Chloroplasts are crucial players in immunity and photosynthesis. However, how chloroplasts arrange the transition between photosynthesis and immunity and how pathogens manipulate this transition remains elusive. Here we report an effector SsCm1 from Sclerotinia sclerotiorum, one of devastating phytopathogenic fungi, inhibits chloroplast immunity and resistance to pathogens, and alleviates photoinhibition in immune state. This is accomplished through stabilizing the conserved chloroplast protein MORF2, which is degraded during immunization and is a suppressor of photoinhibition, cell death, and chloroplast immunity. Overexpression of SsCm1 or MORF2 in plants represses basic immunity and resistance to pathogens, whereas deletion of SsCm1 reduces S. sclerotiorum virulence. Notably, SsCm1 possesses no chorismate mutase activity, which is different from the previously reported Cm effectors. This work reveals a strategy to fine-tune growth-defense balance in chloroplasts by manipulating MORF2, and a pathogenic strategy to subvert the process and promote infection via enzymatically nonfunctional effector stabilizing MORF2.