I enclose a sequence of three photos of a pear that started to go bad in my fruit basket. I discovered it one evening with a perfect bullseye pattern of mould, as in the left picture. Sixty hours later it had grown more (partial) rings of mould, as shown in the middle photograph. Another 48 hours later it had grown still more partial rings, always separated by the same gap and all still roughly concentric (right-hand pic). At that point it was getting pretty rotten, so I threw it away. What causes the mould to grow in rings like this?Bob Ladd, Edinburgh, UK