Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 155

inoculation available for subsequent infection
and earlier and faster disease epidemics.
Some fungal pathosystems under elevated
CO 2 revealed two important trends. First, delay in
the initial establishment of the pathogen because
of modifi cations in pathogen aggressiveness and/
or host susceptibility. For example, reduction in
the rate of primary penetration of Erysiphe
graminis on barley and a lengthening of latent
period in Maravalia cryptostegiae (rubber vine
rust) has been observed under elevated CO 2 .
Here, host resistance may have increased because
of change in host morphology, physiology, nutri-
ents, and water balance. A decrease in stomatal
density increases resistance to pathogens that
penetrate through stomata. Under elevated CO 2 ,
barley plants were able to mobilize assimilates
into defense structures including the formation of
papillae and accumulation of silicon at sites of
appressorial penetration of E. graminis .
At elevated CO 2 , increased partitioning of
assimilates to roots occurs consistently in crops
such as carrot, sugar beet, and radish. If more car-
bon is stored in roots, losses from soilborne dis-
eases of root crops may be reduced under climate
change. In contrast, for foliage diseases favored by
high temperature and humidity, increases in tem-
perature and precipitation under climate change
may result in increased crop loss. The effects of
enlarged plant canopies from elevated CO 2 could
further increase crop losses from foliar pathogens.
The second important effect is an increase in
the fecundity of pathogens under elevated CO 2 .
Following penetration, established colonies of
Erysiphe graminis grew faster, and sporulation per
unit area of infected tissue was increased several-
fold under elevated CO 2 . It has been also observed
that under elevated CO 2 out of the 10 biotrophic
pathogens studied, disease severity was enhanced
in six and reduced in four and out of 15 necrotro-
phic pathogens, disease severity increased in nine,
reduced in four, and remained unchanged in the
other two (Chakraborty et al. 1998 ).
It has been observed that oats infected with
Barley yellow dwarf virus (BYDV) showed greater
biomass accumulation to CO 2 enrichment than the
healthy plant. Tobacco plants grown at increased
CO 2 concentrations showed a markedly decreased

may have some positive effects, which may likely
offset the negative effects of virus infection.
Some diseases can cause more severe reduction
in plant growth under twice ambient compared to
ambient CO 2 at least in controlled environments.
For example, in barley powdery mildew, an accli-
mation of photosynthesis at elevated CO 2 and an
infection-induced reduction in net photosynthesis
caused larger reductions in plant growth at ele-
vated CO 2 (Hibberd et al. 1996b ).
Thompson et al. ( 1993 ) reported a signifi cant
reduction in wheat powdery mildew at twice-
ambient CO 2 , but fi nal severity was dependent on
nitrogen and water status of plants.
Pathogen growth can be affected by higher
CO 2 concentrations resulting in greater fungal
spore production. However, increased CO 2 can
result in physiological changes to the host plant
that can increase host resistance to pathogens
(Coakley et al. 1999 ).
Despite initial delays and reduction in host
penetration, established colonies grow faster
inside host tissues at elevated CO 2 (Hibberd et al.
1996a ; Chakraborty et al. 2000a , b ). Fecundity of
both biotrophs and necrotrophs (Chakraborty
et al. 2000a , b ) studied so far has increased under
elevated CO 2 . A combination of increased fecun-
dity and a favorable microclimate within enlarged
canopies will provide more opportunities for
infection. There is evidence of adaptation for
increased aggressiveness in some pathogens
(Kolmer and Leonard 1986 ) within three sexual
generations, and controlled crossing has shown
that aggressiveness is heritable and may be poly-
genically controlled (Caten et al. 1984 ). For sex-
ually reproducing pathogen populations with
broad genetic diversity, increased population size
and the number of generations in favorable
microclimates would increase the probability of
more damaging pathotypes evolving more rap-
idly (Sutherst et al. 1996 ).

8.4 Elevated Temperatures

Harvell et al. ( 2002 ) considered the consequences
of warmer temperatures on host-pathogen inter-
actions and concluded that there will be three
main effects: