Principles of Plant Genetics and Breeding

368 CHAPTER 20
Weed control is a major activity in crop production.
However, crop tolerance to weeds is seldom, if ever, a
breeding objective, and hence is not discussed in this
chapter.
Biological and economic
effects of plant pests
Disease is basically a change from the state of
metabolism necessary for the normal development and
functioning of an organism. An abnormal growth and
development of the plant will cause a reduction in
biological yield and invariably in economic yield, hence,
the need to control pathogens and insect pests in crop
production. There are four basic ways by which diseases
and insect pests adversely affect plant yield and general
performance, and, eventually, reduce economic value.
1 Complete plant death. Certain parasites sooner or
later will completely kill the afflicted plant. When this
occurs early in crop production, gaps are left in the
crop stand. Where such gaps cannot be compensated
for by plants in the vicinity, additional nutrients
may become available to the existing plants due to
reduced competition. However, the reduced plant
density results in reduced biomass. Crop standreducing
pests include those that cause mildews,
vascular wilts, and insects such as cutworms that
cause a seedling to fall over and die.
2 Stunted growth. Viruses are known to reduce the
metabolic performance of plants without killing them
outright. Afflicted plants grow to only a fraction of
their normal size, and usually cause severely reduced
economic yield of plants.
3 Partial plant death. Some diseases that afflict adult
plants do not completely kill them. Rather, only
certain parts of the plant (e.g., branches) are killed
(e.g., as observed in fungal diebacks).
4 Direct product damage. The three effects of disease
discussed above indirectly affect the economic or
harvested product if it is the fruit, seed, or modified
organs (tubers, bulbs). Some pests directly injure
these products completely (e.g., by causing rotting
of tissue) or reducing quality (e.g., by causing blemishes,
holes).
It is a principle in pest control that a method of
control (especially a chemical method) is warranted
only when economic loss is eminent. It does not make
economic sense to spend $1,000 in pest control to save
$100 of harvestable product.
Overview of the methods of
control of plant parasites
Four strategies are available for controlling plant pathogens
and pests, as summarized here:
1 Exclusion of pathogen from the host. This strategy
may use methods such as legislation (plant quarantine,
crop inspection) or crop isolation to prevent
the pathogen or pest from making initial contact
with the host plant. Quarantine laws help to prevent
the introduction of new pathogens into a production
region. The laws are commonly enforced at the points
of entry for people and goods into a country.
2 Reduction or elimination of the pathogen’s
inoculum. In the event that a pathogen gains access
into a production area, various methods may be used
to remove it or reduce the inoculum to contain it. A
method such as crop rotation reduces disease buildup
in the field, while observance of sanitation (e.g.,
removing diseased plants and burning them) also
reduces the spread of the pathogen. The producer
may also implement management practices that discourage
the growth and spread of parasites (e.g., soil
drainage, weeding, soil sterilization, seed treatment).
3 Improvement of host resistance. This is the strategy
of most concern to plant breeders. It entails breeding
to introduce genetic resistance into adapted cultivars.
This is the primary subject of this chapter.
4 Protection of the host. Economic plants may be
protected from parasites by using chemicals (pesticides).
While this is widely used, the method is environmentally
intrusive and expensive.
Concept of resistance in breeding
Breeding for yield and other morphological traits, as
well as breeding for resistance to abiotic stress, are conceptually
different from breeding for resistance to biotic
stresses. Breeding in the former cases entails the manipulation
of one genetic system – plants. Breeding for
resistance to biotic stresses on the other hand involves
the manipulation of two genetic systems – one for plants
(host) and the other for the organism (parasite) – not
independently, but with regard to the interaction
between the two systems. The breeder needs to understand
the interrelationships between plants and their
parasites that have persisted through coevolution and
coexistence.
Resistance is a response to a cause. There are degrees
of resistance. In terms of disease, resistance is always