The prevention of powdery mildew outbreaks is among grape growers' most important management practices. The explosive episodes of powdery mildew growth that are possible when optimal weather conditions prevail pose substantial production risks to growers. The Gubler-Thomas Powdery Mildew Index (PMI) models powdery mildew growth as a function of length of exposure to different temperature ranges in order to help growers to better anticipate outbreaks. This project aims to understand how growers' use of the PMI shapes their powdery mildew control strategies according to their operational environmental and how these potentially heterogeneous forecast responses determine the environmental impact of the PMI. We find that the environmental impact of the PMI may actually be negative because growers' adjust their powdery mildew management strategies along several dimensions in response to the information–including shifting to higher potency synthetic fungicides, increasing dosage rates, and using multiple products when the risk is high according to the PMI.