Agriculture Reference
In-Depth Information
•
As a 'vagile' disease that spreads quickly and far and as a consequence is
difficult to control by isolation, roguing and other phytosanitary practices.
'Crowd diseases' such as cacao swollen shoot differ in that they do not spread
quickly or far in any considerable amount and they are relatively easy to control
by such methods (Thresh
et al.,
1988).
•
As one of the many diseases that have become particularly prevalent in times of
war or civil strife that facilitate spread and impede the development and
adoption of control measures. Cacao swollen shoot disease in Ghana and plum
pox disease in Europe during the Second World War (1939-1945) are other
examples (Thresh, 1985).
•
As a disease that has caused particular problems following the emergence of a
novel highly virulent form of the causal pathogen which displaces the form(s)
occurring previously.
•
Of the way in which disease incidence and severity are closely associated with
the vulnerability of the cultivars grown. There are many other examples of
disease problems associated with the widespread cultivation of particular
genotypes (Thresh, 1985, 1990). Indeed, Simmonds (1962) commented that
'
disease patterns are to a great extent a product of our plant breeding and
agricultural practices
'.
•
Of the way in which a disease can necessitate the introduction of resistant
genotypes and provide a powerful incentive for their adoption and use on a
large scale and in a shorter period than is usual for agricultural innovations.
Sugarbeet curly-top disease in the south-west states of USA and sugarcane
mosaic disease in many tropical and sub-tropical countries are other examples
of this type (Thresh, 1990).
•
Of the way in which farmers exploit host genetic diversity to provide a degree
of stability and resilience that is lost when there is an over-reliance on relatively
few closely related genotypes (Day, 1977).
•
Of the extent and rapidity with which a long-standing, stable equilibrium
between a pathogen and its host(s) can be disrupted during a perturbation, as
caused by an increase in the abundance of the vector, or in the virulence of the
pathogen. Some of the most striking examples of this type have occurred
following the first appearance of a new virus strain or vector species or biotype
and they have many of the features of a biological invasion. Ecologists have
given much attention to such events (Kornberg and Williamson, 1987;
Hengeveld, 1989) and an ecological approach has been advantageous in seeking
to interpret the Ugandan epidemic.
•
Of the focal expansion of disease as a 'travelling wave' progressing uniformly
at a rate that approaches a constant value (van den Bosch
et al.,
1988; Zadoks
and van den Bosch, 1994). Spread in this way can be visualized as a disease
profile that moves through space at a constant velocity without changing shape;
there are other examples in the epidemiological literature (Zadoks and van den
Bosch, 1994).
•
Of a disease capable of causing a pandemic in the sense of an epidemic that
progresses over a period of years to affect very large areas (Gäumann, 1946).
