Willows (Salix spp.) are a very diverse group of catkin-bearing trees and shrubs that are widely distributed across temperate regions of the globe. Some species respond well to being grown in short rotation coppice (SRC) cycles, which are much shorter than conventional forestry. Coppicing reinvigorates growth and the biomass rapidly accumulated can be used as a source of renewable carbon for bioenergy and biofuels. As SRC willows re-distribute nutrients during the perennial cycle they require only minimal nitrogen fertilizer for growth. This results in fuel chains with potentially high greenhouse gas reductions. To exploit their potential for renewable energy, willows need to be kept free of pests and diseases and yields need to be improved without significantly increasing the requirements for fertilizers and water. The biomass composition needs to be optimized for different end-uses. Yields also need to be sustainable on land less productive for food crops to reduce conflicts over land use. Advances in understanding the physiology and growth of willow, and in the identification of genes underlying key traits, are now at the stage where they can start to be used in breeding programs to help achieve these goals.
This chapter covers the following: variability in virulence and pathotypes within Melampsora larici-populina on poplar; variability in the pathogenicity of M. larici-populina, M. allii-populina and M. medusae and its effects on host resistance; effects of the breakdown of complete resistance; adaptation of poplar cultivation to the evolution of pathotype populations; and evaluation of resistance in clones considering the diversity in virulence within M. allii-populina and M. larici-populina.
There are eight Melampsora species infecting poplars in Europe (Pinon, 1973; Cellerino, 1999). Three of them, M. laricipopulina, M. allii-populina, and M. medusae are pathogenic on the poplars of the sections Aigeiros and Tacamahaca, i.e. P. nigra, P. deltoides, P. trichocarpa, and their interspecific hybrids. Most of the commercial poplar cultivation in Europe is made of P. × euramericana (P. deltoides × P. nigra) and P. × interamericana (P. trichocarpa × P. deltoides) hybrids. Rust was the most damaging disease of poplars in the past decade (Chapter 12, this volume). Five other Melampsora species, M. pinitorqua, M. larici-tremulae, M. rostrupii, M. magnusiana, and M. pulcherrima, are pathogenic on species of the Populus (formerly Leuce) section, i.e. P. alba, P. tremula, and their hybrids. Although these five species are difficult to distinguish, since they have in common evenly echinulated urediniospores, Pinon (1973) proposed a diagnosis key based on the morphology of the urediniospores and the paraphyses. Some authors follow the proposal of Wilson and Henderson (1966) to adopt M. populnea as a
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