Breeding Willow for Short Rotation Coppice Energy Cropping - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 73

breeding effort so far. The challenge will be to maintain this rate of gain. Modern
molecular breeding techniques offer such an opportunity.
Disease control has also been a major breeding objective. Leaf rust ( Melampsora
spp.) resistance has been an ever-present challenge and a primary target. Leaf rust
can severely defoliate willows and reduce yields by up to 40 %. Early resistant
cultivars soon saw their resistance mechanism overcome by the fungal pathogen.
Most recently the high-yielding hybrid Ashton Stott has suffered complete break-
down in resistance. However, successful breeding of resistance derived from
S. schwerinii (or possibly an S. schwerinii x S. viminalis hybrid) collected in Siberia
(L79069) has persisted largely unbroken for more than 20 years (see later).
Other major advances have been the identification of QTLs affecting important
agronomic traits, particularly yield and component traits (stem diameters and
heights, shoot numbers). Many yield traits have been found to collocate to the
same regions of the chromosomes [ 12 ]. Markers for these QTLs are already being
used in marker-assisted selection at Rothamsted.
Target Traits and Current Breeding Goals
Breeding objectives fall into two categories. The first is the creation of high
potential yield and the ability to achieve that yield under stress from biotic and
abiotic challenges. This incorporates many of the traits that breeders of a diverse
range of crops would seek to improve such as pest and disease resistance and
drought tolerance. Secondly, there are quality traits that are specific to the
bioenergy sector. Minimizing inorganic components of the harvested biomass is
important when thermochemical conversion is the target market, while when
biological conversion technologies are targeted, the organic composition of the
biomass and the ease with which biological processes may access the energy
contained therein take precedence.
Yield and the Protection of Yield Potential
Early willow breeding work was relatively simple. Willows are dioecious and are
heterozygous. Crosses give rise to highly variable F 1 progeny from which it was
possible to select the highest yielders and propagate them vegetatively for trialling
and selection. Often early selections were based on visual assessments; subse-
quently, when sufficient planting material was generated to carry out designed
experiments, yield could be directly measured. However, willows are highly var-
iable, and previous studies suggest that more than one growth strategy may result in
high biomass. Stott [ 13 ] noted that some high-yielding willows had large numbers
of thin stems, while others had fewer stems with large diameters. Recent research in
the United States on 32 different willows concluded that there are at least two