Plant Virus Disease Spread Through Insect Vectors and Their Management 153
Epidemiology of Virus Transmission
Spread of aphid-borne nonpersistent viruses fol-
lows the seasonality of vectors; especially the
dominant species (Mora-Aguilera et al. 1993 ;
Basky 1986 ). Alate aphids land on available
plants regardless of species and are unable to
distinguish host plants from nonhost prior to
landing. Host selection occurs after arriving on
plant surface and after ingestion of plant sap.
Virus transmission occurs if the aphid is carry-
ing specific virus, even though the aphid does not
colonize the plant. The acceptance or rejection
of plant host by a vector with piercing-sucking
mouth parts is performed by a series of brief
probes into multiple plant epidermal cells, which
is sufficient to inoculate the nonpersistent vi-
ruses. If brief feeding probes designate the plant
as an acceptable host or food source, the vector
is likely to initiate prolonged feeding. However,
in case of nonregular host plant, there is little
chance of initiating prolonged feeding and in
this case the number of efficient vectors involved
decides the epidemiology of virus incidence.
Thus, in papaya ring spot virus (PRSV), the rapid
spread of virus occurs within a short period when
the vector population is high and it appears that
the fresh incidence of PRSV coincided with num-
ber of aphids trapped 2 weeks prior to infection
suggesting a strong link between aphid vectors
and PRSV incidence (Krisna Kumar et al. 2010 ).
Similarly, to understand ecological factors medi-
ating the spread of insect-borne plant pathogens,
vector species for these pathogens need to be
identified, e.g., different strains of grapevine leaf
roll virus were found to be borne by two mealy-
bug, Planococcus ficus Signoret and Pseudococ-
cus longispinus (Tsai et al. 2010 ).
Transmission Specificity of Plant
Viruses by Vectors
Virus transmission by a vector is often charac-
terized by some degree of specificity. Numerous
studies suggest the involvement of a virus–ligand
interaction in transmission specificity. The coat
protein (CP) and its derivatives and nonstructural
proteins, such as a helper component (HC) or a
transmission factor, are major viral determinants
of transmission specificity. The CP or its de-
rivatives, in the case of Luteoviruses, Cucumber
mosaic virus (CMV), Cucumber necrosis virus
(CNV), and Grape vine fan leaf virus (GFLV),
and a HC or a transmission factor in Potyviruses,
Caulimoviruses, and Waikaviruses have a pro-
found role in transmission specificity (Andret-
Link and Fuchs 2005 ).Cross Protection
A strain of propogative virus already present in
a vector may cross protect that vector against ac-
quisition and transmission of second strain., e.g.,
Aster yellows and corn stunt.Ingestion–Egestion Mechanism of
Transmission
According to Harris ( 1977 ), the transmission
process is based in epidermal and intracellular
zones of the host tissue. The insect vectors dur-
ing their sap sampling and host selection routine,
take in the cell sap or protoplasm to their fore
alimentary canal and in case the host is infected,
the virus-laden material (cell sap or protoplasm)
contaminate the free alimentary canal. This is the
ingestion step. The virus gets transmitted and the
transmission cycle is completed in the next step,
all or part of the virus-infected material is egested
during subsequent sap sampling probes by the
same vector in healthy plants.Horizontal and Vertical Transmission
Spread of viruses by vector in a field or in differ-
ent areas is horizontal transmission while trans-
versal transmission is the vertical transmission of
viruses.