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sawaji varun
How to enable or disable the textfield when selecting item from picklist on the standard page
How to enable or disable the textfield when selecting item from picklist on the standard page like leads, accounts etc.
(1) Override the standard page with Visualforce - overkill for this I'd say
(2) Wait until the edit record home page feature becomes GA in the lightning experience, and you'll be able to replace just the elements that you want to add functionality to.
At present, I'd be handling this via a validation rule until option 2 is available.