Agriculture Reference
In-Depth Information
8.7.3
Elevated Ultraviolet-B
et al. 1999 ). For example, heavy rains reduce fun-
gicide residue. Crop plants growing under ele-
vated CO 2 could be altered morphologically or
physiologically, affecting uptake, translocation,
and metabolism of systemic fungicides. For
example, increased thickness of the epicuticular
wax layer on leaves could result in slower and/or
reduced uptake by the host, while increased can-
opy size could negatively affect spray coverage.
Conversely, if higher temperatures increase
plants' metabolic rates, they may take up chemi-
cals more quickly, which may result in greater
toxicity (Coakley et al. 1999 ).
New dimensions of climate change may add
extra uncertainty in management strategies for
diseases caused by different pathogens. Impacts
of climate change on plant pathosystems would
occur chiefl y through infl uences on host resis-
tance or chemical and biological control agents.
Delayed planting to avoid a pathogen may
become less reliable. Particular attention is
needed to identify cases where the effi cacy of
disease management may be reduced under cli-
mate change.
Ultraviolet (UV) light has long been known to
infl uence plant pathogenic fungi. This light may
stimulate spore production in a wide range of
fungi but may also reduce spore survival during
dispersal or early stages of infection (Paul et al.
1998 ). Although an increase in solar UV-B radia-
tion due to ozone depletion could promote sporu-
lation of pathogenic fungi in a way that could
greatly increase the frequency and intensity of
epidemics, normal daylight already contains
enough UV light to stimulate sporulation of light-
dependent fungi.
There is considerable information on the
effects of increased UV-B on crops and natural
vegetation and on the growth and life cycle of
pathogenic organisms such as fungi. Studies indi-
cate that the UV-B component of solar radiation
plays a natural regulation on plant diseases.
Stimulatory effect of near-UV light on reproduc-
tion of many fungi and spore production in
Leptosphaerulina trifolii peaks at 287 nm are
reported. Fungi differ in their sensitivity to
UV-B. Some strains of Septoria tritici are more
sensitive to UV-B than others, and S. nodorum , as
a species, is more sensitive than S. tritici . UV-B
radiation can modify the relative composition of
phylloplane organisms, such as pink and white
yeast. Continued exposure to enhanced UV-B
radiation lowers the level of antifungal com-
pounds in foliar parts. UV-B has been shown to
reduce tolerance of rice to blast ( Pyricularia gri-
sea ) and although higher UV-B reduced plant
biomass and leaf area, there was no increase in
blast severity. There are some evidences that sun-
light can infl uence pathogen by causing accumu-
lation of phytoalexins or protective pigments in
plant tissues. Therefore, UV-B may affect plant
diseases directly via the pathogen or indirectly
via the host.
8.8.1
Host Resistance
Cultivar resistance to pathogens may become
more effective because of increased static and
dynamic defenses from changes in physiology,
nutritional status, and water availability. Durability
of resistance may be threatened, however, if the
number of infection cycles within a growing sea-
son increases because of one or more of the fol-
lowing factors: increased fecundity, more
pathogen generations per season, or a more suit-
able microclimate for disease development. This
may lead to more rapid evolution of aggressive
pathogen races. In a pilot study, evolution of
Colletotrichum gloeosporioides on Stylosanthes
scabra under elevated CO 2 was monitored. A sus-
ceptible cultivar was grown in a controlled envi-
ronment under 1x or 2xCO 2 and inoculated with
three isolates of the pathogen. For each isolate,
conidia collected from infected host tissue were
used to inoculate a second group of plants of the
same cultivar. Successive groups of plants were
8.8
Disease Management
Climate change can affect disease management
by altering effi cacy of biological and chemical
control options (Chakraborty et al. 1998 ; Coakley
 
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