Principles of Plant Genetics and Breeding

376 CHAPTER 20
especially disease resistance. The success and effectiveness
of introgression of disease-resistance genes into
crop species from wild relatives varies by crop. Factors
to consider include the amount of diversity within the
crop species, ease of hybridization with wild relatives,
and the complexity of the genetic control of the trait.
Some crop breeders (e.g., tomato breeders) use genes
from wild relatives more frequently than other breeders,
such as sorghum breeders, who appear to find their
needs in adapted species. In tomato, wild relatives provided
genes for enhancing the nutritional value (vitamin
C and beta-carotene) and solids content, significantly
boosting the commercial value of the crop. The impact
of introgression of genes from the wild into adapted
cultivars has been dramatic in some crops. For example,
the resistance to the devastating late blight of potato was
found in a wild species. Similarly, resistance to the root
knot nematode in peanut was obtained from three wild
species. A wild relative of rice, Oryza nivara, growing in
the wild in Uttar Pradesh was found to have one single
gene for resistance to the grassy stunt virus, a disease
that devastated the crop in South and South East Asia in
the 1970s.
Screening techniques in disease-
and pest-resistance breeding
One of the critical activities in breeding for resistance to
diseases and insect pests is screening or testing for resistance.
Various facilities, techniques, and approaches are
used, depending on the parasite and host characteristics.
Facilities
Disease, as previously indicated, depends on the interaction
among three factors – pathogen, host, and environment.
Whereas field screening has the advantage of
representing the conditions under which the resistant
cultivar would be produced, it has its limitations. The
weather (or the environmental component of the disease
triangle) is unpredictable, making it difficult to
have the uniformity and consistency of the parasite
population. In some years, the weather may not favor
an adequate pathogen population for an effective evaluation
of plants. Controlled environmental tests provide
reliable, uniform, and consistent evaluation of disease,
but it has less field correspondence. Screening for resistance
to mobile pests is challenging, requiring special
provisions to confine the parasites. The distribution of
the pest on the plants is often uniform.
Factors affecting expression of disease and
insect resistance
Certain specific factors may complicate breeding for resistance
that may be environmental or biological in nature.
Environmental factors
1 Temperature. Low or high temperature over a period
of time may cause loss of resistance.
2 Light. Light intensity affects the chemical composition
of plants that is related to pest resistance (e.g.,
glycoside in potato).
3 Soil fertility. High soil fertility makes plants more
succulent and more susceptible to disease development.
Biological factors
1 Age. The response of a plant to a pathogen or insect
pest may vary with age. Some diseases are more
intense at the early stage in plant growth than others.
2 New pathotypes or biotypes. New variants of the
parasite that overcome the current resistance in the host
may exhibit a different kind of disease expression.
Breeding procedures
The first step in breeding for resistance to pathogens or
insect pests is to assemble and maintain resistance genes.
The sources of resistance genes include commercial cultivars,
landraces, wild progenitors, related species and
genera, mutagenesis, and biotechnology. As indicated
elsewhere in the book, obsolete and current commercial
cultivars are most preferred because they have minimum
undesirable traits. Once a desirable source has been
found, the backcross method of breeding is commonly
used to transfer resistance genes into adapted cultivars.
As previously discussed, there should be an effective
and efficient screening technique for disease-resistance
breeding. For cross-pollinated species, recurrent selection
is effective for increasing the level of resistance in a
population of genetically heterogeneous population.
Applications of biotechnology in
pest-resistance breeding
One of the successful applications of agricultural
biotechnology is in pest-resistance breeding. The first
disease-resistance gene, Pto (binds with products of the