148 V. M. Chavan
Introduction
Most of the devastating plant diseases are at-
tributed to viruses transmitted by vectors. In the
absence of vectors, these diseases would be of
little importance. Therefore, the control mea-
sure is dependent on vectors. Arthropod-borne
plant viruses are among the most important,
most complex, and most extensively distributed
plant disease agents in the world. They feed on
plants and move from one plant to the other, dur-
ing which viruses get effectively transmitted.
The vectors of any one plant virus are always
restricted to one of the major taxa, such as the
aphids, the leafhoppers, the whiteflies, the thrips,
the mites, or the nematodes. The first vectors that
are recognized as being associated with a plant
virus disease were insects when Takata ( 1895 )
confirmed observation by a rice farmer that the
rice stunt virus disease was found transmitted by
leafhoppers Deltocephalis dorsalis Motschulsky.
Later on, Nephottetix apicalis Motschulsky was
found to be very efficient vector of rice stunt
viral disease. Subsequently, aphids were also
identified as vectors of plant viruses (Doolittle
1916). The first authentic report of plant hoppers,
Peregrinus madis (Ashmead) as vectors of corn
mosaic virus was given by Kunkel ( 1922 ). The
first report of whitefly Bemisia tobaci (Genna-
dius) as virus vector transmitting mosaic disease
of abutilon was demonstrated by Orlando and
Silberschmidt ( 1946 ) in Brazil. Not all vectors
are insects, and some viruses are transmitted
by leaf and bud-feeding eriophyoid mites, e.g.,
Phytoptus ribis Nal. transmitted current rever-
sion agent (Amos et al. 1927 ). More recently,
vectors of viruses have been found among soil-
inhabiting organisms, a dorylaimid nematode
by Hewitt et al. ( 1958 ) and a chytrid fungus by
Teakle ( 1960 ). All these vectors transmit more
than 283 viruses and other similar pathogens
(Harris 1981 ). More than half of the nearly 550
vectors transmitted virus species recorded are
disseminated by aphids (55 %), 11 % by leafhop-
pers, 1 1 % by beetles, 9 % by whiteflies, 7 % by
nematodes, 5 % by fungi and plasmodiophorids,
and the remaining 2 % by thrips, mites, mirids, or
mealybugs (Astier et al. 2001 ).
All above plant viruses’ vectors show a com-
mon feature i.e., they all penetrate unwounded
plant cells, usually when feeding on the plants
and thus have an opportunity to acquire virus
from an infected plant and inoculating it. These
processes occur in very different ways with dif-
ferent vectors and viruses, making a fascinating
series of biological adaptation. The most threat-
ening property of the insect vector is its wide
host range. Natesan et al. ( 1996 ) studied the host
range, vector relation, and serological relation-
ship of cotton leaf curl virus in South India and
found that the virus was transmitted by whitefly,
B. tabaci to 24 plant species in 6 families. The
major hosts included were bean, cotton, tobacco,
tomato, and several other weed hosts of the in-
sect.Relationship Between Plant Viruses
and Vectors
Kennedy et al. ( 1962 ) introduced terms which are
descriptive of the relationship between vectors
and viruses and give indication of the location
and route followed by the virus–vector relation-
ship.Terms Used in Virus–Vector Relationship
Acquisition Access Period
The time for which, initially, a virus-free vector
is allowed to access a virus source and could de-
sire to feed on that source.Acquisition Feeding Period or Acquisition
Threshold
The time for which, initially, a virus-free vector
actually feeds on the virus source.Inoculation Access Period
The time for which the virus-carrying vector is
allowed to access a virus-free plant and feed on it.