Agriculture Reference
In-Depth Information
Fig. 8.4 Disease severity (on a scale from 0 to 9) caused
by Colletotrichum gloeosporioides on susceptible
Stylosanthes scabra plants under 2xCO 2 over eight cycles
of infection. For each of the three isolates (indicated by
different symbols and lines ), successive groups of plants
were inoculated with conidia arising from the previous
infection cycle to simulate polycyclic disease develop-
ment and pathogen evolution over time. The regression
lines for the three isolates were signifi cantly different
from zero ( P < 0.05)
inoculated with conidia arising from the previous
infection cycle to simulate polycyclic disease
development and pathogen evolution over time.
After each cycle, measurements were made on
components of pathogen aggressiveness, such as
fecundity, lesion size, lesion number, and disease
severity. Preliminary results suggested a signifi -
cant trend towards increased disease severity
(Fig. 8.4 ); further, two of the three isolates showed
a gradual increase in fecundity under elevated
CO 2 after eight infection cycles.
Elevated temperature may cause the breakdown
of temperature-sensitive resistance in oat cultivars
with Pg3 and 4 genes (Martens et al. 1967 ).
suggest precipitation during the post-application
period (Schepers 1996 ) is critical. Precipitation fol-
lowing fungicide application may improve its dis-
tribution (Schepers 1996 ), but an increase in rainfall
intensity can deplete fungicide residue on the foli-
age (Neuhaus et al. 1974 ). The interactions of pre-
cipitation frequency, intensity, and fungicide
dynamics are complex, and for certain fungicides
precipitation, following application may result in
enhanced disease control because of a redistribu-
tion of the active ingredient on the foliage (Schepers
1996 ). Neuhaus et al. ( 1974 ) applied simulated rain
to potato foliage at two intensities (6 and 30 mm/
ha) and found that the higher rate signifi cantly
reduced the fungicide residue that could be mea-
sured with a chemical assay, but that there was no
difference in disease between the two treatments
when the leaves were challenged in a bioassay with
Phytophthora infestans .
Second, morphological or physiological
changes in crop plants resulting from growth
under elevated CO 2 could affect uptake, translo-
cation, and metabolism of systemic fungicides.
For example, increased thickness of the epicutic-
ular wax layer on leaves (Wolfe 1995 ) could
result in slower and/or reduced uptake by the
host, whereas increased canopy size could
8.8.2
Chemical Control
Climate change could affect the effi cacy of crop
protection chemicals in one of two ways. First,
changes in temperature and precipitation may alter
the dynamics of fungicide residues on the crop foli-
age. Globally, climate change models project an
increase in the frequency of intense rainfall events
(Fowler and Hennessy 1995 ), which could result in
increased fungicide wash-off and reduced control.
Data from fi eld experiments and modeling studies
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