Agriculture Reference
In-Depth Information
A further example of the effect of nitrogen on disease severity may be seen in
fruit: Utkhede and Smith (1995) showed that nitrogen as ammonium nitrate (240 g
per tree per year), applied as a single or split dose, increased the incidence of crown
and root rot caused by
Phytophthora cactorum.
By contrast, foliar sprays of urea or
ground applications of nitrogen as monoammonium phosphate had no effect on
disease incidence.
Source of nitrogen has also been shown to influence the severity of take-all in
cereals. The application of ammonium nitrogen reduces infection by the pathogen by
way of its effect on other soil micro-organisms. Nitrate nitrogen, on the other hand,
increases the susceptibility of roots to infection. However, nitrate N also increases
the number of roots produced by the plant and this generally more than offsets its
negative effects. This provides a good example of a treatment which favours the
epidemic development of a pathogen but, by also increasing the resilience of the
plant, can actually reduce the damage caused by the disease.
11.4 CONTROL STRATEGIES
11.4.1 General considerations
Good farm hygiene (e.g. destruction of haulm on dumps of discarded potatoes), the
use of tested, disease-free seeds (or other organs of propagation) and sound
rotational practice that should be an integral part of any sustainable system will all
help to keep inoculum levels to a minimum and thus reduce disease risk. Complete
eradication of inoculum is possible only for a very few diseases, so action is also
necessary to retard the development of epidemics (the '
r
' of Van der Plank's
equation) within growing crops. The most effective way of achieving this is by the
use of genetically resistant cultivars - but these cannot be wholly relied on as
resistance is seldom complete and, when it is, selection pressure on the fungal
population usually results in the eventual emergence of resistance-breaking strains
of the pathogen. Exploitation of genetic resistance is thus a key element in any inte-
grated disease control strategy, but until genetic manipulation produces more
durable resistance it must be enhanced by other measures to reduce the crop's
susceptibility to infection. Any inherent resistance shown by a crop variety can be
augmented by ensuring balanced nutrition, avoiding the deficiencies and excesses
that render plants susceptible to infection.
An exciting development for the future is the advent of chemicals which, though
themselves are not biocidal, have the ability to enhance the plant's own resistance to
pathogen attack. This advance could revolutionize our attitude to the control of
fungal diseases. It will, however, necessitate treatment before disease is present and
may thus lead to a reappraisal of risk assessment criteria for disease control. The
latter problem would be avoided by the use of non-chemical methods of stimulating
the production of phytoalexins. Percival, Karim and Dixon (1998), for example,
have shown if potato tubers are exposed to light before infection an increase in
their glycoalkaloid content increases their resistance to infection by
Fusarium
sulphureum
and
F. solani
var.
coeruleum.
