The rotting of fleshy vegetables, such as potatoes, is typically due to infestations of Erwinia carotovora var bacteria. These bacteria form spores and are facultatively anaerobic, so they can grow in the presence or absence of oxygen. Erwinia carotovora var can grow at temperatures ranging from 3 to 36 degrees Celsius, which is approximately 37 to 95 degrees Fahrenheit. This bacteria is spread among potatoes via water and insects.
A good science fair project should follow the steps of the scientific method. The first part is observation, which involves doing some background research to better design the project and to formulate the underlying question that the project will try to answer. The hypothesis is a prediction of the solution to your question. The experimental design is how you will test your hypothesis. Finally, you will have the results of your experiment and your conclusion, in which you answer the question based on the results you obtained.
One possible topic for a science fair project involving rotting potatoes is finding the temperature at which potatoes will rot the quickest. For this project, the hypothesis would be that some temperature in between 5 and 35 degrees Celsius, such as 15 degrees Celsius, will cause potatoes to rot the fastest. Your method will involve storing your potatoes at various temperatures, such as at room temperature, the temperature under a refrigerator or under a heat lamp, and watching them for signs of rot. For the best results, make sure that your potatoes are roughly the same size and shape. Then record when the first signs of rot begin on each potato as well as the area of the potato affected by rot over time.
Another experiment that can be done, either separately from or in conjunction with the temperature experiment, is studying whether some types of potatoes rot more quickly than others. This project will be largely the same as the temperature one but instead of varying the temperature you will vary the types of potatoes examined. With this experiment it is essential that all the potatoes be stored in the same place to avoid changes in temperature or humidity that could otherwise affect rotting rate.