Yellow Leaf Spot
Definition - What does Yellow Leaf Spot mean?
Yellow leaf spot is a generic term for a fungal disease that can affect cannabis plants. The scientific name for yellow leaf spot is leaf septoria. In infected plants, uniform spots or lesions appear first on lower leaves. These spots will eventually cause death of the leaves.
'Yellow leaf spot' should not be confused with other things that might cause yellowed leaves, including chlorosis, a general yellowing of the leaves brought on by a nutrient deficiency. Nor should it be confused with interveinal chlorosis, a yellowing of the veins of the leaves. Other things that can cause yellow spots on leaves include burn from excessive nutrients, as well as burn marks from lights that are too close. However, 'yellow leaf spot' is actually a fungal disease.
Hydrolife explains Yellow Leaf Spot
Yellow leaf spot is more common on outdoor cannabis crops that are grown in the same location for multiple seasons, as the fungal pathogen can overwinter in the soil and spreads by wind and rain. Yellow leaf spot can also appear in greenhouse grown cannabis if the pathogen is inadvertently introduced.
It is far easier to prevent yellow leaf spot on cannabis than to control it once it has appeared. Removing the conditions favorable to fungal develop will reduce the chance to allow it to show up on your cannabis leaves. Improve the air circulation in your grow space. This can be achieved by selective pruning and thinning, especially of lower branches and by making sure the air in your grow space is in motion.
Try to avoid wetting the foliage when watering your plants; wet the soil only if possible. Preventative fungicidal sprays can also be applied.
If yellow leaf spot appears, remove and destroy the affected branches. Sterilize the cutting instrument before, during and after the cuts if removing multiple branches. You can also spray with a fungicide such as neem oil, avoiding the buds as it will negatively affect them if contact is made.