What characterizes contact fungicides?

The defining characteristic of contact fungicides is that they require direct contact with the fungus to be effective. Unlike systemic fungicides that are absorbed into the plant and provide protection from within, contact fungicides work by creating a barrier on the plant's surface that inhibits the fungus when the fungicide is applied directly to the affected areas. This means that for contact fungicides to be effective, they must thoroughly cover the parts of the plant that are susceptible to fungal infections.

The other answer options reference different modes of action or behaviors that do not pertain to contact fungicides. For instance, the notion of penetrating plant tissues or being absorbed by plant roots refers to systemic fungicides, which are designed to be taken up by the plant and provide internal protection against diseases. Additionally, stating that contact fungicides work only during rainfall mischaracterizes their application; they can be effective in dry conditions as long as they make direct contact with the target fungus.