Agriculture Reference
In-Depth Information
foraging) although in many cases it is difficult to imag-
ine how biological plant resistance can greatly reduce
such damage (cashew trees being knocked over by
hippopotami is difficult to breed against!).
It can be assumed that pests and diseases will cause
damage and almost all important diseases have been
given attention by plant breeders. Crops differ greatly
in the number of diseases that attack them and simi-
larly in the exact damage that infection can cause. Small
grain cereals are particularly susceptible to air-borne
fungal epidemics; most solanaceous crops are especially
affected by viruses, while cotton is particularly affected
by insect attack.
It is difficult to assign importance to any class of
plant diseases. In economic terms the soil and air-borne
fungi may be more important than all other diseases. So
much so that many breeding text books consider breed-
ing for disease resistance, to actually be simply breeding
for resistance to fungal disease. This is, of course, an
over-generalization, and there is no doubt that other
disease types also have potentially significant impacts
on breeding objectives and goals, depending on the
crop being bred. Indeed, it is recognized that virus dis-
eases and many soil infestations are problematic because
there are few treatments (especially agro-chemicals)
that can be used to treat crops once plants become
infected.
Any breeder trying to develop new cultivars with
specific disease resistance must have knowledge of the
particular disease or pest and its effect on the crop. One
of the obvious, and most important effects of almost
all crop pests and diseases, is reduction in yield. This is
caused in four main ways:
result from other factors like increased weed infestation
through reduced crop competition.
Other impacts of plant pests and diseases relate to
damage to the end-use product of the crop. These
infestations are usually initiated in the field but often
become more apparent after harvest (e.g. cereal smuts,
various rots and insect boring of fruits and tubers).
Many pests/diseases also reduce the quality of harvested
crops (e.g. insect damage in fruit or fungal blemishes of
fruit or tubers).
The first task, which must often be carried out prior
to screening for natural resistance to diseases, is to
determine:
Which diseases can affect the crop?
•
What is the effect of these diseases on yield or quality?
•
Others who have been working with the crop in par-
ticular areas can often answer the first question. For
example, if a relatively well established crop is to be
bred (e.g. wheat in the Pacific Northwest of USA) there
will already be a large body of data that has been col-
lected regarding particular diseases and an indication of
the frequency of disease attack.
The exact yield, quality or economic effect that dif-
ferent pests or diseases have on a crop can be used to
partition the degree of effort that is exerted in breed-
ing for resistance. Obviously, if a particular disease does
not exist within the region there may be little point
in devoting a large effort towards screening for natu-
ral resistance. Similarly, if a certain pathogen does not
recognize your crop as a host, any attempt to increase
resistance would be a waste of time and effort. In reality,
the availability of a cheap and effective control measure
will also decrease the priority a breeder assigns to tack-
ling the resistance or tolerance to that particular disease
or pest.
The most common means to determine the effect
of a disease is to grow a series of genotypes under
conditions where disease is artificially managed. In
most cases the simplest way that this is done is to
grow plots where disease is chemically controlled next
to others where disease is allowed to occur naturally
(or indeed artificially infected to ensure high disease
pressure).
For example, the effect on yield caused by infesta-
tion by cabbage seedpod weevil (Figure 3.3) on four
Brassica
species was examined in field trials in 1992 and
Destruction of leaf tissue and hence reducing plant
photosynthesis capacity or efficiency (e.g. many rusts,
mildew and blight).
•
Stunting plants by metabolic disturbance, nutrient
drain or root damage (e.g. many viruses, aphids or
eelworm).
•
•
Reducing plant stands by killing whole plants and
leaving gaps in the crop which cannot be com-
pensated with increased productivity of neighbours
(e.g.
vascular
wilts,
soil-born
fungi
and
boring
insects).
•
Killing parts of plants (e.g. boring or feeding insects).
Killing plant tissue or causing reduced plant vigour
can reduce yields
per se
, although reduced yield can
