Principles of Plant Genetics and Breeding

496 CHAPTER 28
by W. A. Russel as: longer grain filling period (i.e.,
early flowering, delayed senescence), rapid grain
filling, increased sink size (i.e., more kernels per
unit area, larger kernels, induced bareness), higher
harvest index and shelling percentage, short plant
and tassel, upright leaves, shorter anthesis to silking
interval, better standability, and better tolerance to
abiotic stress. Further, modern-day cultivars are more
efficient at exploiting high soil nitrogen, and also are
amenable to cultivation under high plant density.
Another approach to yield breeding corn is to
select for prolificacy (plant has more than one ear).
Multiple ears per plant are known to improve standability
under adverse conditions.
2 Yield stability. It is desirable for cultivars to have
stable yield in the production region.
3 Agromorphological traits:
(a) Lodging resistance. To accomplish lodging
resistance or good standability in corn, the
specific focus of corn breeders is improved stalk
quality. Lodging in corn may be at the root
level or the stalk level. Rind thickness and crushing
strength are indicators used to select stalk
strength. Tall genotypes will have ears too high
on the stem and become prone to lodging. Stalk
breakage is caused by boring insects, such as the
European corn borer (Ostrinia nubilalis). One
of the most successful applications of genetic
engineering in plant breeding is the development
and deployment of Bt corn varieties. These
genetically modified (GM) varieties are resistant
to attack by the borer. As previously indicated,
the use of the brachytic 2 gene in breeding is
limited.
(b) Resistance to ear dropping. When harvesting
is delayed, mature corn may break off and drop
to the ground before harvesting. Sometimes,
the impact of the combine harvester may cause
such ear drop to occur, leading to significant
field losses. Disease may promote ear dropping.
Genotypes with large, heavy ears are more susceptible
to ear dropping. Long ear shanks are
more susceptible to corn borer attack and are
structurally weak and more prone to ear dropping.
Plant breeders hence select for shorter and
stronger shanks. However, it is desirable for the
ear to bend downward when mature to accelerate
drying.
(c) Husk covering. The ear of corn is covered by
a husk that has a protective role (against pests
and weather) and also promotes rapid drying.
Breeders select for a long husk or complete covering
of the tip.
(d) Dry-down. The husk and grain must dry before
harvesting of corn. The more rapid the drying
process, the better, as it allows for early harvesting
of the crop.
4 Adaptation:
(a) Early maturity. Maturity in maize is a qualitative
trait. Corn is frost insensitive. Hence, early maturity
is an important objective in temperate climates.
Early maturing cultivars are usable in areas of
shorter growing season and higher altitudes.
Maturity traits used in breeding corn includes
days to silk, tassel, and brown husk. The black
layer is an indicator of physiological maturity.
(b) Drought resistance. Corn is adapted to both
temperate and tropical climates. However,
excessive temperature and drought at flowering
promotes reduced seed set and bareness.
Drought-resistance traits in maize include small
male tassel, small leaf area, prolificacy, leaf elongation,
heat tolerance, high abscisic acid, and
low temperature.
(c) Cold tolerance. Cold tolerance is important
for germination under unseasonably cold soil
temperatures, and also for quick dry-down in
cold weather.
5 Disease resistance. Corn is plagued by over 100 diseases
and over 100 insect pests worldwide, some of
which have global importance.
(a) Seed rots and seedling blights. Corn is plagued
by many pathogens that cause rots and blights,
the common ones including Diplodia maydis,
Fusarium moniliforme, and Penicillium spp.
(b) Root, stalk, and ear rots. Agents of common
root rots include Fusarium spp. and Pythium
spp. Common stalk rots include Diplodia rots
and Fusarium rots. Corn is also attacked by ear
rots caused by Diplodia and Aspegillus.
(c) Leaf blight or spots. Leaf blights or spots of
economic importance include bacterial wilt (caused
by Bacterium stewarti) and Helminthosporium
leaf diseases like H. maydis (causes southern corn
leaf blight), H. carbonum, and H. turcicum
(causes northern corn leaf blight). Common
rusts include common corn rust (caused by
Puccinia sorghi) and southern corn rust (caused
by P. polysora). Important viral diseases of corn
include maize dwarf mosaic virus and maize
chloric dwarf virus.
(d) Smuts. Important smuts include common smut
caused by the fungus Ustilago maydis or U. zeae,
and head smut caused by Sphacelotheca reiliana.
6 Insect resistance. Most of the research emphasis has
been on breeding resistance to the European corn