Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 160

some regions, the bacterium caused serious
losses. Similarly, the coffee rust epidemic in Asia
during the late 1800s was facilitated by growing
an introduced susceptible host, Coffea arabica , in
a region where the native pathogen, Hemileia
vastatrix , was already present on alternative hosts
in the forests outlining the coffee plantations
(Carefoot and Sprott 1967 ). Expansion of host
range may even occur in specialized biotrophs, as
geographical proximity is as important as phylo-
genetic relatedness in infl uencing the host range
of some rusts (Savile and Urban 1982 ). As plants
in both natural and agricultural communities can
be symptomless carriers of pathogens, any early
predictions of impending damage will be diffi cult
(Dinoor 1974 ).

other pathogens. Of the 49 bacteria and fungal
pathogens examined, exposure to elevated ozone
concentration enhanced disease in 25, did not
affect 10, and reduced 14. Pollutant concentra-
tions, which inhibit pathogen development, also
injure the host. Similarly, infection by plant patho-
gens can alter ozone sensitivity of plants. Exposure
to 5-10 ppm ozone for a few hours can cause vis-
ible injury to sensitive crops like barley, tomato,
onion, potato, soybean, tobacco, and wheat.
Elevated ozone concentrations may change
the structure and properties of leaf surfaces in
ways that may affect the inoculation and infec-
tion process. Ozone enhances senescence pro-
cesses, may encourage necrosis, and seems to
promote attacks on plants by necrotrophic fungi.
Current climate change scenarios predict a
further increase of tropospheric ozone, which is
well known to inhibit plant photosynthesis and
growth process. Ozone can also predispose plants
to enhanced biotic attack, as proposed in particu-
lar for necrotrophic fungi, root rot fungi, and
black beetles. However, at present it does not
seem possible to predict whether increased ambi-
ent ozone will lead to higher or lower disease
likelihood in particular plant-pathogen system.
Several root pathogens show a preference for
stressed trees, although the direct role of ozone is
not always evident. Onions injured by ozone
exposure were more susceptible to Botrytis cine-
rea , but not to B. squamosa . Increased onion
yields and reduced dieback when fi lters removed
ambient ozone have been also observed in some
experimental studies.

8.7 Elevated Levels

of Atmospheric Pollutants

8.7.1 Ozone

Ozone is considered to be the most phytotoxic of
the common air pollutants. It can cause chlorotic
and necrotic lesions on sensitive plant species,
and even in the absence of visible symptoms, pho-
tosynthesis and growth can be inhibited. Ozone
damage can lead to reduced competitive fi tness of
plants, and reduced vitality makes plants more
susceptible to plant pathogens (Sandermann
2000 ). Direct effects of ozone on fungal patho-
gens are not signifi cant (Manning and von
Tiedemann 1995 ), although interactions between
ozone damage and infection by Alternaria solani ,
the causal agent of early blight of potato, have
been reported (Holley et al. 1985a , b ). Researchers
have reported both increased (Sandermann 2000 )
and decreased (Coleman et al. 1988 ) disease sus-
ceptibility in plants after ozone exposure.
According to von Tiedemann and Firsching
( 2000 ), ozone effects on plant disease susceptibil-
ity may be strongly altered by interfering factors
such as plant developmental stage, nutrient sup-
ply, and other atmospheric trace gases.
Most air pollutants indirectly infl uence diseases
through their effect on host. Ozone induces reac-
tions similar to those normally elicited by viral and

8.7.2 Acid Rain

Most studies on the effect of acid rain were done
with simulated acid rain since it is not easy to
establish experiments under fi eld conditions. In
fi rst year of experiment, no effect of acid rain has
been observed on any of four pathosystems: alfalfa
leaf spot, peanut leaf spot (PLS), potato late blight
(PLB), and soybean brown spot. In the second
year, PLS severity decreased with increasing acid-
ity and the dose response was linear; PLB severity
showed a curvilinear response to acid rain.