Contact fungicides are chemical agents used to control fungal diseases on plants. They work by forming a protective barrier on the surface of leaves and stems, preventing fungi from penetrating and causing harm. These fungicides are typically applied directly to the plant and are effective as long as they remain on the surface. Unlike systemic fungicides, which are absorbed by the plant and provide internal protection, contact fungicides require thorough coverage for optimal effectiveness. They are often used in agricultural practices to manage diseases like powdery mildew and rust, helping to maintain healthy crops and improve yields.