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Who Done It? Or what's that brown fuzzy stuff on my plum? 
(Koch's Postulates for Proof of Pathogenicity)

A safe and simple exercise that uses Koch's postulates to prove that an observed fungus is the cause of  fruit disease. This lesson is suitable for grades 7-12 and does not require the purchase or maintenance of special cultures. Since the fungus that causes brown rot of stone fruit (e.g., apricots, peaches, nectarines, plums, cherries) is present naturally on the surface of these fruit, stone fruit purchased from the supermarket or farm stand will usually develop the disease. Italian or prune plums are usually the least expensive stone fruit to buy and work very well in this exercise. The fungi responsible for brown rot of stone fruit are not human pathogens. This lab requires dissecting and compound microscopes. A simplified exercise, without cultures, to demonstrate the Germ Theory also is described.

Jasalavich, C.A., and G.L. Schumann. 2001. Who Done It? Or what's that brown fuzzy stuff on my plum? The Plant Health Instructor. DOI: 10.1094/PHI-K-2001-1128-01
Updated, 2006.