Does host genotype diversity affect the distribution of insect and disease damage in willow cropping systems?
Planting of willow genotype mixtures for biomass production has been suggested as a non-chemical strategy for pest management. Basic information on spatial dynamics of important insects and pathogens is necessary for the effective deployment of host genotypes in a plantation. In 1998 and 1999, in Bristol, UK, the degree and spatial distribution of damage by Melampsora spp. and Phratora vulgatissima were studied concurrently in a field trial containing monocultures of willow Salix genotypes with different willow rust and beetle susceptibilities, and two design mixtures (random or regular) of three or five genotypes (Salix burjatica cv. Korso, S. dasyclados, S. stipularis and S. viminalis cultivars Bowles Hybrid and Mullatin). For both years, there was more rust and beetle damage on plants in monocultures than in mixtures. There were significant differences in the vertical distribution of beetle damage along stems between plantation designs for the 2 years, yet only in 1999 for rust. Rust severity along stems was significantly correlated between leaves, indicating localized spread of disease. In contrast, beetle damage severity along stems was weakly correlated between alternate leaves and leaves farther apart, suggesting beetle movement from tree to tree. In 1998, spatial distribution of rust was aggregated in 67% of plots studied and in 40% for beetles. There was no significant difference in the distribution of beetle damage between planting design, but rust was aggregated in 75% of mixtures and 33% of monocultures. While beetle damage distribution was similar between years, rust was aggregated in all monoculture plots but in only 8% of mixtures in 1999. The difference between years and design for rust was probably because of the later stage of rust development in 1999 and the delaying effect of mixtures on this development. In 1998, there were significant negative correlations between the extent of rust and beetle damage on individual trees. However, only 20% of plots showed a significant spatial dissociation between these two types of damage. There were fewer discernible interactions in 1999. Plantation design provides the basis for integrated control of rust and beetle damage within willow cropping systems by delaying the spread and development of both organisms. Both pest and disease can be assessed successfully simultaneously under field conditions, a major saving in human resources. Initial selection of willow mixture configuration should primarily take into account the effects of spatial design on rust development.