Principles of Plant Genetics and Breeding

elative to a benchmark. A genotype that is immune to a
disease is completely or totally resistant to it. In practice,
plant breeders arbitrarily set the susceptibility–resistance
scale. A resistant cultivar has less disease than the standard
cultivar – it could be a little or a lot less diseased.
True disease resistance has a genetic basis and hence
is amenable to plant breeding methodologies. It is
manifested in two basic forms: inhibition of infection
or inhibition of subsequent growth of the pathogen,
the former being the more common form of resistance.
Resistance can be a qualitative or quantitative trait.
As A. L. Hooker observed, breeding for pest resistance
is an integral part of any system of crop pest
control. If yield and other desirable traits are maintained,
resistance (host resistance) is an ideal method
of breeding to control plant pests. The development
and use of resistant cultivars has several advantages. It
is relatively inexpensive to deploy (versus use of other
pest control measures) and has no adverse environmental
consequences. Also, as Hooker indicated, it is
convenient for farmers to use, requiring no additional
production costs or decision-making during the production
season. However, when host resistance is not
satisfactory, farmers may need to supplement host resistance
with other pest management practices (e.g., use of
pesticides).
Concepts of pathogen and host
The pathogen
The pathogen is a living organism that is capable of
inflicting a distinct disease or disorder in another organism
(the host). The capacity of the pathogen to cause
disease or disorder in a member of a host species is
called its pathogenecity, while the extent of disorder
or disease development it causes is its virulence. The
pathogenecity and virulence of a pathogen vary among
pathogen types (races or pathotypes). Races or pathotypes
that fail to cause disease symptoms or successfully
attack a given host are said to be avirulent. It should
be pointed out that the presence of a pathogen on a
susceptible host is not enough to cause disease symptoms
to occur. A third factor – favorable environment –
is needed, the trio (pathogen plus susceptible host
plus favorable environment) referred to as the disease
triangle. Pathotypes or races of pathogens may also be
described in terms of aggressiveness or non-aggressiveness
in relation to the rate at which they produce
disease symptoms.
BREEDING FOR RESISTANCE TO DISEASES AND INSECT PESTS 369
The host
The host (genotype, plant) is the organism in which a
pathogen may produce disease symptoms. A susceptible
host is one in which a pathotype or race can manifest a
disease symptom. A host may employ one of several
mechanisms (defense mechanisms) to resist pathogens.
These mechanisms may be pre-existing or induced upon
infection.
1 Pre-existing defense mechanisms. These include
morphological features that pose as barriers to the
penetration of the pathogen into the plant (e.g., presence
of lignin, cork layer, callose layers), or secondary
metabolites (phenols, alkaloids, glycosides) that have
antimicrobial properties.
2 Infection-induced defense mechanisms. Upon
infection, the host quickly produces chemical products
(e.g., peroxidases, hydrolases, phytoalexins, etc.)
to combat the infection.
Mechanisms of defense in plants
against pests
Plants exhibit a wide variety of strategies and mechanisms
of defense against pathogens and insects pest that
may be classified into three major groups – avoidance,
resistance, and tolerance. The mechanisms are given
different names as pertain to insect pests, as will be
discussed.
Avoidance
Also described as escape, avoidance is a mechanism that
reduces the probability of contact between pathogens or
insect pests and the plant. That is, it operates prior to the
establishment of any intimate host–pest/pathogen contact.
This mechanism is rare in pathogens and hence is
primarily applicable to insect pests where entomologists
describe it as antixenosis (or non-preference). A plant
using this mechanism has characteristics (morphological
or chemical) that make it unattractive to insects for feeding,
oviposition, or shelter. In the cabbage, a variant leaf
color makes the plant unattractive to the cabbage aphid
(Brevicoryne brassicae), while pubescence in cotton prevents
oviposition in the cotton boll weevil (Heliothis
zea). Also, in cotton, a repulsive odor deters the boll
weevil from feeding on the plant.
Avoidance or non-preference may not be effective
if there are no alternative hosts. Non-preference