Out of the tragedy of the Irish potato famine was to emerge a major new discipline in science - plant pathology. Infectious micro-organisms would come to be accepted as a cause of disease rather than its result. Kathy Willis hears from Kew's head of mycology, Brin Dentinger, on the significance of German botanist Antony de Bary's experiments that would lead to a new understanding of the causes of potato blight. Insights into the life cycle and behaviour of fungal spores required detailed and repetitive observations. Some of the most important insights in the 19th century came from children's story writer and natural history illustrator Beatrix Potter. Historian Jim Endersby explains how her careful observations contributed to the controversial idea that many fungi, far from being destructive, live in symbiosis with a host of plants. Kew mycologist Martin Bidartondo studies this relationship and we hear how thanks to new technology enabling researchers to identify fungal DNA we're on the brink of elucidating the real importance of fungi in today's ecosystems. Producer Adrian Washbourne Presenter: KATHY WILLIS is director of science at Royal Botanic Gardens at Kew. She is also professor of long-term ecology and a fellow of Merton College, both at Oxford University. Winner of several awards, she has spent over 20 years researching and teaching biodiversity and conservation at Oxford and Cambridge.