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.
Twenty inter-and intra-species genotypes of willows (Salix spp.) were grown in large mono-plots and incorporated into five, 10, 15 and 20-way mixtures. In each growing season from 1996 to 2001 the level of rust disease caused by Melampsora epitea was recorded on each genotype where it was growing as a mono-plot or as part of a mixture. Three genotypes, S. schwerinii × viminalis × dasyclados 'V7531', S. schwerinii × aquatica 'V7533' and S. viminalis 'Gigantea' remained virtually rust free through the six yr of the trial. It is, however, argued that there are benefits of including such genotypes in a mixture in order to reduce the risk of them developing rust susceptibility. S. burjatica 'Germany', S. dasyclados × aquatica 'V7511' and S. dasyclados × caprea 'V794' were severely affected by rust, although levels tended to be less when included in mixtures. The levels of rust on S. mollissima-undulata 'SQ83' were much higher in the first three-year harvest cycle than during the second cycle. On a number of genotypes, e.g. S. burjatica 'Germany' and S. dasyclados x aquatica 'V7511' rust was more severe on the regrowth from freshly coppiced stools.
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