Cecidology time again

Autumn is a time when naturalists may turn their attention to plant galls hunting - or cecidology. Adult insects are becoming more scarce and the more exciting flowers have mostly gone to seed. Our keen eyes now search out leaf mines - the patterns left by tiny tunneling insect larvae - or galls - amazing plant growths formed by unbelievably complex processes. Most plant galls, like the familiar oak apples, marble galls and spangle galls, are caused by mites, bugs, wasps, flies or other insects, but some are caused by fungi. This is the case with this colourful little gall found on pear leaves. This barnacle-like growth on the underside of the leaves is home to one stage of a rust fungus called Gymnosporangium sabinae, also helpfully known as 'pear-leaf cluster-cups'. The upper side of the leaf displays a bright yellow-red blotch, as can be seen in the background. As with many rusts, this one has a sort of alternation of generations. The spores that are eventually released from the hanging  brush-like structures are blown on the wind and need to land on an introduced variety of juniper bush in order to germinate and develop into the next stage of the life cycle of this fungus.