152 V. M. Chavan
Transmission by Beetles
Nearly 40 viruses are transmitted by beetles
which are sap-transmissible, relatively stable,
highly antigenic, and icosahedral except TMV
and PSTV. They always cause mosaic diseases.
Beetles have biting mouthparts and transmit
virus mechanically. Viruses are acquired by vec-
tors within short acquisition feeding period of
few minutes, have no incubation period and are
always transmitted immediately after acquisition
period. Some of the viruses are retained by vec-
tors for short time usually ranging from 24–48 h,
while others are retained by vectors for prolonged
periods ranging from 7 to 20 days. Virus vector
relationship of these viruses is of nonpersistent
type except that some viruses are retained for
many days.
Transmission by Mites
Some eriophyid mites are firmly established as
virus-vector. Mites possess mouthparts that are
spherically adopted for piercing and sucking pro-
cess. Relationship of wheat streak virus with its
mite vector is of persistent type but not circula-
tive because of long acquisition, long inocula-
tion, and retention period of virus in its vector
through moulting. Virus particles mostly exist in
densely packed bundles in greatest concentration
in the lumen of mid-gut and rectum-like sac of
hind-gut. Plants could possibly get infected by
the virus by its back flow from gut to mouth parts
during feeding or by introduction of defected
virus into plant cells through feeding punctures
or abrasions caused by anal setae or anal suckers.
Transmission by Nematodes
About 25 viruses belonging to two groups are
transmitted by three genera of soil inhabiting
plant parasitic nematodes. The nepoviruses,
which have isometric particles about 30 nm in di-
ameter, and the tobra viruses, which are straight
tubular ones. Nepoviruses are transmitted by spe-
cies of Xiphinema or Longidorus (Dorylaimidae),
whereas vectors of tobra viruses are Trichodorus
species (Trichodoridae). The three species are
cosmopolitan in distribution and are considered
as obligatory but migratory ectoparasite getting
their food mainly from root tips. Both adults and
larval stages of most nematode vector species
can transmit viruses equally efficiently and there
is no transovarial transmission.Important Aspects of Virus
Transmission by Vectors
Sequential Spread of Viruses in Insects
Propogative viruses occur systematically in al-
most all body parts of vector. They follow a defi-
nite route and sequence to spread systematically
within the body of a vector after ingestion. The
virus is sucked into the gut which is generally
the primary site of virus multiplication, passes
into hemolymph through some part of alimen-
tary track, enters the blood stream, is carried to
salivary glands by circulating blood and is finally
injected into healthy plant with salivary liquid
during feeding.Transovarial Spread of Viruses
Propogative viruses occur systematically in inter-
nal parts including presumably, ovaries and eggs
of vector. Progeny of viruliferous females will be
infected. This is called as transovarial passage of
viruses.Dependent Transmission
Aphids sometimes transmit virus from an in-
fected plant only if the later is also infected by
second virus. In this dependent transmission, the
presence of second virus, i.e., helper virus is es-
sential, e.g., potato virus C is transmitted only in
presence of potato virus Y.