In recent years, there has been increased interest in growing willow and poplar trees, as fast-growing species that have several purposes, including use as biofuels for energy production. However, silviculture of these trees has been constrained by diseases such as Melampsora rusts. This book provides a comprehensive review of over two decades of extensive study of the rust diseases affecting willow and poplar. It provides insights into the population biology of Melampsora rusts in Europe, China, India and Chile, the genetics of their resistance, and their interaction with their hosts. The book offers information essential to the development of effective and sustainable disease control measures including the use of willow genotype mixtures and biological control agents.