This chapter considers the challenges associated with the use of biocontrol agents in foliar spray applications, including the equipment, spray parameters, formulation and their impact on treatment efficacy. It discusses the various factors that affect effective foliar treatment and provides examples of how they can be optimised. These factors include the persistence of biocontrol agents, the impact of spray systems on surface coverage, the potential of equipment and adjuvants to develop more targeted foliar spray applications, and finally the rainfastness of biocontrol agent.