Agriculture Reference
In-Depth Information
12.4
Addressing the PVY Problem
(2011) used the designation PVY
Q
to distin-
guish phylogenetic “PVY
O
” from biological
PVY
O
, and suggested the use of PVY
N:Q
for the
phylogenetic grouping PVY
N:O
. An alternative
to using distinguishing letters for phylogenetic
PVY groupings would be to employ Latinized
numerals (e.g. I-V) and number subclades
(e.g. I-
1,
I-
2,
etc.) instead, as currently done
with PVX (see above). This approach would
help eliminate the problems caused by the cur-
rent phylogenetic PVY strain nomenclature,
and provide a clearer picture of the biological
characteristics of isolates based on potato
differentials. Karasev and Gray (2013a) em-
phasize that further refinement of PVY classi-
fication systems may be expected in the future,
once more information becomes available. Fu-
ture research on PVY diversity should include
PVY isolates from the Andean potato domesti-
cation center, as it is also the center of diver-
sity of common potato viruses (e.g. Jones,
1981a; Spetz
et al
., 2003). Sequencing of
Andean PVY isolates and their strain group
typing with potato differentials would help to
establish the extent of this diversity and iden-
tify any additional PVY strains groups, for
example the unknown PVY variant that re-
combined with Y
N
within isolate NE-
11
(Lorenzen
et al
., 2006).
Providing a final resolution to the PVY
strain nomenclature system issue is of critical
importance to the PVY research community,
and of considerable importance to post-entry
quarantine testing and breeding to incorporate
strain-specific PVY resistance genes (e.g.
Ny
)
into new potato cultivars. However, it has less
relevance to situations where large-scale, rou-
tine testing for PVY is undertaken, such as in
standard seed potato certification, diagnostic la-
boratories, or breeding to incorporate extreme
resistance gene
Ry
. This is because the priority
for such activities is normally to establish virus
occurrence regardless of strain, rather than
determine what PVY strain is actually present.
For this purpose, large-scale serological tests
using generic PVY monoclonal or polyclonal
antibodies are generally adequate to detect the
virus reliably. In the future, as technological in-
novation advances, alternative approaches to
large-scale, routine testing by serology are likely
to evolve more efficient and sensitive assays that
displace it (Boonham
et al
., 2014; Jones, 2014).
The factors responsible for the evolution of re-
combinant PVY strains that cause tuber necro-
sis and the current upsurge in PVY incidence in
potato crops in many countries have been dis-
cussed in recent reviews and research articles
(e.g. Valkonen, 2007; Kerlan and Moury, 2008;
Rolland
et al
., 2008; Singh
et al
., 2008; Gray
et al
., 2010; Karasev and Gray, 2013a,b; Visser
et al
., 2013; MacKenzie
et al
., 2014). The main
contributory factors in different parts of the
world were: (i) lack of sufficient potato cultivars
carrying comprehensive (non-strain-specific)
PVY resistance (gene
Ry
); (ii) increased survival
due to milder winters of volunteer potatoes that
acted as PVY sources in seed potato fields; (iii) in-
creased aphid vector numbers late in the grow-
ing season, resulting in unforeseen late PVY
spread to seed tubers; (iv) complacency over the
effectiveness of seed potato programs, resulting
in less emphasis being placed on traditional
virus control measures; (v) widespread release
by potato breeding programs of new potato cul-
tivars that were symptomless PVY carriers or de-
veloped very mild foliage symptoms; and (vi) an
alteration in the genetic composition of PVY
strains, leading to the development of genetic re-
combinants that caused mild or symptomless in-
fection in potato foliage. Factors (iii-vi) meant
that PVY infection was often missed on visual
inspection of seed crops, resulting in virus inci-
dences that built up undetected in each succes-
sive seed crop and spread to other cultivars,
including sensitive ones in which substantial
yield losses occurred (e.g. Jones, 2006; Valko-
nen, 2007; Gray
et al
., 2010).
Controlling PVY effectively during seed po-
tato production needs to focus on removing or
minimizing the initial virus infection source
and protecting the crop from migrant virulifer-
ous aphids that can reintroduce the virus from
external sources and spread it within seed crops.
Relaxation of traditional virus control meas-
ures in response to commercial pressures to re-
duce seed production costs, especially isolation
distances between generations or from non-
certified potato crops, and lack of strict adher-
ence to virus tolerance limits should be resisted
at all costs (Jones, 2006). Wherever possible,
seed production should be moved away from
farms producing ware crops in more aphid-prone
