The following answer was selected and edited by New Scientist staffThe pear shown in the photograph is suffering from brown rot disease which is caused by the pathogenic fungus Monilinia fructigena. This is a very common and widespread disease of apples, pears and stone fruits and spreads through the air as spores. The spores germinate on areas of damaged fruit, attacking it where the fungus has easy access to the unprotected, nutrient-rich fleshy parts inside.The fungal threads, or hyphae, grow and branch within the tissue and degrade the flesh. At first, the disease is invisible to the naked eye, but as it spreads, the pear responds with the typical "browning" reaction seen in the photograph which gives the disease its name.As it grows, daylight prompts the fungus to produce more spores on specialised hyphae which grow back out of the skin, forming the grey-brown pustules you can see in the photograph.A new crop of fungal spores is therefore produced with each period of daylight, and the fungus continues to grow through the flesh forming successively larger rings each day, giving this typical appearance.New spores are produced with each period of daylight leading to the appearance of ringsA parallel situation can be seen in the "fairy rings" of dense, green grass growth and toadstools that appear in lawns - again it is a visible manifestation of a microscopic fungus growing beneath the surface.In this case, however, it is the fungus breaking down organic matter in the soil which causes the release of nutrients to stimulate grass growth and provide the essential energy to form the spore structures of the fairy ring toadstool.Peter Jeffries, Faculty of Science, Technology and Medical Studies, University of Kent, Canterbury, UK

but why a ring? why not a random pattern?

The fungus infects the flesh starting at some point, then grows as fast as it can in all directions. Assuming the fruit is uniform through its volume, and your home is about the same temperature at the same time of day, each day's growth will extend the outer edge of the infected zone by the same amount. The best-nourished hyphae are the ones that do the reproducing; that is, the ones that have fed most recently on the freshest food.

The damage patterns is most likely caused by the transportation of the fruit. The basket or paper it was transported in caused some damage wich the fungus uses to grow.

I found a similar pattern on a rotting apple still on my tree. It's interesting to note that the circular pattern only appeared on the side of the apple facing the sunlight. The shaded side had a more random covering of mould.Picture here: http://www.jelbert.com/mouldyapple.jpg RegardsRichard