Dr. Christine Hawkes discusses plant symbiotic fungi can mediate plant stress tolerance, changing the way we think about controllers of plant physiology. Understanding these interactions is particularly important given forecasts for a future with more frequent and extreme drought and heat. We focus on foliar fungal endophytes in C4 grasses, which are widespread and horizontally transmitted. Using a combination of field and greenhouse studies, we discovered a broad array of host-fungal relationships and traits to predict how individual fungi affect plant survival and productivity. However, fungal effects on the plant host vary with both the abiotic environment and biotic interactions, challenging our ability to scale from simple to complex systems. Although symbiotic fungi like foliar endophytes represent potential new management tools for agricultural ecosystems, their implementation in real-world systems requires a better mechanistic understanding of drivers of both community assembly and function.