Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 163

8.8.4 Microbial Interactions

negatively affect spray coverage and lead to a
dilution of the active ingredient in the host tissue.
Both factors would suggest lowered control effi -
cacy at higher concentrations of CO 2 . Conversely,
increased metabolic rates because of higher tem-
peratures could result in faster uptake by and
greater toxicity to the target organism. Despite
the potential for important interactions, no simi-
lar studies evaluating the impacts of climate
change variables on physiological aspects have
been published for fungicides.
The more frequent rainfall events predicted by
climate change models could result in farmers
fi nding it diffi cult to keep residues of contact fun-
gicides on plants, triggering more frequent appli-
cations. Systemic fungicides could be affected
negatively by physiological changes that slow
uptake rates, such as smaller stomatal opening or
thicker epicuticular waxes in crop plants grown
under higher temperatures.

Climate change may alter the composition and
dynamics of microbial communities in aerial and
soil environments suffi ciently to infl uence the
health of plant organs (Gunasekera et al. 1997 ).
Changed microbial population in the phyllo-
sphere and rhizosphere may infl uence plant dis-
ease through natural and augmented biological
control agents. A direct effect of elevated CO 2 is
unlikely in the soil environment as the microfl ora
there is regularly exposed to levels 10-15 times
higher than atmospheric CO 2 .
Trees grown in soils of poor nutrient status,
especially nitrogen, favor colonization of roots
by arbuscular mycorrhizal fungi (Klironomos
et al. 1997 ). The relationship between elevated
CO 2 and mycorrhizae is not well understood
(Singer 1996 ), and there are confl icting reports
on how it may be infl uenced by the nutrient status
of the plant and soil. If a lower nitrogen status of
plant tissue under increased CO 2 results in more
mycorrhizal colonization, this could improve
plant health through improved nutrient uptake.
Similar confusion exists on the potential role of
arbuscular mycorrhizal fungi and ectomycorrhi-
zae in the suppression and biological control of
plant pathogens. Mycorrhizae can have positive,
negative, or neutral effects on plant disease, and
their role is not well understood despite numer-
ous studies on the subject (Pfl eger and Linderman
1994 ). Clearly, the infl uence of mycorrhizae on
plant health under climate change requires fur-
ther research.

8.8.3 Biological Control

There may be problems with applications of bio-
control agents in the fi eld because of the vulner-
ability of biocontrol agent populations to
environmental variations and environmental
extremes. If appropriate temperature and mois-
ture are not consistently available, biocontrol
agent populations may reach densities that are
too small to have important effects and may not
recover as rapidly as pathogen populations when
congenial conditions reoccur.
In the rhizosphere, elevated CO 2 would inter-
act with nitrogen and other soil factors to modify
the number and type of mycorrhizal fungi to
infl uence root health and nutrient uptake. Some
short-term studies under controlled conditions
have shown that elevated CO 2 can stimulate
mycorrhizal colonization (Staddon and Fitter
1998 ) due to faster plant growth. Colonization of
roots by arbuscular mycorrhizal fungi is favored
in soils of poor nutritional status (Klironomos
et al. 1997 ). It is not clear if increases in soil car-
bon storage due to greater root and mycorrhizal
growth under high CO 2 will infl uence mycorrhizal
colonization.

8.8.5 Quarantine and Exclusion

Management of climate change will put addi-
tional pressure on agencies responsible for exclu-
sion as a plant disease control strategy (Kahn
1991 ). In some regions, certain diseases of eco-
nomic concern do not currently occur because
the climate has precluded the causal agents from
becoming established. Use of geographical
information systems and climate-matching tools
may assist quarantine agencies in determining
the threat posed by a given pathogen under