Leaf septoria or yellow leaf spot is caused by Septoria cannabis and S. neocannabina, both of which are fungal pathogens, targeting cannabis in particular. It occurs on outdoor grown marijuana, triggered by warm weather and rain. It can also appear in plants lacking sufficient nitrogen. Yellow leaf spot will first appear on the lower leaves of the marijuana plant. The spots can range in color from white to grayish brown to yellow, first appearing as small round lesions. As yellow leaf spot matures, it takes on a pimple-like appearance, expanding in size.
Yellow leaf spot will damage the leaves and sometimes the stems of your cannabis plants, but will not kill the crop. However, if left untreated it will reduce yield. Infected leaves dry at the site of the spot, eventually crumbling and creating a hole where the spot began. Heavily infected leaves will turn completely yellow, dry up and drop off. If the fallen leaves are not removed, you have just created a landing ground for spores to overwinter. This means they lay low until warmer weather arrives which is when they do their damage. Yellow leaf spot spores rely on the wind, water and hitchhiking onto passersby for their travels.
Yellow leaf spot can be prevented from attacking your cannabis garden through crop rotation. This means choosing a different planting site after each harvest. If this is not possible, then once you have harvested a marijuana crop completely remove any evidence of the previous vegetation and till the soil well, removing all debris. Prepare the site with sterilized compost and work into the soil. This should provide enough nitrogen to give the new crop a healthy starting ground. In the event you have no available compost (every gardener should get into the habit of creating a compost pile) then add blood meal, fish meal or soybean meal to the existing dirt. This will provide the nitrogen needed for your cannabis. Be sure to sterilize all gardening tools with peroxide or rubbing alcohol before storing.
Another option when starting a new marijuana bed is to apply a fungicide to the planting site before adding seed or seedlings. There are various natural solutions you can either make yourself, or buy already mixed. Adding beneficial bacteria, such as Bacillus Pumilis or beneficial fungi such as Trichoderma will benefit the soil if you have no compost to incorporate. You can also spray the area with essential oil mixtures containing cinnamon, clove, coriander, neem, or sesame. Spraying the soil with compost tea or a baking soda solution can help to prevent yellow leaf spot spores from settling in the area.
Water your cannabis plants in the early morning or early evening. Water the soil, not the leaves. In so doing, the sun's rays won't burn the leaves. This practice should be adhered to no matter what you are growing.
In between harvests, if yellow leaf spots find your marijuana patch, the fungus can be controlled in several ways:
- Physically remove all affected leaves and destroy them. Do not add any diseased plant material to the compost pile.
- Spray the affected areas with a mild solution of ½ teaspoon baking soda per gallon of water. Do this in the early morning or in the evening so the leaves dry before the sun heats the day.
- Apply a Bordeaux mixture to discourage the disease from spreading.
- Spray the plants with a neem oil solution in the early morning or evening to avoid the sun burning the plant leaves.
- Use something like Marijuana Booster to avoid nitrogen deficiencies.