Agriculture Reference
In-Depth Information
growth inhibition can be controlled by either qualitative
or quantitative genes.
Another complication in screening and determining
plant disease resistance is related to
tolerance
. Tolerance
is the ability of a genotype to be infected by a disease
and yet not have a marked reduction in productivity as a
result. Therefore, despite plants showing disease symp-
toms (e.g. fungal lesions or insect damage) the plant
compensates for the infection or damage. Tolerance to
disease has been related to plant vigor, which may be
associated with other physiological stresses. For exam-
ple, genotypic tolerance in potato to infection by potato
cyst nematode, late blight, early blight (Alternaria) and
wilt (verticillium) are all highly correlated to drought
stress or salinity tolerance.
One other factor needs to be considered in relation
to disease resistance, and that is
escape
. This is where
a genotype (although not having any resistance genes)
is not affected by a disease because the infective agent
of the disease is not present during the growth period
of the genotype. Disease escape is most often related to
maturity or other growth parameters of the plant and
phenology of the pest. For example, potato cultivars
that initiate tubers and mature early are unlikely to be
affected by potato late blight (
Phytophthora infestans
)as
the plants are mature before the disease normally reaches
epidemic levels.
pathogens of the air-borne fungi is commonly
detected on seedlings in the greenhouse or on
detached leaves in the laboratory. Resistance to viruses
is often carried out under greenhouse conditions
where plants are 'hand-infected' with virus and plant
response noted. Similarly, resistance to potato cyst
nematode (both qualitative and quantitative resis-
tance) can be effectively screened in a greenhouse
by growing test plants in soil with high cyst counts.
These methods in general demand rather precise
pathological control but are usually quick, accurate
and give clear results. If greenhouse or laboratory test-
ing is to be used it is essential that the test results
relate to actual resistance under field conditions. For
example, seedling resistance to powdery mildew in
barley is not always correlated with adult plant resis-
tance observed under field conditions. The simplest
method to authenticate small scale testing is to carry
out direct comparisons with field trials as an initial
part of setting up the testing regime.
•
In vitro
testing and screening has been used, where the
diseases that infect plant tissue is a result of toxins pro-
duced by the pathogens. These toxins can be extracted
and
in vitro
plantlets, or plant tissue, can be subjected
to the toxin. As with testing in a greenhouse (above)
it must be clearly shown that the
in vitro
plant tissue
reaction to disease toxin is indeed related to whole
plant resistance. It is never sufficient to observe phe-
notypic variation of
in vitro
material and to assume
that this variation is useful
in vivo. In vitro
evalu-
ation can be particularly effective if there are other
reasons to propagate genotypes
in vitro
. For example
if embryo rescue is necessary (as in some interspecific
crosses), if whole plants are being regenerated from
single cells (protoplast fusion or transformation), or
if initial plant propagation is carried out
in vitro
to
avoid disease (e.g. with potato).
Testing plant resistance
In order to select for plant resistance to pests it is nec-
essary to have a well established disease testing scheme,
one that truly mimics the disease as it exists in an
agricultural crop. If a plant's resistance to a pathogen
cannot be reliably measured, it will not be possible to
screen germplasm for differential resistance levels. Nor
will it be possible to select resistant lines from amongst
segregating populations.
Methods used for assessing disease resistance in plant
breeding are extremely varied but may conveniently be
grouped into three categories.
Field testing, using natural infection, artificial infec-
tion, or commonly, a combination of natural and
artificial infection, is widely carried out. The object is
usually to ensure that the initial infection rates are not
limiting so that those estimates of genotypic resistance
levels are more reliable (rather than simply a result of
being escapes due to lack of infection). In the field
there can be no precise control of the pathogen in
terms of race composition or level of infection. Arti-
ficial infection may be achieved by inoculating seed
or planting infecting material before planting the test
•
•
Plants can be artificially infected in a greenhouse or
laboratory. This can be especially effective in screen-
ing for vertical resistance (single gene resistance)
where an all-or-nothing reaction is expected. There-
fore a simple resistance or susceptible rating to
