It looks like a fungus, probably the beginning of botrytis or powdery mildew. I thought I saw white spots on the leaves, so it’s likely a bit of oidium. Anyway, without definitely determining the type, I’m convinced it’s a fungal attack... At the same time, it’s the season. In any case, I invite you to check Google for various solutions, although given your progress in growing, I think the best option is to harvest...