Strategy for PRSV Disease Management
PRSV is themost destructive viral disease of papaya. Control
of PRSV includes rouging infected plants and spraying them
with aphicides. However, rouging cannot stop the spread of
the disease once it is established. Similarly, spraying with
aphicides is often ineffective since the virus is transmitted
to the plants before the aphids are killed [49]. The PRSV
diseasemanagement has been focused on developing tolerant
or resistant varieties of papaya, but these varieties are rarely
planted due to poor fruit quality and vigour [50]. PRSVresistant
gene is available in some wild varieties related to
the Carica species. But the development of PRSV-resistant
varieties through conventional breeding methods has been
complicated due to the sexual incompatibility of wild species
and cultivated papaya [51, 52]. Disease tolerance in back
crosses with commercial papaya also limits this approach
for PRSV disease management. Cross protection was used
to control PRSV which involved the use of a mild virus
strain against economic damage caused by severe strains
of the same virus [6, 53]. The cross protection strategy of
inoculating papaya with a mild strain of PRSV provides resistance
against severe PRSV strain infection in Taiwan [54].
Cross protection depends on the availability of mild strains
that can be used for effective protection against the target
virus. Cross protection needs extra agricultural practice and
care. However, strain specificity and the technical difficulties
associated with propagating pure strains of mild forms of
the virus and the unavailability of such mild strains limit
the benefits of this approach [55]. Field evaluations revealed
that cross protection was marginally effective for PRSV
management evaluation in the field [8]. Researchers from
Cornell University and the University of Hawaii initiated the
development of PRSV-resistant papaya by gene technology.
The concept of pathogen derived resistance was proposed by
Sanford and Johnston [56] for developing resistance against
pathogens. This research group has applied the concept of
pathogen derived resistance which has stimulated research
into obtaining virus resistance through gene technology.
Pathogen derived resistance is governed either by proteinmediated
or RNA-mediatedmethods. An alternative strategy
using RNA-mediated gene silencing with transgenic plants
expressing viral genes has been developed [57]. Resistance
levels of PRSV differ with environmental factors and plant
development stages despite of the success with this approach.
Broad spectrum resistance against different PRSV isolates
depends on the homology of transgenes with viral target
genes and the genetic divergence of different PRSV strains
which are correlated with their geographical distribution
[58]. The transgenic papaya varieties resistant to PRSV
against different viral strains must be developed individually
for various papaya growing regions. The development of
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PRSV-resistant lines is generally considered the best strategy
for efficient PRSV disease control in papaya for long-term
protection [20].