Chemicals are single man made products or compounds, not found in nature. They are used to force things we want, that would not happen in a natural system. Chemicals often disrupt the natural balance.
Chemicals are helpful in fighting fungi, insects and plant diseases in general. They are also used to fertilize soil. When used too much of it, it starts to become a problem. Binding phosphates in the soil, creating resistant bugs, creating super weeds, poisoning people who eat chemical residue, creating hard pan and kill soil life, etc. etc.
See also: www.fallarmyworm.eu
Spraying Rebearth on plants and soil supports and stimulates the natural system to take care of itself. Nature’s design is perfect in its own way, but we did not know how to handle the system for a long time. Since the invention of Rebearth we know a lot more about how nature works and how to support it. If you support the plant’s own system, it can take care of fungi, pests, insects and moulds most of the time.