Hi,
Sometimes called "yellow leaf spot" or "leaf septoria," this condition is caused by a fungus (or sometimes a bacteria) that attacks cannabis plants and usually appears in warm, wet weather. The symptoms first appear on the bottom leaves of the plant.
Although the spots often appear yellow like in the picture you showed, they will start turning brown over time. So if you see round brown spots on your cannabis leaves it could also be caused by leaf septoria.
Solution to Leaf Septoria
This fungus spores may stay in the soil over the winter, and attack your plants in the summer. Spores are also easily spread by wind and rain.
Immediately (and carefully) remove all affected leaves to get rid of as many spores as possible.
Prune any leafy parts of the plant to improve circulation through the plant
Avoid getting water on the leaves or laying on top of each other – you want to avoid moisture
Make sure to keep the ground under your plant clean. Rake away all leaves and vegetation. Adding mulch can also help prevent spores from spreading.
Keep plants healthy, sicks plants are much more susceptible to leaf septoria than healthy plants One way to help prevent this fungus from attacking your plants is to rotate crop sites or move to a new location every year.
A copper based fungicide or a broad spectrum fungicide may be effective at stopping the fungus from spreading though they are best used as a preventative. Depending on exactly what's causing your leaf spot, Neem Oil may be used to help combat the problem. Neem Oil will leave an unpleasant taste/smell on buds when used to treat flowering plants, so don't let this stuff get near your buds! You will need a mister (also called a "One-Hand Pressure Sprayer") to spray all the leaves evenly, since neem oil and water can separate easily. Try spraying just a small part of the plant and see how it reacts first before spraying the whole thing.
Hope this helps.
