Mold and diseases caused by fungi can greatly impact the shelf life of fruits and vegetables. However, some fungi benefit their hosts by aiding plant survival. Colletotrichum tofieldiae (Ct) is a root mold that typically supports continued plant development even when the plant is starved of phosphorus, an important nutrient for photosynthesis and growth. Researchers studied a unique pathogenic strain of the fungi called Ct3, which conversely inhibits plant growth. By comparing the beneficial and harmful Ct strains, they found that activation of a single fungal secondary metabolism gene cluster determined the negative impact of the fungus on the host plant. When the cluster was disrupted, either genetically or by a change in environment, the fungi’s behavior changed from inhibiting growth to promoting it. Understanding mechanisms like this could help us reduce food waste by harnessing the beneficial role fungi can have on food.