Principles of Plant Genetics and Breeding

Types of genetic resistance
The complexity of host–pathogen interaction makes it
difficult to categorize resistance into finite types. A large
number of host–pathogen interaction systems occur at
various stages of coevolution. Nonetheless, some rough
categorization, albeit simplistic, is widely acceptable for
its convenience and practical plant breeding applications.
Resistance reactions may be generally categorized
into two major kinds – vertical or horizontal – based
on their epidemiological status and stability of resistance.
Vertical (specific) or horizontal (non-specific)
effects are expressed in terms of population dynamics,
not in terms of genes.
Vertical resistance
Vertical resistance is known by other terms including
hypersensitivity, race- or pathotype-specific resistance,
major gene or oligogenic resistance, nondurable
or qualitative resistance, and non-uniform
resistance. This reaction is said to occur when a race of a
pathogen produces disease symptoms on some cultivars
of a host but fails to do so on others. That is, there is a
complete display of resistance in some genotypes while
others are susceptible. Vertical resistance is a differential
resistance, or a host × isolate interaction of cultivars
against different races of a pathogen.
However, there are examples of resistance genes that
the pathogen apparently fails to match (e.g., dominant
resistance in Periconia cincinata, the causal agent of the
milo disease of sorghum). Major gene resistance is often
absolute, conferring immunity against the appropriate
pathotype (but not necessarily so). It delays the start of
an epidemic, and disappears when the pathotype virulent
to the vertical resistance gene is prevalent. This type
of resistance is relatively easy to breed because the major
genes are easy to identify and transfer through simple
crosses. These genes control specific races or genotypes
of pests and hence do not protect against new races of
the pests. Cultivars equipped with this narrow-based
genetic protection are often vulnerable when a genetically
variable population of a pathogen occurs in the
environment. They may not succumb immediately to
the new strains of the pathogen but under certain cultural
systems (e.g., monoculture without crop rotation),
the populations of the new strains could build up high
enough to cause economic injury to the crop. This form
of protection is also described as non-durable resistance.
Disease or insect resistance may be conditioned
BREEDING FOR RESISTANCE TO DISEASES AND INSECT PESTS 371
by dominant or recessive genes. Dominant genes occur
in resistance to, for example, rusts, viruses, mildews, and
nematodes.
Horizontal resistance
Horizontal resistance is also known as slow rusting,
partial resistance, field resistance, race-non-specific
resistance, minor gene reactions, or polygenic inheritance.
There is a lack of differential reaction of isolates
on different cultivars. The resistance is effective against
all genotypes of the parasite species without cultivar ×
isolate interaction (i.e., race-non-specific). Following
the initial establishment of the pathogen, the plant may
resist its spread and reproduction such that the disease
develops at a slower rate. Unlike vertical resistance,
evolution of new races due to selection pressure against
any specific race is absent under horizontal resistance,
making this type of resistance more stable.
Horizontal resistance is controlled by polygenes.
Each of the genes that condition the disease contributes
toward the level of resistance, and hence resistance is
also called minor gene resistance. Polygenic resistance
is widespread and provides protection against a wide
range of races of a pest and hence is less susceptible to
being overcome by a new race. Polygenic resistance protects
plants by slowing down the spread of disease and
the development of epidemics in the field. Breeding
polygenic resistance is more challenging. The many
minor genes cannot be individually identified and consequently
cannot be transferred through crossing in a
predictable fashion. However, the heritability for polygenic
resistance is high.
Durability of resistance
An aspect of resistance of concern to plant breeders
is the durability. Resistance that is durable is one
that remains effective in a widely grown cultivar for a
long time in an environment that favors the parasite.
Horizontal resistance is said to be durable whereas
vertical resistance is not. Adaptations of the parasite
to newly introduced resistant cultivars (i.e., breakdown
of resistance) occurs frequently in pathogens but less so
in insect pests. Such cultivars lack durable resistance.
Durability of resistance is variable even within pathosystems.
For example in one study involving resistance to
stripe rust in wheat, durability ranged from 1 to 18
years, depending on the cultivars.