Page 8 - Volume 2,Issue 2, July 2009

© Benaki Phytopathological Institute
Van der Vlugt 52
nt 4742 until the 3’ poly(A) tail the US2 se-
quence again resembles the Ch2 sequence.
Whether this represents a true recombina-
tion event in the US2 isolate or is an artefact
from the sequence assembly process (both
US1 and US2 sequences were obtained from
one RNA sample) remains unclear. Unfortu-
nately, the original infectious material is no
longer available.
Currently four different strains of PepMV
isolates are recognized:
The Peruvian (PE) strain, originally found 1.
on pepino (S. muricatum) and wild Lyco-
persicon spp.
The EU-tomato ( 2. S. lycopersicum) strain.
The US1/Ch1 strain. 3.
The Ch2 strain. 4.
Several studies have shown that isolates
from the EU-tomato and Ch2 strains can read-
ily occur simultaneously in a mixed infection
in plants (Pagán et al., 2006; Hanssen et al.,
2008; Van der Vlugt, unpublished). Detailed
sequence analysis showed different recom-
bination events between the EU-tomato and
Ch2 genotypes. The findings of these recom-
binants in different geographical locations
with different recombination points, suggest
that genetic recombination between PepMV
strains may not be a rare event.
The differences in symptom expres-
sion between isolates of one strain as well
as between isolates of different strains, do
not allow biological differentiation between
strains. Serological differentiation is also not
possible with the currently available (poly-
clonal) antisera. Given the clear genetic dif-
ferences between strains, a number of mo-
lecular tests have been developed that can
differentiate between strains. These are
based on strain-specific RFLP patterns (Mar-
tínez-Culebras et al., 2002) or strain-specific
RT-PCR primer sets (Ling et al. , 2007; Hans-
sen et al., 2008; Mumford, unpublished; Van
der Vlugt, unpublished) or a combination of
both (Alfaro-Fernández et al., 2009).
Virus epidemiology
The origin of PepMV clearly lies in South
America. Studies showed that the virus is
likely to be widespread in wild Lycopersi-
con spp. in Peru, even in isolated wild pop-
ulations (Soler et al., 2002), suggesting that
Peru might be its centre of origin. Interest-
ingly most infected wild Lycopersicon spp.
showed no distinct symptoms, which might
suggest a long term relationship with the vi-
rus. Several studies have confirmed that the
original pepino isolate is characterised by
the absence of, or only very mild symptoms
on commercial tomato crops (Van der Vlugt
et al., 2002). The strain now known as the EU-
tomato strain of PepMV caused more dis-
tinct symptoms on tomato although these
were still relatively mild.
Detailed population studies from Spain
on material collected between 1998 and
2004 confirmed the prevalence and ho-
mogeneity of the tomato strain isolates as
well as their presence since 1998 (Pagán et
al., 2006). However, there were clear indica-
tions for several independent introductions
of this virus strain both in the Canary Islands
and the Spanish mainland. Interestingly, in
addition to the tomato strain, the original
pepino strain and the US2/Ch2 strain were
also found, but always in mixed infections
with the tomato strain. Both strains were
shown to be already present in Spain since
2000. This is surprising since the Ch2 strain
was only recognised for the first time in
2006. Recently the US1 strain was also found
on the Canary Islands (Alfaro-Fernández et
al., 2008), and this is the first official report
on the occurrence of this strain outside the
US.
The above data suggest that different
strains of PepMV, at different occasions,
have spread from South America, most like-
ly from different host plants, such as pepi-
no, wild Lycopersicon spp. or other as yet
unidentified hosts, resulting in different in-
troductions of the virus in different parts of
the world. A number of virus characteristics
are likely to have contributed to this:
The original pepino strain is (nearly) symp- 
tomless in commercial tomato.
Proper diagnostics for the virus only be- 
came available after the recognition of