Agriculture Reference
In-Depth Information
contract to areas of higher latitude or higher ele-
vations in mountainous regions within the trop-
ics and subtropics, including ones previously too
cold for the potato crop. For vector-borne potato
viruses, such alterations would be limited if the
ranges of their vectors were to remain un-
changed, but the opposite scenario is predicted for
key temperate region potato virus insect vectors,
such as aphids, or warmer climate vectors, such
as whitefly or thrips (Jones and Barbetti, 2012).
have different thermal thresholds, a change in
species composition is projected from global
warming. Thus, the range of the tropical vector
species,
Thrips palmi
, is expected to expand into
areas formerly too cold for it, displacing vector
thrips species adapted to cooler temperatures
(Pappu
et al
., 2009). This, in turn, is likely to
cause expansion of damaging potato-infecting
tospoviruses transmitted by
T. palmi
(e.g. GBNV)
into regions formerly too cool for them.
Epidemics of potato viruses transmitted by
fungi or nematodes in the soil are projected to
alter in response to altered temperature and
rainfall. For example, epidemics of the fun-
gus-transmitted virus, PMTV, are likely to be-
come prevalent over increasingly wide areas in
temperate regions (Jones, 2009). This is because
of increased activity and movement of vector
zoospores resulting from increased soil moisture
and temperature.
Some research has been undertaken on the
effects of unusually high temperatures on symp-
toms or virus content of virus-infected potato
plants or tubers. For example, PVX was detected
when potato plants were held at 25°C after in-
oculation, but not at 30°C (Adams
et al
., 1986).
Also, PLRV foliar symptom severity was dimin-
ished in potato plants kept above 20°C (Jones,
1981b), and PLRV,
Alfalfa mosaic virus
, and
To-
mato black ring virus
were eliminated from dis-
eased tubers of several potato cultivars by hot-air
treatment at 37°C, but similar hot-air treat-
ments for up to
10
weeks did not eradicate PVY
(Kassanis, 1949; Kaiser, 1980). Unusually high
air temperatures can also reduce insect vector
populations (Jones and Barbetti, 2012). Thus,
there is a possibility that, in warm countries,
prolonged heatwaves resulting from climate
change might decrease the incidences of some
viruses in surviving potato crops or in seed
tubers stored without refrigeration. There was
no evidence of temperature-sensitive resistance
to PVX in potato plants (Adams
et al
., 1986).
There is apparently no information on the
responses of potato plants to virus infection
when elevated greenhouse gas concentrations
are present in the atmosphere, but elevated CO
2
increased resistance to PVY infection in tobacco
plants (Matros
et al
., 2006). Elevated CO
2
con-
centrations have the potential to influence insect
vector numbers, but experiments with elevated
ambient CO
2
had mixed effects on aphids,
Effects on vectors and potato viruses
Aphid vectors react strongly to small changes in
mean temperatures due to their low develop-
mental threshold temperatures, short gener-
ation times, and great capacity for reproduction
(Harrington, 2002; Harrington
et al
., 2007). An
additional five generations of aphids per year is
predicted in temperate zones from a warming of
2°C (Yamamura and Kiritani, 1998). The risk of
serious epidemics of aphid-transmitted potato
viruses therefore increases as their populations
and activities increase. For example, PLRV is pro-
jected to become more widespread in temperate
regions under increasing mean winter and sum-
mer temperature scenarios (Boland
et al
., 2004;
Jones, 2009). Whitefly vectors also react strongly
to climatic changes due to their short generation
times and great capacity for reproduction. With
the important whitefly virus vector species,
Be-
misia tabaci
,
25-
28°C is optimal for development
(Wagner, 1995), and much shorter adult-to-
adult generation times occur at high (
31-
33°C)
rather than low (17°C) temperatures (Muniz
and Nombela, 2001). Thus, rising mean temper-
atures increase the risk of damaging epidemics
of viruses transmitted by
B. tabaci
in formerly
cooler regions. Interestingly, at lower altitudes in
the Andean region,
B. tabaci
is displacing an-
other whitefly species,
Trialurodes vaporariorum
,
while the latter species is expanding its range at
higher altitudes. In turn, this shift in vector dis-
tribution is being reflected in the distribution of
PYVV, which is transmitted by
T. vaporariorum
but not
B. tabaci
(Salazar
et al
., 2000; Barker
et al
.,
2007). Increased mean temperatures can also
increase thrips vector populations by hastening
their development rates, leading to more gener-
ations per year. However, as different thrips species
