Some grafters suggest using a plastic bag to create a micro-climate after grafting. I have found it time-wasting and counter-productive in my warmer climate and the warm and moist atmosphere with little or no air movement inside is a recipe for fungi and needs constant attention. Fungi usually start when the leaves inside are in contact with the plastic bag and insects can also be trapped inside. And breed!
It is better to have a misting bed with timed foggers of the smallest water droplets that can be switched off in damp or humid conditions.