Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 165

whether this may relate to recent increases in mean
and minimum temperatures (Yang et al. 1998 ).
Rice blast and wheat scab have increased sharply
since the 1970s. The wheat acreage infected with
powdery mildew has become more extensive,
whereas stripe rust has decreased steadily (Yang
et al. 1998 ). This may be related to the increased
spring and early summer temperatures and would
be consistent with the changes in stripe rust
observed in the Pacifi c Northwest associated with
climate variability (Coakley et al. 1988 ).
Jahn et al. ( 1996 ) utilized long-term plant
disease-monitoring records collected by the State
Plant Protection Service in the former German
Democratic Republic (GDR) to develop empiri-
cal climate-disease models for 15 individual
host-pathogen combinations. These models were
then used with various climate change scenarios
to predict possible changes in “infestation levels”
in a future climate. Calculations with the most
realistic scenario (a temperature increase of
1 ± °C combined with a decrease in precipitation
of 30 %) indicated that leaf rusts of wheat and
barley and powdery mildew of sugar beet could
increase substantially, reaching levels between
two and fi ve times as high as under the current
climate. Infestation levels on small grains by
powdery mildews would remain virtually
unchanged, whereas those caused by foot rots
and leaf blotch diseases would decrease. Most
notable was a decrease in potato late blight to a
mere 16 % of its current level. The authors cau-
tioned against over-interpreting their results,
which were based on calculations with data from
only 1 of 14 regions in the former GDR.

dry matter, and simulating the effects of late
blight on tuber dry matter through a reduction in
green leaf area, assuming that disease reduced
leaf area to zero within 14 days after the pre-
dicted outbreak. Model parameters were obtained
and model validation was done using data from a
3-year fi eld and greenhouse study. The combined
model was then used with various temperature
change scenarios to predict possible changes in
potato yield and yield losses caused by late blight
in a warmer climate. The results suggested that
tuber yield could increase by 2 t/ha per 1 ± °C
warming in the absence of late blight. This poten-
tial yield gain was almost completely offset when
late blight was considered, chiefl y because late
blight outbreaks occurred 4-7 days earlier and
the period during which the crop was susceptible
was lengthened by 10-20 days per 1 ± °C warm-
ing. This study did not consider possible yield-
enhancing effects of elevated CO 2 , nor did it
incorporate the effects of changes in precipitation
on late blight.

8.9.4 Simulation Models

Simulation models have been used extensively to
predict yields of various crops in different agro-
ecological zones under climate change (Riha et al.
1996 ). Biotic yield-reducing factors such as
insects, pathogens, and weeds have, however, been
largely ignored in these simulations (Teng et al.
1996 ). Because of this shortcoming, the develop-
ment of linked disease-crop models is an impor-
tant objective within the overall goal of developing
a predictive capability for agricultural impact
assessment and mitigation (Scherm 1999). For at
least one key crop, rice, preliminary analyses con-
sidering the combined effects on yield of increased
temperature, elevated UV-B radiation, and rice
blast disease ( Pyricularia grisea ) have been done
using a coupled simulation model (Luo et al.
1997 ). The model consisted of a physiological rice
growth model and a leaf blast epidemic simulator,
linked via the quantitative effects of leaf blast on
photosynthesis and biomass production (Luo et al.
1997 ). Climate change was imposed by increasing
mean temperature in fi xed increments and by

8.9.3 Population Models

A very different conclusion regarding the impor-
tance of potato late blight under climate change
was reached by Kaukoranta ( 1996 ). This author
developed degree-day models for the emergence
of potatoes and the date of late blight outbreaks
in Finland. The two models were coupled and
extended by including leaf area expansion of the
crop as a function of thermal time, calculating
radiation interception as a function of leaf area,