Shrub Willow - Bioenergy Crop Plants - page 699

using 0.5% sulfuric acid at 200°C followed by enzymatic hydrolysis resulted in high sugar recoveries
of 92% of glucose and 86% of xylose, which yielded 79% of the theoretical maximal production of
ethanol after simultaneous saccharification and fermentation (Sassner et al. 2008). Production of
ethanol could be further enhanced by use of a microbe that could ferment all of the available sugars
(Sassner et al. 2006). Because there is considerable variation in the chemical composition of the
biomass produced from different genotypes of willow, it may well be possible to select varieties
that have biomass compositional characteristics that are optimized for particular pretreatment and
fermentation technologies (Serapiglia et al. 2008, 2009). Alternatively, willow biomass can be
used as a feedstock for thermal conversion processes (Demirbas 2007), including flash pyrolysis or
co-pyrolysis for the production of bio-oil (Cornelissen et al. 2008; Lievens et al. 2009) and gasification
or hydrogasification for the production of synthetic natural gas (Porada 2009).
28.6 Pests and dIseases oF WIlloW BIoenerGy croPs
Rust is one of the most damaging diseases of shrub willow. Most rusts that infect willow have been
identified as Melampsora epitea , which uses larch ( Larix decidua Mill.) as an alternate host. Rust
appears to the naked eye as small (1-2 mm), orange or brown, discrete, slightly raised pustules
of teliospores on the surface of the leaf (Figure 28.6). With the onset of severe infections, rust
defoliates the plant prematurely and reduces yield significantly (Dawson and McCracken 1994).
Furthermore, rust infections can predispose a plant to infection by secondary pathogens, which
may lead to further reductions in yield or premature death. Most willow rusts only infect leaves;
however, strains of Melampsora that infect young leaves and green stems have been identified in
some willows, including S. viminalis and S. caprea (Pei 1997). Species used in bioenergy plantations
that display particular susceptibility to rust include S. burjatica and S. viminalis in Europe and
S. eriocephala in North America.
Rusts evolve rapidly through hybridization. They usually complete a complex sexual life-cycle
that involves five different spore stages, including one that lives on an alternative host, such as
European larch. Rust populations multiply through infection and repeated reinfection of the leaf
during the summer, finally producing teliospores that overwinter on the dead infected leaves in the
litter. More than 12 pathotypes have been identified in England, Scotland, and Northern Ireland (Pei
et al. 1993, 1999), some of which have been designated into formae specialis ; the host range for each
f. sp. appears to be confined to particular species of willow (Pei et al. 1996). The genetic relationship
among ff. spp. is complicated (Pei et al. 2005). The pathotypes occurring in rust populations in the
United Kingdom and Sweden are similar, but were shown at one point to be different from those
occurring in North America (Royle and Ostry 1995). Moist, cool summers and mild, temperate
winters are ideal environmental conditions for Melampsora growth. Such conditions are often
found in Northern Ireland, England, and Sweden where rust has proven to be a major problem in
willow plantations (McCracken and Dawson 1992; Hunter et al. 1996).
Resistance to willow rust is under strong genetic control (Rönnberg-Wästljung and Gullberg
1999; Cameron et al. 2008); therefore, selection and breeding programs in Europe and the United
States emphasize rust resistance as the primary management for this disease. In Northern Ireland,
planting shrub willow in mixed-culture stands has effectively reduced inocula and has proven to
be an effective method of nonfungicide control of rust disease (McCracken and Dawson 1997;
McCracken et al. 2001; Begley et al. 2009). These stands have been planted by continuously loading
the mechanical planter with whips of several different varieties as it proceeds down a row. The
physical barrier provided by resistant varieties limits the movement of the inocula and slows the
build-up, often delaying the onset of disease. This strategy has successfully slowed the spread of
disease and reduced the impact of rust overall on willow plantations (McCracken and Dawson 1997;
McCracken et al. 2001, 2005).
Other diseases, such as crown gall disease caused by Agrobacterium tumefaciens , anthracnose tip
blight caused by Colletotrichum spp., black cancer caused by Glomerella miyabeana, have not had