Agriculture Reference
In-Depth Information
The infection sources for spring-planted crops were overwintered diseased plants.
By introducing a time gap in the annual production cycle, so that newly planted
crops were not contemporaneous with older, infected crops, the transmission
cycle for the virus was broken and the disease was controlled. The same strategy
has successfully controlled OYDV in several areas. The removal of any diseased
'volunteer' plants remaining from previous crops is important, and crops should
be 'rogued' early in the season to remove any viral plants that might act as a
disease source. Also, leek and onion seedling production and drilled crops should
be isolated from virus sources in shallots, garlic and overwintering infected
alliums in home gardens. Onion sets must be raised in isolation from infected
plants and must be monitored during production for freedom from virus.
The lack of alternative hosts showing differential susceptibility to, and
transmission of, the different component viruses in the mixtures found in garlic
made it difficult to identify the species in the mixture by the traditional methods
of plant virology. Most of the viruses are flexuous rods and look similar under the
electron microscope; several are transmitted by aphids, so these features could
not resolve the mixture. In the 1970s virologists in The Netherlands dis-
tinguished OYDV, LYSV and SLV as separate viruses, and prepared antisera that
reacted specifically with the coat proteins of the purified viruses. These antisera
made it possible to use ELISA and immunosorbent electron microscopy (ISEM)
to identify and distinguish these viruses (Walkey, 1990).
More recently, molecular methods have been applied to determine the
sequence of bases in the RNA (i.e. the genetic code) of most of these viruses
(Salomon, 2002; ICTVdB Management, 2006). The number of bases in their
RNA strand ranges from 8363 for Garlic Virus X to about 17,500 in the
Tospoviruses. The genetic codes were the basis for developing the technique of
reverse transcriptase-polymerase chain reaction (RT-PCR) identification of
allium viruses (Salomon, 2002). This involves separating the RNA from
infected plants and making DNA complementary to the RNA using the reverse
transcriptase enzyme. Short 'primer' sequences of DNA specific and diagnostic
for the complementary DNA of each virus have been designed, based on their
genetic codes (Salomon, 2002). If a virus is present, its primers initiate the
amplification of its complementary DNA many-fold in the PCR reaction, and
diagnostic bands of amplified DNA can be separated and identified by
electrophoresis (see Fig. 5.10). In this way the presence or absence of particular
viruses in a complex mixture can be determined (Tsuneyoshi and Sumi, 1996;
Takaichi
et al
., 1998). RT-PCR can be up to 10,000 times more sensitive than
ELISA for garlic virus detection (Salomon, 2002; Lunello
et al
., 2005).
Thanks to these immunological and molecular methods it has been
possible to distinguish the different viruses in Table 5.3 and make rapid
progress in determining the component viruses causing what, for many years,
was simply called 'garlic mosaic' disease.
In France, a long series of investigations has been aimed at overcoming the
problem of garlic mosaic (Messiaen
et al
., 1993). Systematic observations of
