If you want to keep growing the Rosemary in water, you need to use a hydroponic system, which provides nutrition and deters infection, though Rosemary is more prone to botrytis when grown in this way; whilst Rosemary will root in water, it's usual for it then to be potted up into soil. Information on growing Rosemary hydroponically here https://gardeningtips.in/growing-rosemary-hydroponically-a-full-guide
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