I was referring to KK's. Was yours outside, bigger older leaves that yellowed some first and then got degraded and the rest of the plant was healthy it would be the same. If they got sickly and spotted while green and it involved leaves all over the plant than that would be a different problem.
I have seen black spot disease before. It usually starts by infecting leaves in the same area big and small. Plants with black spot look bare and straggly during late summer or early fall and the other plants look normal. It will flower but the flowers hardly develop and get discolored.
I am sure it can look different from case to case and plant species to plant species. So I don't doubt your assessment. If a person thinks they have black spot, even if they are wrong about the exact one it is still probably another fungus. So when they treat for black spot it knocks out the fungus they have.
Yours just doesn't look like black spot disease to me. They have some black spots the size of a pencil eraser in the green and then yellows around them while half or more of the leaf can still be green. Then the whole leaf can yellow.
If you have a 6 foot pot plant the lower leaves are going to be 3 feet off the ground. It is hard to get splashed. A person should be pruning lower growth and all surrounding vegetation under their plant anyways to keep air circulating.
Why do you think you had black spot disease?
I am fairly certain, even though its initial presentation was so odd, for several reasons. But as I followed the progression of symptoms, I kept coming back to some species of Septoria.
Indoor grown plant, cut gifted to me from someone in Cali's delta, which is an area that's been farmed hard and has lots and lots of different diseases going on. Started with the lower leaves yellowing with the spots staying green. Eventually (like after several weeks) it became the classic necrotic spotting of lowers, working its way up the plant and eventually defoliating.
I spent a
long time trying to fix deficiency after deficiency, consulting grower after grower and getting nowhere, all the while the symptom profile never changed, which to me is another indicator that I had a different problem going on. It wasn't until I began a regimen of Kop-R-Safe (which was after spending a lot of time on a bunch of ag/edu sites) that it began to abate, and it wasn't until I doubled the dosage rate (not against label instrux) that I finally eradicated it.
I can't tell you exactly what species of Septoria this cut came with, but I feel I nailed it closely enough to call it. I haven't found another group of funguses (fungi?) that so closely match the general disease progression, that's why I generalize as a Septoria spp.