What term describes a substance used to control harmful organisms on plants?

The term that describes a substance used to control harmful organisms on plants is "pesticide." Pesticides are a broad category of chemicals used to manage pests, including insects, weeds, fungi, and other organisms that can damage crops or harm plant health.

Pesticides can be formulated to target specific types of pests, and they encompass various subcategories such as herbicides (which control weeds), insecticides (which control insects), and fungicides (which control fungal diseases). The broad nature of the term "pesticide" allows it to cover any substance that serves this purpose, making it the most comprehensive and appropriate choice in this context.

Fertilizers, while important for plant growth and health, are not used to control harmful organisms; instead, they provide essential nutrients to plants. Hence, although beneficial for plant growth, fertilizers do not fall under the definition of substances that control harmful organisms. Similarly, herbicides and insecticides are types of pesticides that specifically target weeds and insects, respectively, but they do not encompass the full range of substances aimed at pest control like the broader term "pesticide" does.