Complex sentence: Grapes typically become infected with "Botrytis" when they are ripe, but when then exposed to drier conditions become partially raisined and the form of infection brought about by the partial drying process is known as noble rot.
Split into simpler sentences: Grapes typically become infected with "Botrytis" when they are ripe. If they are then exposed to drier conditions and become partially raisined this form of infection brought about by the partial drying process is known as noble rot.