Gardeners must always be on the lookout for diseases caused by fungi. Viruses, bacteria, and other organisms can cause problems, but fungi and “fungal-like organisms” cause more than half of all plant diseases.
Fungi are difficult to kill off. Most can live in the dirt or on dead foliage during the winter, and return to cause problems in the spring or summer. Those that can’t survive are simply replaced by new fungi carried on spring air currents from warmer climates.
Weed plants are particularly susceptible to fungi. And one specific fungus causes the yellow spots that can signal damage to, or eventual destruction of, a cannabis crop.
It’s commonly known as septoria leaf spot or leaf septoria, and it affects other plants as well. The pathogen adapts so well, however, that different species of septoria have evolved to attack each type of plant.
Septoria cannabis is the one that hits weed plants, and it can be deadly.
What Leaf Septoria Does to Weed Plants
The fungus that causes septoria leaf spot is known to biologists as Septoria lycopersici, and it’s a survivor.
It overwinters on plant debris, in the dirt, or even on garden hardware, and then comes right back in the spring. Septoria leaf spot doesn’t usually hit cannabis plants hard until the flowering stage, but the fungus is patient and waits for its opportunity to attack.
When the weather is warmest, the fungus releases spores to be carried to plants by the wind or rain. The spores land on leaves and release their pathogens, consuming the plant’s nutrients and spreading quickly. Eventually, the plant is stressed and its growth is stunted, resulting in smaller plants that produce much less harvestable flowers. In some cases, the plants may even die.
Thankfully, septoria leaf disease is easy to spot (sorry about the pun) and it can be successfully treated — if you catch it early.
How to Diagnose Leaf Septoria
Since you’re reading this article about yellow spots on weed plants, you’re probably already familiar with the sign that your plants have been infected with Cannabis septoria.
In truth, the spots may be brown if you haven’t been paying close attention to your grow. They usually start as small yellow circles but can turn brown over time.
Septoria spots are distinctive. Some will have dark edges, and the spots will normally be uniform in size with a hard, dark “pimple” in the middle of each circle. That growth is actually the mold spore that triggered the problem. As the fungus spreads, more and more circles will appear on more and more leaves. Infested leaves eventually turn yellow, then brown, and then they die and fall off.
Leaf septoria is most likely to develop and spread during hot, wet weather, and it’s most likely to initially appear on the lower part of plants. From there, it quickly works its way up to the top. The disease won’t specifically damage buds but its presence wreaks havoc with productivity — even if it doesn’t kill the plant.
That makes it imperative to find the yellow spots on your weed plants and treat the issue immediately.
How to Treat Leaf Septoria
Any guide to growing cannabis will emphasize that growers need to carefully inspect their plants regularly. If they can’t check the crop every day, they should certainly do an inspection several times a week. That’s the only way to discover disease and insect infestations before it’s too late.
It’s particularly important to constantly check the plants’ leaves when it’s hot and humid, and once they’ve entered the flowering stage. When you do, it’s hard to miss the yellow circles caused by Cannabis septoria.
Your next steps will depend on how advanced the disease has become.
Early Stages of Septoria Leaf Disease
Here’s what to do if the yellow spots are limited to lower portions of the plant.
1. Remove Infected Leaves
Immediately remove all of the affected leaves. Handle them carefully and bag them immediately, because they can still spread spores if they’re in proximity to the rest of the plant (or neighboring ones) before they’re taken out of the grow.
Dispose of the bag well away from the grow room or garden, wash your hands, and disinfect any tools you used.
2. Enhance Air Circulation
Once you’ve removed the leaves with yellow spots, you want to make sure that conditions in your grow aren’t conducive to new outbreaks.
Prune all of the plants’ excess growth, particularly in areas that have become bushy. You don’t want leaves touching each other, and you want to allow as much room as possible for air to pass between the leaves and the plants.
If you have an indoor grow, add ventilation fans. If the plants are outdoors, remove any barriers that might obstruct breezes from flowing through them.
3. Pay More Attention to Temperature, Humidity, and Water
This obviously applies primarily to indoor grow rooms, although if you’re growing outdoors be sure to water the soil and not the leaves. Spraying plants from above can allow mold spores to ride on the water drops. You might also want to erect rain shields for outdoor gardens.
Watering the soil is important indoors as well, but there’s more you can do. Lower the temperature and humidity levels in the grow room, without going so low that it harms the plants. You’re more likely to see the yellow spots of leaf septoria in hot, humid conditions.
4. Clear the Growing Medium
This is particularly important outdoors. Spores can remain on weeds and dead foliage, ready to hop back onto your plants at their first opportunity. Get rid of old leaves, weeds, and anything else in the soil that could harbor fungi, and then add a layer of mulch on top.
Later Stages of Septoria Leaf Disease
If the majority of a plant’s leaves are infected, more drastic action is necessary. Improving air circulation and the grow’s environment is still important, but that won’t be enough.
That means applying fungicides.
Since your plants are probably flowering at this point, you’d do best to stay away from chemical fungicides. Many of the chemicals in those products can work their way into your bud and you’ll end up consuming them. Organic fungicides that contain copper bicarbonate or potassium will usually be effective against leaf septoria, and horticultural oils like neem oil may also do the job.
Don’t expect the yellow spots on your weed plants to go away, though. You’re treating the disease, not the symptoms — your goal is to prevent the fungus from spreading any further.
And be vigilant. If there are Cannabis septoria spores in the area, they’re just waiting for an opportunity to return to your plants.
Yellow Spots on Cannabis Plants: FAQ
Q: What should you do at the start of a grow to minimize the possibility of plants developing septoria leaf disease?
A: Change your soil or growing medium (or rotate your crops) every season, and make sure there are no weeds or old vegetation in the soil. Add organic fungicide, horticultural oil, and/or beneficial bacteria to the soil before planting. Most importantly, follow all of the guidance we’ve mentioned regarding airflow and pruning, watering, temperature and humidity throughout the grow.
Q: If leaves turn yellow, is that the same thing as seeing yellow spots on them?
A: Not usually. Leaves affected by leaf septoria may eventually turn yellow before they die, but if leaves turn yellow without first developing yellow spots, the problem is more likely to be a nutritional deficiency.
Rahnama, M., Szarka, D., Li, H., Dixon, E., Castlebury, L., & Ward Gauthier, N. A. (2021). Reemergence of Septoria leaf spot caused by Septoria cannabis on hemp in Kentucky. Plant disease, 105(9), 2286-2289.