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.

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Three nurseries produced apple rootstocks (M9) and budwood (cv. Royal Gala), which they exchanged at the end of the first year. Each nursery then budded its own budwood onto the rootstocks it had produced and that from the other two nurseries. Budded trees were grown on for a further year before being planted at HRI, East Malling in southern England; NIHPBS, Loughgall in Northern Ireland; and ADAS, Rosemaund in the West Midlands of England. Canker development was monitored twice a year. The position of the infected trees within the orchard was recorded, as was the position of the canker on each tree (main-stem or peripheral). Nectria galligena was isolated from representative cankers and analysed using molecular techniques. At the sites in Northern Ireland and HRI there was a strong positional effect, especially of peripheral cankers, indicating that most of the inoculum was external and had been spread from neighbouring orchards. There was little or no positional effect on main-stem cankers at any of the three sites. The proportions of different isolates taken from peripheral cankers was different in Northern Ireland from that in England, suggesting different populations associated with the geographic areas. In contrast, the populations of N. galligena obtained from main-stem cankers were very similar in England and Northern Ireland. It was concluded that a small proportion of trees developing canker were infected during propagation, with no symptom development until after planting. In a second trial it was demonstrated that trees infected during the propagation phase, and particularly at budding and heading back, could develop canker up to 3 years later. While it is clear that some canker developing in the orchard can be associated with the nursery of production, in climatic conditions conducive to the formation and dissemination of conidia, inoculum from surrounding infected orchards is the primary source of the pathogen. Aerial spread is therefore an essential element of the epidemiology of N. galligena , and its control is a crucial part of any canker-control programme.

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

SuinmaryA review is given of the use of clonal mixtures in the development of disease control strategics in short rotation coppice willow in Northern Ireland. Salix burjatiea 'Korso' had been grown sueeessfully for over 10 years when, in 1985, Melampsora epitea var. epitea caused serious problems. Although fungieides were effeetive, their use was not praetieal for environmental and economic reasons. Therefore, in 1987 large scale field experiments were initiated to investigate the use of elonal mixtures as a disease control strategy. Increased yields were consistently reeorded from mixed stands when eompared to either the mean yield of eomponent clones or the individual yields of any of the component clones grown in monoculture. Part of this increased yield was due to a reduetion in the impact of rust disease. Investigations are currently being conducted on the effeet of number of clones within a mixture, the best clones to use and the optimum planting density. It is essential that there is a range of susceptibilities to M. epitea var. epitea pathotypes within the components of the mixture.