Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 157

to rust diseases with increased temperature; but
some forage species become more resistant to
fungi with increased temperature (Coakley et al.
1999 ). Many mathematical models that have
been useful for forecasting plant disease epidem-
ics are based on increases in pathogen growth
and infection within specifi ed temperature
ranges. Generally, fungi that cause plant disease
grow best in moderate temperature ranges.
Temperate climate zones that include seasons
with cold average temperatures are likely to
experience longer periods of temperatures suit-
able for pathogen growth and reproduction if cli-
mates warm. For example, predictive models for
potato and tomato late blight (caused by
Phytophthora infestans ) show that the fungus
infects and reproduces most successfully during
periods of high moisture that occur when tem-
peratures are between 7.2 and 26.8 °C (Wallin
and Waggoner 1950 ). Earlier onset of warm tem-
peratures could result in an earlier threat from
late blight with the potential for more severe epi-
demics and increases in the number of fungicide
applications needed for control.
A simulation model was developed for rice
leaf blast epidemics in Japan, China, Thailand,
South Korea, and the Philippines under increas-
ing temperature and ultraviolet-B (UV-B)
radiation. Elevated CO 2 was not considered. The
simulation showed that in the cooler regions of
Japan and northern China, a temperature increase
might lead to more severe blast epidemics, while
in humid tropics and warm humid tropics, this
risk might decrease. The authors concluded that
in these regions, blast development is inhibited
by high temperatures. UV-B radiation will
enhance the severity of blast, but more in cooler
than in warmer regions (Luo et al. 1995 ).
Kaukoranta ( 1996 ) simulated yield loss in
potato based on a 3-year-long controlled environ-
ment study of late blight at ambient temperature
and at 3 °C higher than ambient. This study sug-
gested that increases in yield loss of unprotected
potato crop at the high temperature would wipe
out any benefi ts from yield increases of around
2 t/ha dry matter per degree of warming.
Agricultural crops and plants in natural com-
munities may harbor pathogens as symptomless

carriers (Dinoor 1974 ), and disease may develop
if plants are stressed in a warmer climate. Host
stress is an especially important factor in decline
of various forest species. Climate extremes such
as drought may increase invasion by Armillaria
spp. that are not normally very pathogenic
(Lonsdale and Gibbs 1996 ). High temperatures
may increase the damage caused by diseases
such as Scleroderris canker on lodgepole pine
(Lonsdale and Gibbs 1996 ). Such projections,
however, do not consider other factors that can
enhance the resilience of forest ecosystems to cli-
mate change, which led Loehle ( 1996 ) to con-
clude that there is “a systematic bias toward
alarmist predictions” in projections of tree health
response to climate change.
Most plant viruses are transmitted by vectors
and majority by insects. Particularly aphids are
expected to react strongly to environmental
changes because of their short generation time,
low developmental threshold temperatures, and
ability to survive mild winters without winter
storms. An increase in the number of insect vec-
tors will inevitably lead to a higher risk for viral
infection of plants. The aphid transmissible com-
plex of barley yellow dwarf viruses in cereals and
potato virus Y in potato is amenable to show
potential effects on the prevalence of infection
because of climate change. In mild winters, high
intensity of aphid movement during spring and a
high frequency of PVY-infected potatoes have
been reported. The severity of viral diseases is
determined in large part by the amount of inocu-
lum and the time of infection. The amount of
virus inoculum is infl uenced by winter survival
of its hosts. For some viruses, higher tempera-
tures also cause more severe symptoms develop-
ment. Aphids are expected to have increased
survival with milder winter temperatures, and
higher spring and summer temperatures will
increase their development and reproductive
rates and lead to more severe disease. Milder
winters are also expected to increase survival of
alternate weed hosts of viruses. Increases in fre-
quency and intensity of summer storms with high
winds, rain, and hail will increase wounding of
plants and result in increased transmission of
viruses by mechanical means. Therefore, with