What is the primary function of contact fungicides?

The primary function of contact fungicides is indeed to create a barrier on the plant's surface. These fungicides work by remaining on the outside of the plant tissues, preventing fungal spores from germinating and establishing infections. When applied correctly, contact fungicides protect the treated surfaces from fungal pathogens by effectively blocking their access to the plant.

This mode of action makes contact fungicides particularly valuable for managing diseases caused by various fungi, as they can prevent initial infections before they begin. In contrast, systemic fungicides are absorbed by the plant and can move throughout the tissues, targeting established infections from within, which is why option describing systemic action does not apply to contact fungicides.

The other choices focus on roles that are not applicable to contact fungicides. For instance, inhibiting the growth of bacteria is more aligned with antibacterial treatments, and preventing insect infestations pertains to insecticides rather than fungicides. Overall, the effectiveness of contact fungicides lies in their ability to provide a protective barrier that helps maintain the health of plants against fungal threats.