I’ve battled this and it’s cousin that usually follows gray mold for years along the coast.
It always pops up late June when we get daily rain. I’ve tried everything and then some with very little success.
Recently I tried Ed Rosenthals method of mixing water with pH up till I hit 8.2 and spraying the plants. To my surprise it works and works well. Potassium bicarbonate works on light cases but a spray made of 8.2 pH water then followed by a hydrogen peroxide spray effectively and quickly stops leaf Septoria. We are having a severe issues with leaf Septoria this year from Texas to Florida due to the constant daily rain.
I hope this now tried and proven method that’s evidently been known by others for years helps others battling this leaf spot as it works.
Leaf Septoria always, always proceeds Grey Mold in Dixie especially along the coast. Grey Mold seems to hit plants that are weakened by Leaf Septoria especially hard.
It’s not looking good if these constant daily rains don’t stop as the Indicas are already flowering and the Sativa’s are starting. I hope high pH water has the same effect on Grey Mold as LS y’all try it as it’s safe, simple and effective