Rust occurs on many species of plant, but in most cases any one species of rust can only infect one species of plant. The following describes the infection process of asexual spores.
- Spore Attachment
When a rust spore lands on a plant surface it needs to attach to it, or it would simply be washed off. First, weak, hydrophobic interactions are formed between the spore and the plant cell surface. Then unknown signals cause the production of a substance called adhesin. This will stick the spore irreversibly to the plant surface. Once attached, the spore will germinate.
- Germ Tube Elongation
Rust fungi penetrate the plant by using the natural pores on the underside of a leaf, but first the growing germ tube must locate it. The germ tube grows in a random manner until it reaches a ridge. At this point it will start to grow perpendicular to the ridge, greatly increasing its chances of locating a pore (called a stomata).
- Appresorium Formation
The stomata is the site of a structure called an appresorium that functions to both firmly anchor the fungus and aid in penetration. From the appresorium an infection peg grows down into the plant and between the leaf cells.
- The Haustoria
Rust fungi gain their nutrients from living cells. This requires a specialized penetration of the fungi into a living plant cell called a haustoria. This develops from a haustorial mother cell. The plant cell membrane surrounds the main haustorial body. An iron- and phosphorus-rich neck band bridges the plant and fungal membranes and acts as a seal preventing the escape of nutrients into the plant. The rust fungi will then continue to grow and invade the plant until it is ready to make new spores.
