Which substance is primarily used to kill fungi?

The primary use of a fungicide is to specifically target and kill fungi, including various types of fungal pathogens that can affect plants, crops, and even humans. Fungicides work by disrupting the cellular processes of fungi, preventing their growth and reproduction. This is particularly important in agriculture and horticulture, where fungal diseases can cause significant damage to crops.

In contrast, herbicides are designed to kill unwanted plants or weeds, while insecticides target insects. Adulticides are a specific type of insecticide that focuses on killing adult insect pests. Since the question specifically asks about a substance used to kill fungi, fungicide is the correct choice, as it directly addresses the issue of fungal infestation and control.