Horticultural oils are generally classified as having a physical mode of action, rather than a chemical one like synthetic insecticides. The most common way that horticultural oils work is to coat and clog the spiracles, which are pores that insects use to breathe. Additional effects such as disruption of membrane functions and interfering with the sucking or piercing actions that some insects use to eat.
Fungal diseases like powdery mildew and sooty mold can be controlled with horticultural oils in three ways:
Controlling the insects that carry them, trapping fungal spores, which prevents them from spreading, and coating plant surfaces, making it difficult for fungi to adhere to the host plant.