Which characteristic best defines a fumigant?

The characteristic that best defines a fumigant is that it acts as a gas but is formulated in solid or liquid form. Fumigants are unique because they are designed to volatilize and penetrate soil or other materials as a gas, effectively targeting pests, diseases, and weeds in enclosed spaces. This gaseous state allows for deep penetration into the substrate, which is crucial for effectively controlling pests that may be located below the surface.

This characteristic highlights the versatility of fumigants as they can be applied in various settings, including soil, storage areas, and structures, ultimately ensuring the intended pest control effect. The ability to be formulated in solid or liquid form before application allows for easier handling and application, while their conversion to gas gives them their primary mode of action.

The other options provided do not accurately capture the defining nature of fumigants. Although some can be applied through irrigation systems, this does not exclusively define them; and visibility during application does not contribute to their key characteristics. Furthermore, while fumigants are often available as solid formulations, their defining feature is their transformation into gas, not their physical state before application.