Principles of Plant Genetics and Breeding

5 The plant or cell overproduces the target compound
for the phytotoxin in large amounts such that it
would take a high concentration of the herbicide to
overcome it.
Epidemics and plant breeding
Modern agriculture is drastically different to that done
by early plant domesticators. Plant breeding, in adapting
wild species for cultivation, often removes natural
means of crop protection needed for survival in the
wild, but undesirable in modern production. Starting
with the early domesticators, mass production of a few
desirable genotypes has become the norm of crop production.
The monocultures of modern agriculture are
extreme versions of mass production, in which genetic
diversity is further restricted. It is not difficult to see
how such breeding and production practices predispose
crops to widespread disease and insect pest damage.
With the arrival of a devastating pest attack, breeders
counter with the production of resistant cultivars, thus
setting the stage for sequential cycles of pest resistance
and pest susceptibility of crop plants.
The patterns of disease incidence vary from one class
of pathogen to another. Soil-borne pathogens tend to
be localized and persistent in the soil, season after season.
Airborne pathogens have a different biology and
spread pattern, as well as the potential to become epidemic.
An epidemic may be defined as an outbreak of
disease characterized by an infection that starts from a
low level and then progresses to a high one. Airborne
pathogens tend to have annual epidemic cycles. These
cycles depend on seasonal weather patterns and the
presence or absence of a susceptible host (crop). Soilborne
pathogens usually stabilize in a new location
after several years, and persist in the chronic phase.
Alam, S.N., and M.B. Cohen. 1998. Detection and analysis of
QTLs for resistance to the brown planthopper, Nilaparvata
lugens, in a doubled haploid rice population. Theor. Appl.
Genet. 97:1370–1379.
Bogdanove, A.J. 2002. Protein–protein interactions in pathogen
recognition in plants. Plant Mol. Biol. 50:981–989.
Flor, H.H. 1956. The complementary genic system in flax and
flax rust. Adv. Genet. 8:29–54.
Flor, H.H. 1971. Current status of the gene-for-gene concept.
Ann. Rev. Phytopathol. 9:275–296.
BREEDING FOR RESISTANCE TO DISEASES AND INSECT PESTS 383
Each type of disease pathogen and the epidemic it causes
is characterized by its own ecology and spatial and
temporal dynamics. Plant fungal diseases are especially
significantly impacted by weather conditions and climatic
patterns. Epidemics in agricultural crop production
impact US agriculture by affecting the economic value,
quantity, and quality of food and fiber produced.
J. E. Van der Plank provided the underlying scientific
principles of disease epidemics. An epidemic occurs in
phases. It starts with a small inoculum of the pathogen
(that may be from overwintering spores, or freshly introduced
into the area from outside). The initial rate of
infection is proportional to the infection level. As time
proceeds, the infection rate slows down as uninfected
tissue decreases. Van der Plank expressed the initial
infection rate by this mathematical relationship:
dI/dt = rI or I = I 0 exp rt
References and suggested reading
where I = infection at time t, I 0 = initial infection, and
r = multiplication rate of the pathogen. A general and
more realistic expression is:
dI = rI(1 − I)
I = 1/[1 + (exp −rt ) or log e (1/1 − I) = rt + log e (1/C)
where C is a constant that depends upon I 0 but is not
equal to it.
In plant breeding, the breeder’s goal in countering a
disease epidemic is to reduce r (the multiplication rate of
the pathogen) to a level such that, at the time of crop
maturity, the final attack by the pathogen can inflict only
a small and acceptable yield loss. It may be best if r = 0
(i.e., immunity), but this is not necessary. In practice,
breeders do not aim for immunity but for reducing
r below that of the current commercial cultivar that is
susceptible to the pathogen.
Hill, C.B., Y. Li, and G.L. Hartman. 2004. Resistance to the
soybean aphid in soybean germplasm. Crop Sci. 44:98–106.
Hooker, A.L. 1980. Breeding to control pests. In: Crop
breeding (Wood, D.R., ed.). American Society of
Agronomy/Crop Science Society of America, Madison,
WI.
Khush, G.S., and D.S. Brar. 1991. Genetics of resistance to
insects in crop plants. Adv. Agron. 45:223–274.
Niks, R.E., P.R. Ellis, and J.E. Parlevliet. 1993. Resistance
to parasites. In: Plant breeding: Principles and prospects