How to Identify and Treat Septoria Leaf Spot in Cannabis Plants

If you’ve noticed small brown spots on your marijuana leaves, or that the lower foliage on your plants is changing color, wilting, and falling, Septoria fungus could be affecting your crop. This is a very common and harmful fungal disease that damages the quality, size, and yields of your marijuana plants. But fear not, as there are several ways to treat and prevent septoriosis and we are here to provide you with those solutions.

Septoria leaf spot (also known as yellow or brown leaf spot) is one of the most commonly reported fungal diseases in cannabis, especially in outdoor crops. While not as prevalent as botrytis or powdery mildew, septoriosis is another silent foe facing growers everywhere.

This leaf infection can spread rapidly through plantations during periods of heavy rain or high humidity. Outdoor growers usually notice the first signs in the late stages of the vegetative phase, in the early summer months, although it can appear at any time. Under extreme conditions, plants can lose 50% to 90% of their leaves in the lower to middle part of the canopy. So, if left unchecked, it can destroy entire crops just as they begin to bloom.

Identifying Septoria in cannabis leaves

The symptoms of the disease begin on the lower leaves and within the inner canopy, where humidity levels are highest. Small, irregular spots measuring 15 mm to 30 mm, with bright-yellow edges, form on the leaves, both on the upper side and on the underside, subsequently expanding to approximately half a centimeter in diameter and turning brown with a prominent yellow halo. As the spots grow, the brown areas become more irregular and merge with the yellow ones, developing centers ranging from gray to white.

Small black structures (pycnids) resembling peppercorns may also be present on the spots. Leaf spot by Septoria will spread extremely fast, and you will notice that the spots grow until they eventually merge, covering whole leaves, which wilt and fall. At this point the problem has evolved to the point where you have to act incredibly fast if you want to save your plants.

Septoriasis: cause and development

Septoria leaf spot is caused by the fungus Septoria spp., which infects a specific group of hosts. For example, Septoria lycopersici, which causes leaf spot on the tomato, affects the tomato and also other solanaceous plants, such as potatoes and eggplants; and Septoria tritici causes the spot on wheat, affecting it and also oats, barley and rye. Septoria cannabis is known to cause disease only in cannabis.

Septoriosis is spread by asexual spores (called conidia) that develop within capsule-shaped structures (called pycnidia) containing several hundred conidia. When there is excess moisture, the conidia are released from the pycnidia in a curly, exudative formation. They are moved short distances by rain splashes, and long distances by wind, then accessing the plant from the ground.

The most conditions most favoring the development of Septoria leaf spot are humid and warm areas with temperatures between 15 and 25 ºC. During the early summer months outdoor growers should pay special attention to the weather, as frequent rains and high temperatures will create settings ideal for its spread.

Conidia, capable of creating new infections

Conidia are capable of creating leaf spots that develop in 7 to 10 days from the time of infection. When humidity is high the life cycles of the fungi are more intense and the disease becomes more severe. During rainy summers the disease can spread quickly through fields.

Septoria species can spend the winter in infected fields or among the debris of dead leaves and weeds, which serve as “green bridges” to help the pathogen survive from one season to the next. Pycnids function as survival structures and protect conidia from extreme environmental conditions, such as desiccation. Under cool conditions with low relative humidity, pycnids can remain viable for up to 9 months. In contrast, the survival of pycnids decreases drastically in warm climates and, buried in the ground, they survive for less than 1 month.

Septoriosis vs calcium deficiency

Septoriosis and calcium deficiency often show similar signs, making it difficult to tell them apart. Some factors can help in this regard:

• Leaf color: leaf septoria causes the leaves to turn yellow at first, and then brown. Calcium deficiency causes plants to take on a yellowish green color at first. As the disease progresses, the plant turns reddish-brown.
• Weak roots and branches: in cases of calcium deficiency, the roots and branches are weak, and the plant may fall. Septoria fungus does not weaken the marijuana plant, at first.
• Position of the spots: the Septoria fungus starts attacking leaves from the bottom. If a plant does not have enough calcium, random yellow-green spots can appear on any leaf.

How to prevent Septoria leaf spot

Environmental monitoring and proper sterilization are the easiest ways to mitigate the risk of septoriosis persisting between crops.

• Clear the area of diseased plants and plant debris to help prevent Septoria leaf spot.
• Overwatering will cause the disease to spread, so try to keep the relative humidity level below 50%.
• Leave space between your plants to ensure good air circulation.
• Use mulch to prevent Septoria spores in the soil from reaching the lower leaves.
• If a previous indoor cannabis crop had septoriosis problems, discard the substrate and do not reuse it.
• Practice deep tillage at the end of the season, and again before planting, to completely bury the remains of previous crops.
• Rotate your marijuana crops from season to season; that is, don’t grow your plants in the same place every year.
• Whatever you do, don’t put infected plants in your fertilizer, as spores can survive and infect future crops.

How to treat and eliminate septoriosis

Septoria leaf spot thrives under humid and warm conditions, so the first thing to do is to decrease the humidity levels and temperatures in your growing space. Once you have internalized this, act as follows:

• All infected leaves should be removed immediately, and should not touch other plants. If the problem is with just one plant, it may even be worth eliminating it altogether. Always wash your hands and thoroughly clean any tools before touching healthy plants.
• Organic fungicides containing potassium bicarbonate or copper are an effective treatment as long as the septoriosis is not yet too severe. Chemical treatments are not recommended, as you may not be able to consume the marijuana. Biological products including the active ingredient Bacillus (Bacillus amyloliquefaciens, B. mycoides, B. subtilis) have shown a suppressive capacity in laboratory and greenhouse experiments.
• If your plants are outdoors, elevating them slightly will improve air circulation. The spores of the Septoria leaf spot germinate and spread under conditions of excess moisture, so keep the soil moist, but not wet. And always avoid watering from above, which means splashes.
• Rake the outside soil to remove infected vegetation. The most common hosts of Septoria leaf spot spores are nettle and blackberry grass. It is necessary to remove any weeds that may harbor the disease.

Conclusion

Although botrytis and powdery mildew may get all the attention on cannabis grower forums, septoriosis remains an equally serious concern. Knowing the early signs of it, and treatment options, is critical to reducing the risk. And, let us emphasize how important planning and preventative maintenance are for successful cultivation. There are many tools available to cannabis growers, and there’s no reason you can’t stay one step ahead of all these pesky fungal diseases.