What structures do fungi use to grow on their host?

Fungi grow and infiltrate their host through specialized structures known as hyphae and mycelium. Hyphae are thin, thread-like filaments that extend out from the fungal organism, allowing it to absorb nutrients from the environment or its host. When these hyphae grow and form a mass, they create mycelium, which is the vegetative part of the fungus. This extensive network of hyphae increases the surface area for absorption, enhancing the fungus's ability to thrive on its host.

In the context of the options provided, roots and shoots pertain to plants, while conidia and spores relate to fungal reproductive structures, not the ones involved in growth on a host. Leaves and branches also belong to the plant kingdom. Therefore, the most accurate choice regarding the structures fungi use for growth on their host is hyphae and mycelium.