Pathogens, known to most people as germs, are viruses, bacteria and fungi that cause disease when they infect other organisms. Because pathogens are microscopic they present special problems for detection and management.
We have trouble preventing or controlling disease outbreaks that threaten human life (think bird flu or SARS) let alone germs that attack trees!
Phytopthera cinnamoni, aka the "botanical bulldozer", is a fungus introduced by Europeans bringing new plants into the colony. It is currently bulldozing entire species of Australian plants to the brink of extinction.
Exotic pathogens are a serious threat to species and the integrity of ecosystems on land as well as fresh and marine waters. They can devastate industries as well as the environment on which we all depend.