Shrub Willow - Bioenergy Crop Plants - page 701

host-specific, such as Chrysomelid beetles, including Phratora vitellinae and Phratora vulgatis-
sima (brassy and blue willow beetles) in the United Kingdom and Plagiodera versicolor (imported
willow beetle) in the United States (Peacock et al. 2002, 2004; Nordman et al. 2005). Leaf-sucking
insects, such as the potato leaf hopper ( Empoasca fabae ), and stem-sucking insects, such as the
giant willow aphid ( Tuberolachnus salignus ; Figure 28.6) and the black willow aphid ( Pterocomma
salicis ), can also debilitate fast-growing willows (Collins et al. 2001). Planting fields with a random
mixture of multiple, genetically diverse, structurally different varieties has been shown to be effec-
tive in limiting the build-up of local populations of insects, which would reduce the need to use
pesticides (Peacock et al. 2001; Dalin et al. 2009).
28.7 GenetIc ImProvement For Increased yIeld and resIstance
Increases in yield achieved through genetic improvement will reduce the overall cost of producing
willow biomass and will encourage wider adoption and long-term sustainability of the crop. Willow
breeding began in North America in the early 1980s at the University of Toronto in Ontario, Canada
under the direction of L. Zsuffa (1988) using S. eriocephala , S. exigua , S. lucida , S. amygdaloides ,
S. bebbiana , S. pellita , S. petiolaris , and S. discolor . They verified that variations observed in
plant biomass, moisture content, and specific gravity were traits associated with species differences
(Mosseler et al. 1988). Furthermore, Zsuffa and Mosseler investigated species hybridization,
including pollination barriers, crossability relationships, and hybrid performance (Mosseler and
Papadopol 1989; Mosseler and Zsuffa 1989; Mosseler 1990).
Breeding programs for willow bioenergy crops also emerged in Sweden in the 1980s, including
the industrial breeding program directed by S. Larsson at Svalöf Weibull AB and the academic
research programs including U. Gullberg and A. Rönnberg-Wästljung at the Swedish University of
Agricultural Science in Uppsala. The original emphasis was placed on producing clones with high
biomass and good form using various species. S. viminalis , S. dasyclados , S. schwerinii , S. triandra ,
S. caprea , S. daphnoides , and S. eriocephala were the most common species used (Larsson 1997).
The main gene pool for the Swedish breeding program was collected from Europe, central Russia,
and Siberia. Experiencing rapid progress and success, they produced varieties with improved yield,
erect growth form, and resistance to Melampsora rust and insects (Larsson 1998). By 2001, six
varieties of shrub willow were available for commercial production with yields averaging 6-14 odt/
ha per year over multiple harvests(Larsson 2001). Since the early 1990s, Svalöf Weibull AB has
also been very interested in developing varieties of willow that are frost tolerant with low moisture
content. To that end, two varieties, Loden and Gudrun, were developed and commercialized. Most
recently, Svalöf Weibull has placed emphasis on developing varieties that display tolerance to high
salt concentrations and toxic pollution to be used to remediate polluted soils and leachate water from
landfills (Larsson 2001) and has marketed at least 13 varieties for commercial production.
The cultivation and use of willows has a long history in the United Kingdom, and the National
Willow Collection, a collection of 1,100 varieties comprising over 120 different species that was
managed by Stott for many years, has been a vital resource for ongoing breeding (Stott 1984).
Willow breeding by Stott and Lindegaard at the Long Ashton Research Station became focused
on bioenergy in the 1980s and 1990s and was formalized by funding that supported the European
Willow Breeding Program involving Long Ashton, Svalöf Weibull, and Murray Carter in the mid-
1990s (Lindegaard and Barker 1997). The fundamental aim of the program was similar to that of the
Swedish program: to produce and commercialize high-yielding, disease- and pest-resistant varieties
with a wide and diverse genetic base. Rust, caused by Melampsora epitea , was a major problem
in the United Kingdom, so special emphasis was placed on rust resistance and developing mixed
varietal plantations to prevent rust outbreaks (Lindegaard and Barker 1997). In the early 2000s,
the Long Ashton Station was shut down, the National Willow Collection was moved to Rothamsted
Research, and willow breeding was reinitiated in the United Kingdom with funding that formed
the Biomass for Energy Genetic Improvement Network. Over 300 crosses were attempted in the