopersici. If you’re seeing yellow spots on leaves, it could be this. Luckily, it’s completely treatable.
- What causes leaf septoria on cannabis?
- How does leaf septoria affect weed plants?
- What are the symptoms of cannabis leaf septoria?
- 1. Yellow spots on cannabis leaves
- 2. Growth of pycnidia
- 3. Stunted growth
- Don’t confuse leaf septoria with other cannabis problems
- How to treat yellow spots on cannabis leaves
- 1. Remove any affected cannabis leaves
- 2. Adjust airflow
- 3. Reduce moisture
- 4. Adjust temperature and humidity
- 5. For severe cases, use fungicide
- How to prevent cannabis leaf septoria
- Tips for outdoor growers
- Spot the spots before it’s too late
What causes leaf septoria on cannabis?
The fungal spores that cause yellow spots to form on leaves are typically spread through splashing water, wind, or contaminated tools and, once the spores land on a plant, they can quickly germinate and penetrate the plant tissue, leading to infection. While leaf septoria is more common on outdoor plants, indoor plants can still be affected if the grow area isn’t kept clean.
How does leaf septoria affect weed plants?
Leaf septoria spends its winters “hibernating” on fallen leaves in cool, wet environments—and not just cannabis leaves but any leaf matter sitting outside all winter could host this fungus. Once spring rolls around and new plants start growing, these spores activate and seek out a new home. Most commonly, Septoria spores are spread via water splashing or wind and, once it’s found a new home, they get to work.
This fungus starts by penetrating the outer walls of the leaf and then begins to multiply from there. Once an infection has started, it will continue to spread until the leaf develops yellow spots that quickly turn brown before the leaf dies. While septoria doesn’t affect the actual flowers on your cannabis plant, the damage to the leaves can greatly reduce your yield, make the plant more susceptible to other issues, and possibly even kill your plant if left unchecked.
Knowing what to look for and how to tell cannabis leaf septoria from other issues is key to timely intervention.
What are the symptoms of cannabis leaf septoria?
1. Yellow spots on cannabis leaves
The single most common symptom of leaf septoria is irregular yellow spots developing across a patch of cannabis leaves. Since this infection takes time to show signs, these spots are most likely to appear on older growth first, though they can pop up anywhere.
These spots can be either circular or elliptical, but they will almost always have irregular shapes and edges and won’t have a consistent colour. Typically, they will be yellow on the outer edges, light brown inner rings, and dark brown/grey in the centre. These spots will continue to expand until the fungus has consumed the entire leaf.
2. Growth of pycnidia
If left untreated, the fungus inside of the leaf will begin to sprout fruiting bodies from the centre of the yellow spots. These will appear as small black dots (known as “pycnidia”) that may be visible to the naked eye, though a magnifying glass will help you spot them earlier. If you see these, you need to take immediate action as the infection may already be spreading.
3. Stunted growth
Since infected cannabis leaves are less able to absorb light, the entire plant will slow its growth to a crawl. However, this symptom is hardly unique to leaf septoria. Further, septoria is more common in flowering cannabis plants, so you may not notice a lack of vertical growth as the plant may have already finished by the time signs appear. While this disease doesn’t affect buds directly, an infected plant will produce far less since it cannot photosynthesise properly.
Related post
Don’t confuse leaf septoria with other cannabis problems
Yellowing leaves are perhaps the most common sign of something wrong with a cannabis plant. While a deficiency or excess of almost any nutrient can lead to yellowing, calcium deficiency is the one most likely to be confused with septoria as they both lead to yellow/brown spots on affected leaves. Knowing which of these you’re treating is the only way to actually fix the issue.
Broadly speaking, there are four main ways to tell if it’s septoria or calcium deficiency:
- Spot location
Septoria often shows up first at the bottom of plants (there are a few reasons for this such as these leaves being closer to the ground and/or having less airflow). In contrast, calcium deficiency can appear anywhere or everywhere from day one.
- Leaf colour
With septoria, the leaves develop yellow spots that slowly spread and turn brown. With a calcium deficiency, leaves tend to slowly turn a yellow-green colour between the veins before gradually developing reddish-brown spots.
- Branch strength
This one can be a bit more subjective, but if the branches are still firm then it is likely septoria while a calcium deficiency will result in weaker, softer branches.
- pH levels
Check the pH of your water, medium, and runoff. If it’s in the ideal range (6.0-7.0 for soil, 5.5-6.5 for hydro) then it’s almost certainly septoria. If the pH is too low, then it’s likely calcium deficiency.
Keep in mind that a plant can be affected by both at the same time, so this isn’t necessarily an either/or scenario.
Related post
How to treat yellow spots on cannabis leaves
Now that you’ve determined that you are dealing with leaf septoria, it’s time to take action. You’ll need to act quickly as the infection will not stop on its own. To treat cannabis leaf septoria, simply follow these steps:
1. Remove any affected cannabis leaves
Once an infection has taken hold of a leaf, that leaf is done for. This is not the time for a careful, measured hand; if a leaf has even a single yellow spot, it has to go. No exceptions. Any spores left behind are a ticking time bomb that will begin to spread eventually.
Use disinfected, sharp scissors (or a razor) to remove the leaves. Be sure to disinfect them after between each use as the spores can live on tools for quite some time. The last thing you want to do at this point is introduce any new pathogens. Once all of the leaves have been removed, they need to be disposed of properly and immediately. DO NOT use these leaves in compost as the fungus will simply go dormant until it has a chance to spread later. Throw them away, and take the trash outside.
