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The ability of a pathogen to cause disease in specific host species is termed virulence. For example, Erwinia amylovora causes a disease in pear trees called fireblight, and Erwinia carotovora causes a disease in potatoes called soft rot. E. amylovora then is considered to be virulent on pear trees but avirulent on potatoes; the opposite is true for E. carotovora. This difference in virulence can be traced to differences in the genetic capabilities of each organism specific for the host it attacks. Often, in a laboratory, this difference cannot be detected readily by doing simple biochemical tests and may require more advanced tests or actual inoculations into different hosts.

When associating a specific organism with a particular disease (and vice versa) Koch's Postulates may be invoked. In the 1870's, in connection with experiments on anthrax and its causative agent (Bacillus anthracis), Robert Koch advanced the following set of logical steps:

1. The organism in question is associated with a particular disease in a particular host.
2. The organism is isolated and should be characterized as fully as possible.
3. The organism is then inoculated into a healthy host, and the same disease is reproduced.
4. The organism is then reisolated and characterized, and it is identified as the same organism as in step 2.

For more information on host microbes interactions and disease, read the chapter on Host-Microbe interactions in the microbiology textbook. In the following experiments, which deal with plant diseases, we will attempt to follow Koch's Postulates.

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