Septoria leaf spot is a fungal disease caused by Septoria cannabis. Spots begin small and yellow, then enlarge into tan, grey or white lesions, often angular where veins limit them, with darker edges and a yellow halo. Inside the lesions tiny black fruiting bodies (pycnidia) can be seen with a hand lens. Heavily spotted leaves yellow and drop.
In practice
Septoria is mainly an outdoor and greenhouse disease that starts on the lowest, oldest leaves and moves upward. Spores splash from infected debris and soil in rain or overhead irrigation, and leaves must stay wet for infection. Removing affected lower leaves, cleaning up fallen debris, watering at the base and mulching to stop splash slow it down. It rarely touches buds directly, but heavy defoliation in late flower reduces yield. The fungus overwinters on plant residue, so rotate sites and remove debris at season end. Check what is registered for cannabis locally.
Don't confuse with
- Nutrient spotting: often symmetrical across leaves, with no pycnidia.
- Powdery mildew: white powder on the surface, not sunken lesions.
- Spider mite stippling: many tiny pale dots without halos.
