Apple trees suffer from a fungal disease called Valsa canker that becomes more severe in cold weather. Scientists discovered that the fungus has a special protein called Vmplc1 that acts like a temperature sensor, telling the fungus to produce more aggressive enzymes when it’s cold. When researchers disabled this protein, the fungus lost its ability to damage apple trees during cold periods. This discovery helps explain why the disease is worse in spring and could lead to better disease management strategies.