Breeding Willow for Short Rotation Coppice Energy Cropping - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 74

functional groups with alternative growth strategies for high biomass production.
The first had a large number of stems (typically 11 per stool), relatively low leaf
area index (LAI) and specific leaf area (SLA), but high foliar N concentrations and
wood-specific gravity. The second group had a small number of large diameter
stems (typically 6 per stool), high LAI and SLA, but low foliar concentrations of N
and low wood-specific gravity [ 14 ]. These findings are reflected in the current
European varieties, with the majority (e.g., Tora) falling into the former class,
and some varieties exhibiting a “wall of stems” (e.g., Terra Nova). From these
studies it would seem that that multiple ideotypes may need to be incorporated into
willow breeding and selection efforts.
It rapidly became evident that leaf-infecting rust caused by Melampsora spp.
was a major threat to achieving the yield potential. Research effort was placed on
the causal fungus and on the genetics of resistance, particularly as some early
sources of resistance proved durable only in the short term.
The most durable source of resistance so far in western Europe has come via the
accession L79069 used in the Sval¨f Weibull breeding program in Sweden
(described earlier). The first bred products were the cultivars Tora and Bjorn.
Subsequently, Bjorn was used as a parent to create a mapping population segregat-
ing for the rust resistance in order to identify the underlying genetic basis. A single
locus with a major effect was identified as responsible for conferring resistance,
although several more loci with minor effects were also identified. The study also
revealed that the vast majority of cultivars on the market were reliant upon the
major locus based resistance mechanism derived from L79069. This resistance has
proved durable so far, but planting a perennial crop with a life expectancy of 20 or
more years that is reliant upon one source of resistance can clearly be considered
high risk. Therefore, identifying and incorporating new, unique sources of resis-
tance into the breeding pool and stacking multiple sources of resistance into
cultivars released to the market are a breeding priority.
Melampsora rust has infected willows in the Northeast United States also.
Damage has been less severe than in parts of northern Europe. Insect pests are a
greater threat to willow cultivation in the Northeastern United States. S. viminalis is
particularly sensitive to potato leaf hopper Empoasca fabae (Harris) damage, but
because it conveys many useful traits for biomass production, it is used in inter-
specific hybrids such as S. viminalis x S. miyabeana . Further north, in Canada, leaf
hopper damage is less of a concern. In continental areas, where the winters are
particularly cold, an early and reliable (in time) autumn senescence is sought.
Native S. eriocephela routinely survive these conditions and certain S. viminalis
genotypes from Europe are proving promising potential to lift the yield potential
under such conditions.