I learned the same way you did a few year ago, and I did what you did, kept feeding more calcium and it kept getting worse
eventually I figured it out I stopped it Dead with liquid copper Fungicide and I also burn sulfur
Recommend you spray the baking soda and spray it again in 48 hours and if indeed it is fungus you'll see no new lesions
Agree sometimes fungus and calcium deficiency look a lot a like but if you look at
calcium deficiency;
black spot with a lighter ring
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but when you see brown spot with a black ring that's septoria
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Yes it will spread to every plant although some strains seem more resistant
You've been feeding extra calcium & it's not helped; spray it with the baking soda and spray it again in 48 hours and see if that helps.. only way to know, good luck!