Applied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast BT9 5PX, Northern Ireland, UK

Background and objectives
Willow grown in short rotation coppice (SRC) offers a potentially important source of renewable energy. It is CO2 neutral, and also provides an alternative commodity to enable farm diversification in situations of over-production. Many of the agronomic problems associated with SRC willow have been solved, and there are several potentially high-yielding clones now available for planting. However, many of the clones currently being grown are susceptible to foliar rust caused by Melampsora epitea var. epitea [1]. This disease can significantly reduce yield, and in severe cases can wipe out a complete clone. Furthermore, the pathogen is continually adapting so that clones which were previously resistant may become susceptible. In Northern Ireland the possibility of using clonal mixtures as a disease control strategy has been investigated over a number of years [2]. The objective of the work described here is to determine the effectiveness of clonal mixtures to reduce the impact of rust and to evaluate the optimum number of clones within a mixture, the effect of planting densities and the performance of more recently released clones in mixtures.

Materials and methods
Large plantations of willow were planted in 1994 and 1995, comprising 20 clones in monoclonal plots and included in five-, 10-, 15- and 20-way mixtures. Planting densities were 10,000, 15,000 and 20,000 stools/ha. In 1996 and 1997, the level of disease was recorded throughout the growing season on individual clones in both monoclonal and polyclonal plots.

Results and conclusions
In the first year of regrowth from freshly coppiced stools, there was significantly less disease on clones growing in mixtures as compared to clones with a medium to high level of rust in monoclonal plots. Where there was a low level of disease, there was no reduction in disease when they were included in a mixture. The number of clones in a mixture had little impact on the level of rust on an individual clone, and no significant differences were recorded at the different planting densities. The pattern of disease development on stools in their second year of regrowth after coppicing (1997) was largely similar.

In the first year of regrowth, yield increases were recorded in the five- and 10-way mixtures compared to the component monoclonal yields. These higher yields were not observed in the 15- and 20-way mixtures. However, when final harvests are done in 1998 on 3-year-old growth, it is expected that the yield patterns will show different trends.

Significant new areas of willow are being planted in various parts of the UK. The use of clonal mixtures can be recommended with confidence as an effective and practical method of reducing the impact of rust disease, ensuring the long-term stability and sustainability of plantations, and reducing the disease pressure on new clones which have a degree of resistance to the rust. There are also significant yield benefits when all stools survive, and the possibility of neighbouring stool compensation when individual stools die out.

McCracken AR, Dawson WM, 1997. European Journal of Forest Pathology 27, 319-329.
McCracken AR, Dawson WM, 1998. European Journal of Forest Pathology, in press.