Sugar Beet, Energy Beet, and Industrial Beet - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 86

genetic loci. Linkage group nomenclature is based on the Butterfass trisomic series
[ 37 ]. Genetic maps of (mostly) sugar beet have been created based on morpholog-
ical, isozyme, and molecular markers [ 27 , 38 ]. Results suggest that single nucleo-
tide polymorphisms occur at 1 per 50-130 bp within genic regions [ 39 , 40 ].
Major Breeding Achievements
Breeding achievements for sugar beet are notable, beyond ancient selection diver-
sifying root and leaf crop types. Perhaps the first character subject to modern
progeny test breeding methods was for increased sucrose content during the latter
1800s, followed by a rapid improvement in technologies for measurement and
selection of fodder beet genotypes with increasing sucrose levels, from ~4-8 % to
12-15 % (fresh weight) over the next 100 years [ 41 ], and continuing at a less rapid
pace today where modern sugar beet hybrids routinely achieve 18 % sucrose in
optimal growing regions. Today, private companies produce sugar beet seed for
commercial planting. Each of these companies has an extensive variety develop-
ment program, and most of the worldwide yield gains over the past 25 years are
directly due to their efforts (Fig. 5.2 ).
Initial varieties were open-pollinated populations and mass selected for
increased root weight (e.g., tons/ha) as well as increased proportion of sucrose in
the root. These values determine the yield of sucrose, minus what is unrecovered
during processing. Two major innovations were developed in the 1940 and 1950s,
and each of these is a staple in sugar beet breeding programs today. The first was
discovery and deployment of the monogerm seed character, a single recessive gene
( m ) that conditions one seed per seedball (the ancestral state is a multigerm seedball
which is a fused group of 2-8 woody flower tissue with one seed per flower,
botanically a utricle). The second major development was cytoplasmic male steril-
ity (CMS) as well as development of fertility restoration genotypes, which are
conditioned by a pair of recessive nuclear genetic loci ( x and z ) that prevent pollen
fertility in CMS mitochondrial genotypes but restore fertility in normal cytoplasmic
genetic backgrounds (e.g., maintainer lines).
There are many diseases and pests affecting beets from the seedling stage
through flowering [ 42 ]. Predominant seedling diseases include Pythium ,
Aphanomyces , and Rhizoctonia , for which some chemical treatments are effective
and genetic resistance (or tolerance) is available, but additional genetic controls are
continuously sought for sustainable long-term disease control. Major fungal root
diseases include Rhizoctonia , Aphanomyces , and Fusarium , and genetic resistance
or tolerance is the only effective control for these diseases. In warmer climates,
should energy beets find a niche, Sclerotium rolfsii could be a yield-limiting
disease, and genetic resistance is not currently available. Rhizomania, “crazy
root,” caused by beet necrotic yellow vein virus (BNYVV) which is transmitted
by the soil organism Polymyxa betae , is perhaps the major root disease of beet
worldwide. At least two genes ( Rz1 and Rz2 ) are deployed singly or in combination
in the majority of sugar beet hybrids; however, these are not effective against