Scientists discovered that a specific protein called SsSR acts as a master switch controlling how dangerous a fungus called Sclerotinia sclerotiorum becomes when attacking plants. Unlike other protein switches that make the fungus grow faster, this one specifically controls the fungus’s ability to cause infection by managing the production of ergosterol, a critical component of the fungus’s cell membranes. This discovery could lead to new ways to protect crops like oilseed rape from this devastating disease.