Principles of Plant Genetics and Breeding

370 CHAPTER 20
conditioned by allelochemicals is not as effective in
modern agriculture where monocropping and breakdown
of resistance are common. However, morphological
traits that interfere with feeding and breeding of
insects are important breeding goals as a first line of
defense against insect pests. Crop producers may help
plants escape pest attack by timing the planting of the
crop (early or late planting) so that the susceptible
growth stage escapes the time when the insect pest is
most abundant.
Resistance
The mechanism of resistance manifests after a host has
been attacked by a pathogen or insect pest. The mechanism
operates to curtail the invasion or to reduce the
growth and/or development of the pathogen. The
nature of the mechanism may be biochemical, physiological,
anatomical, or morphological. The equivalent
term used to describe this mechanism as it relates to
insect pests is antibiosis. As previously discussed, resistance
is a relative term for the genetic-based capacity of a
host to reduce the adverse effect of a pathogenic attack.
It does not imply complete overcoming of the pathogen
and may be quantitative in its expression (i.e., some
genotypes are more resistant than others in the face
of equal amount of initial inoculum). Further, hostresistance
mechanisms may be pre-existing (called passive
resistance). For example, onion is resistant to
smudge disease (caused by Colectotrichum circinaus)
because of the presence of various chemicals (e.g.,
catechol) in its outer scales. The resistance may also
be induced or activated (called active resistance) in
response to an infection.
Resistance reactions may also be described as hypersensitive,
overdevelopment, or underdevelopment.
1 Hypersensitive reaction. This mechanism acts to
prevent pathogen establishment. An infection evokes
a rapid localized reaction whereby the cells immediately
surrounding the point of attack die so that
the infection is contained. The pathogen eventually
dies, leaving a necrotic spot. This infection inhibition
response is the most common and is evoked in diseases
such as leaf spots, blights, cankers, and decays.
2 Overdevelopment of tissue. Meristematic activity
may be stimulated, resulting in excessive abnormal
tissue growth as exemplified by galls and also leaf
curling. In some cases, a layer of suberized cells form
around the invaded tissue to curtail the spread of the
pathogen.
3 Underdevelopment of tissue. Afflicted plants
become stunted in growth, or organs become only
partially developed. Viral infections produce this kind
of effect.
Tolerance
Unlike avoidance and resistance mechanisms that operate
to reduce the levels of infection by the pathogen or
pest, tolerance (or endurance) operates to reduce the
extent of damage inflicted. The afflicted host attempts
to perform normally in spite of the biotic stress. A host
may be highly susceptible (i.e., support a high population
of pathogens or pests) and yet exhibit little reduction
in economic yield (i.e., the plant host is tolerant
of the parasite). Because tolerance is measured in terms
of economic yield, it is not applicable to diseases or
insect pests that directly attack the economic part of the
plant (grain or fruit). The mechanism may be applied
to a situation in which a plant recovers quickly following
a pest attack such as grazing. From the point of view
of virologists, plants with few or no symptoms are
described as tolerant.
The exact mechanism(s) of tolerance is not understood.
However, some attribute it to plant vigor,
compensatory growth, and changes in photosynthate
partitioning, among other factors. Because it is difficult
to determine the yield reduction per unit of infection, it
is very difficult to determine the difference in tolerance
between genotypes.
Specificity of defense mechanisms
Almost all plant species are non-hosts for almost all
parasites. Most avoidance mechanisms are non-specific
(general or broad) in that they are capable of avoiding
groups of parasites with similar ecological requirements.
For example, the closed flowering habit of some barley
cultivars that excludes pathogens that invade through
the open flower, such as Ustilago nuda (causes loose
smut), also excludes Claviceps purpurea (causes ergot).
However, some specificity may occur in the case of
certain repellents. Resistance mechanisms may also be
specific or non-specific. Specificity may not only be at
the pathogen level but even at the race level (cultivar ×
race interaction). Race non-specific resistance can be
highly pathogen-specific. Plant breeders are primarily
interested in resistance in the host–pathogen system that
has the race-specific and race-non-specific effects.