Agriculture Reference
In-Depth Information
plants, by inter-planting 'spreader rows' of plants that
are highly susceptible to the disease, by spraying or
otherwise infecting the crop with the disease or by
artificial infection of the soil with eelworm or fungi.
Uneven inoculation or infection is almost always a
problem, especially with some of the soil pathogens.
It is essential therefore, that these trials use adequate
experimental designs and in many cases suitably high
levels of replication.
CONCLUSIONS
A house builder would not build a house without an
architect first providing a plan, an automobile pro-
ducer would not build an automobile without having
some form of test model. So also a plant breeder will
not produce a successful cultivar development pro-
gramme without suitable breeding objectives. This will
involve:
Examining the whole production and use system
from farmer, through processor to final product user.
Determine the demands or preferences of each group
in the production chain and take these into account
in the selection scheme. Remember that there may be
conflicts in the preferred requirements from different
parts of this chain.
In conclusion, breeding for disease resistance is no
different (in many ways) to breeding and selecting for
other traits. The steps, which must be taken in a disease
breeding scheme, include:
•
Develop a means to evaluate germplasm and breed-
ing lines. In many cases success will be directly
related to how effective the disease screening meth-
ods are at detecting differences in resistance levels.
If it is not possible to differentiate consistently
and accurately the level of disease present, it will
not be possible to identify sources of plant resis-
tance or to screen for resistance within segregating
progeny.
•
•
Examine what is currently known about the crop.
How is this end product produced at present? What
cultivars presently predominate? What are the advan-
tageous characters of these cultivars, and what are
their defects?
•
Examine how the operation of correcting present defi-
ciencies can most effectively be addressed while always
remembering that it is necessary to maintain at least
the same level of acceptability for most other traits.
•
Evaluate germplasm and breeding lines to identify
sources of plant resistance. In the first instance evalu-
ation of the most adapted lines should be carried out.
If no resistance is identified then more primitive or
wild genotypes need to be screened.
In order to set appropriate breeding objectives the
breeder needs to consider incorporating: yield potential,
disease and pest resistance, end-use quality and even the
influence of potential political factors/decisions. Hav-
ing taken these into account, it will be possible to design
a successful plant breeding programme.
•
Examine, if possible the mechanism of resistance to
determine the mode of resistance being exploited
(e.g. avoid infection, limit spread, non-preference
and antibiosis).
Determine the mode of inheritance (i.e. qualita-
tive or quantitative) of any resistance detected. The
mode of inheritance can have a large influence on
the method of introducing the resistance into the
commercially acceptable gene pool (e.g. single gene
would perhaps be best handled by a back-crossing
method).
•
THINK QUESTIONS
(1) When breeding new cultivars it is often necessary to
try and predict events that may occur in the future.
Briefly outline four factors which may influence
cultivar breeding and hence need to be consid-
ered in setting the breeding objectives of a cultivar
development programme.
(2) Discuss the statement that durable resistance to
disease is usually partial rather than complete.
(3) Outline the major features caused by the following
diseases: soil-borne fungi; bacteria; eelworms; air-
borne fungi; insects and viruses.
•
Introgress source of resistance into a new cultivar.
Despite the importance of disease resistance in plant
breeding, it should always be remembered that a new
cultivar will be unlikely to succeed simply because
of disease resistance - new cultivars will also need
to have acceptable levels of expression for the other
important traits.
