Plants cannot run away from pests or pathogens, so they have defences. The first line of defence is a set of physical barriers that stop a pathogen getting into the plant, or stop a pest getting to the tissue it feeds on.
The surface of a leaf is covered by the cuticle, a waxy layer. Pathogens such as fungal spores find it hard to get through. A thicker cuticle makes infection less likely. The wax is waterproof, so rain runs off the leaf instead of forming a film of water where pathogens could grow.
Every plant cell is also surrounded by a cell wall made of cellulose. This is tough, so a pathogen has to break through it to reach the inside of the cell. On trees, bark is a thick layer of dead cells that protects the stem. Some plants also have thorns, spines or hairs that make it harder for animals to eat them.