The natural occurrence of Melampsora rusts in biomass willow plantations for renewable energy was investigated at four sites in the UK during 1992-1994. Three sites, located at Long Ashton, south west England, Loughgall, Northern Ireland, and Craibstone, Scotland, were rust/clone trials that contained standard 24 willow clones and the fourth site at Markington, northern England, had a similar range of willows. Larch-alternating M. epitea (M. lariciepitea) was found to be most prevalent. Melampsora capraearum was predominant on S. caprea hybrids and the stem-infecting form occurred mainly on S. viminalis ＇Bowles Hybrid＇. To identify different pathotypes in larch-alternating M. epitea, field rust samples were tested for pathogenicity in the laboratory using eight willow differentials. A total of 12 pathotypes were identified. More pathotypes were found at Long Ashton, Loughgall and Markington, where extensive, long-term willow plantings were grown, than at Craibstone, where the planting was more isolated. A major breakdown of rust resistance in S. x mollissima ＇Q83＇ in the UK in 1992 was caused by the emergence of a new pathotype. Our results also suggest that the larch-alternating M. epitea found in UK biomass willow plantations may consist of genetically separate populations. This would provide an opportunity to plant willow clones that harbour different mating populations, in mixed plantations in order to enhance the property of mixtures to suppress rust.