Citrus fruit (oranges, tangerines, grapefruit or lemons) purchased in a grocery store can be experimentally infected with readily-available sources of "Penicillium digitatum" to demonstrate the four basic steps of Koch's postulates, also known as proof of pathogenicity. The mould is isolated from naturally-infected citrus fruit into pure culture on artificial agar medium. The mycelium is then reintroduced into uninfected fruit from which the pathogen can be recovered. The entire set of procedures can be completed in as little as four weeks, using easily available equipment. In addition to demonstrating the importance of associating a disease with its causal agent, the exercise also allows for practicing sterile technique. None of the hundreds of students who performed these steps in college lab settings has ever experienced an allergic reaction to the fungus, making this a very safe, quick and inexpensive demonstration.