At this point, it’s probably worth going a little further and removing a few extra cannabis leaves even if they don’t show signs. This is because the fungus can take time to develop and just because a leaf looks healthy now, doesn’t meant that it isn’t already infected. Additionally, maintaining adequate airflow inside of your growing space can help reduce the spread of septoria.
2. Adjust airflow
Septoria fungi thrive in humid, damp, still environments so maintaining proper airflow is crucial in limiting its ability to spread. Proper airflow reduces humidity, dries out leaves (dry leaves are far less susceptible to fungal infection), and generally promotes healthy robust plant growth.
If you’re growing cannabis indoors, make sure that you have at least one (or more in a larger space) good oscillating fan running at all times (this is, of course, on top of your ventilation fan). Additionally, keep your plants as spread out as possible and don’t have excess foliage in the lower areas.
3. Reduce moisture
Related to the above point, moisture is key for fungal growth so keeping it to a minimum is paramount. This can be tricky since weed plants need a lot of water and decently high humidity, but there are steps you can take.
Firstly, avoid overwatering your plants; we’ve got a whole guide on that so take a look. In short, water your plants only when the top centimetre or two has completely dried out. Additionally, avoid watering your plants from any overhead source and try to avoid getting any water directly on the leaves. Pour any water directly onto the soil and water until you start to see runoff.
4. Adjust temperature and humidity
Cannabis plants thrive in temperatures in the 20°C to 30°C range, though this changes according to the stage of the plant. Likewise, the proper humidity changes as the plant grows; 65% to 70% for seedlings, 40% to 70% for vegetating, and 40% to 50% for flowering plants.
Keeping these numbers in the proper range is a good way to give your plants the best chance at avoiding any infections.
5. For severe cases, use fungicide
If you managed to spot leaf septoria early, then these should be enough to keep it from coming back. However, in more extreme cases, these steps simply won’t be enough. If you’ve done everything and you’re still seeing new yellow spots appear, you may need to resort to chemical interventions such as fungicides or certain natural oils.
There are several commercially available fungicides that are safe for use on weed plants that will almost certainly cure your plant, but they aren’t without some risk. For one, while it kills the fungus, it won’t heal your plant; any damage that’s been done is permanent.
If you’re looking for a slightly more natural approach, neem oil is one of the most popular organic pesticides/fungicides used by cannabis growers. A mixture of neem oil and water, sprayed directly onto the leaves, is a great way of preventing many common pests like spider mites and thrips, but its (relatively minor) fungicidal properties can make it a good choice for dealing with septoria if you’re opposed to harsher chemicals.
No matter what you use, do not get it on the flowers. These chemicals will all linger on your buds and can have a tremendously negative affect on the flavour of your finished product. As with anything you’re using on your cannabis plants, follow the manufacturer’s instructions closely, they know what they’re talking about.
How to prevent cannabis leaf septoria
If possible, prevention is always preferable to treatment, especially since damage done by leaf septoria is permanent. Keeping your cannabis plants free from septoria isn’t that hard and largely comes down to proper plant maintenance that any grower should already be practising. Additionally, Septoria spores can linger for months, so make sure the thoroughly clean your entire grow space before and after every grow.
1. Keep your grow space clean
This sounds pretty obvious, but the number of growers who don’t remove dead plant matter never ceases to amaze me. Septoria (like many fungi) grow on dead plant matter, so removing any old or fallen leaves is crucial for keeping your plants healthy. This is generally more important for indoor growers, but I’ll talk a bit about what steps outdoor growers can take a bit further down.
2. Maintain proper conditions
I talked about this in the previous section but its importance can’t be overstated. Keeping your grow space in the optimal range of temperature and humidity with proper airflow will make it far harder for any infection (fungal, bacterial, or pest-based) from taking hold in the first place.
3. Prune your plants
Keeping your plants’ lower branches pruned helps not only improve yield but also fend off diseases. Pruned plants allow for better airflow, lower humidity, and keep leaves away from the soil where fungal spores can live.
Related post
Tips for outdoor growers
Growing weed outdoors has several benefits, but it comes with the major downside of having little to no control over the environment. However, there are a few good tips that outdoor growers can follow to reduce the risk of leaf septoria.
- Pick a good site
You’re going to want to select a location with adequate drainage where water won’t pool after rain. Of course, you’ll want plenty of sunlight, too, but not just for the plants. If the sun regularly dries out the soil, then the odds of a fungal infection are greatly reduced.
- Keep it clean
While it’s impossible to keep any outdoor area truly “clean”, you can take steps to minimise just how dirty it gets. Remove any dead leaves (cannabis or otherwise) that get into the area and remove any non-cannabis plants that might be growing alongside them. While there are reasons for growing certain crops alongside cannabis (e.g., legumes can help regulate nitrogen levels), any other weeds are just going to cause problems.
- Spread things out
Keeping your cannabis plants far apart not only gives them plenty of room to grow, but also reduces the risk of disease spreading form on plant to plant.
- Mulch your topsoil
By spreading a layer of mulch on top of your soil, you put a barrier between your cannabis plant’s leaves and any spores that might be living in the soil.
Spot the spots before it’s too late
To wrap things up, leaf septoria is a fungal disease that can wreak havoc on your cannabis plants if not treated quickly. By keeping a close eye on things, following a few basic precautions, and taking timely intervention, it’s possible to completely avoid any serious damage to your plants.
