Relative humidity (RH) levels influence a plants ability to dilate and contract stomata, located on the undersides of leaves. Among their multiple functions, stomata enable the plant to transpire water and gases into the growing environment. In response to changing conditions, stomata will open or close to moderate moisture loss.
As plants transpire, the humidity surrounding the plants saturates leaves with water vapor. When relative humidity levels are too high or inadequate air circulation exists, plants cannot properly evaporate (part of the transpiration process) or uptake water/nutrients. If these conditions sustain, plants will eventually begin to decay and die.