Here is a photograph of a pear that started to go bad in my fruit basket. I discovered it one evening with a perfect bullseye pattern of mould. Sixty hours later it had grown more (partial) rings of mould. Another 48 hours later it had grown still more partial rings, always separated by the same gap and all still roughly concentric. At that point it was getting pretty rotten, so I threw it away. What causes the mould to grow in rings like this?
鈥� The 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 鈥渂rowning鈥� reaction seen in the photograph which gives the disease its name.
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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.
鈥淣ew spores are produced with each period of daylight leading to the appearance of rings鈥�
A parallel situation can be seen in the 鈥渇airy rings鈥� of dense, green grass growth and toadstools that appear in lawns 鈥� 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