Explain how the insects act as vectors of plant diseases?
Answers
Plant diseases appear as
necrotic areas, usually spots of various
shapes and sizes on leaves, shoots,
and fruit; as cankers on stems; as
blights, wilts, and necrosis of shoots,
branches and entire plants; as
discolorations, malformations, galls, and
root rots, etc. Regardless of their
appearance, plant diseases interfere
with one or more of the physiological
functions of the plant (absorption and
translocation of water and nutrients from
the soil, photosynthesis, etc.), and
thereby reduce the ability of the plant to
grow and produce the product for which
it is cultivated. Plant diseases are
generally caused by microscopic
organisms such as fungi, bacteria,
nematodes, protozoa, and parasitic
green algae, that penetrate, infect, and
feed off one or more types of host
plants; submicroscopic organisms such
as viruses and viroids that enter, infect,
spread systemically and affect the
growth of their host plants; parasitic
higher plants which range from about an
inch to several feet in size and penetrate
and feed off their host plants. Plant
diseases are also caused by abiotic,
environmental factors such as nutrient
deficiencies, extremes in temperature
and soil moisture, etc. that affect the
normal growth and survival of affected
plants.
Of the aforementioned causes of
disease, many of the microscopic
organisms and of the viruses are
transmitted by insects either accidentally
(several fungi and bacteria) or by a
specific insect vector on which the
pathogenic organism (some fungi, some
bacteria, some nematodes, all protozoa
causing disease in plants, and many
viruses) depends on for transmission
from one plant to another, and on which
some pathogens depend on for survival
(Fig. 1).
The importance of insect
transmission of plant diseases has
generally been overlooked and greatly
underestimated. Many plant diseases in
the field or in harvested plant produce
become much more serious and
damaging in the presence of specific or
non-specific insect vectors that spread
the pathogen to new hosts. Many
insects facilitate the entry of a pathogen
into its host through the wounds the
insects make on aboveground or
belowground plant organs. In some
cases, insects help the survival of the
pathogen by allowing it to overseason in
the body of the insect. Finally, in many
cases, insects make possible the
existence of a plant disease by
obtaining, carrying, and delivering into
host plants pathogens that, in the
absence of the insect, would have been
unable to spread, and thereby unable to
cause disease. It is offered as a guess
that 30-40% of the damage and losses
caused by plant diseases is due to the
direct or indirect effects of transmission
and facilitation of pathogens by insects.
Insects and related organisms,
such as mites, are frequently involved in
the transmission of plant pathogens
from one plant organ, or one plant, to
another on which then the pathogens
cause disease.