The effects of UV and elevated temperatures
are synergistic (2 + 2 =
6). Example: Riesling seedlings were
inoculated with PM spores, maintained
at 68°, 77°, or 86°F, and subjected
to outdoor levels of UV for 6
hours/day x 4 days. At both 68° and
77°F, the latent period or generation
time (i.e., time between inoculation
and formation of a mildew colony
producing new spores) increased by
two days relative to other seedlings
at the same temperature that did not
receive the UV. However, at 86°F (near
the upper threshold for mildew development),
UV exposure increased it by
seven days.
Surface temperature and
UV field experiments
In order to separate these two specific
sunlight components, we suspended
a Plexiglas “roof” over both
Chancellor (highly susceptible interspecific
hybrid) and Chardonnay
vines in Geneva, NY. Plexiglas blocks
UV radiation but permits passage
of the sunlight wavelengths that
elevate surface temperature. At the
Chancellor vineyard, we also suspended
shade cloth over other vines
to shield them not only from UV
radiation but also from the heating
effect of direct sunlight.
Clusters were inoculated with PM
spores at 75% capfall. As shown in
Figure 2, we found that removing UV
radiation (Plexiglas filter) increased
disease severity on fruit by 50%
to fivefold, for both varieties. The
Chancellor shade cloth treatment,
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which eliminated both the increase
in surface temperature and UV radiation,
further increased disease severity
in one of the two experiments.
Sunlight manipulation
in the vineyard
Given that UV radiation and sun
exposure reduce PM, how can we use
this information to better manage
the disease? We examined this question
in a young Chardonnay vineyard
in Geneva, NY by comparing
two training systems, Vertical Shoot
Positioning
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(VSP) and Umbrella-
Kniffen (UK), and removing basal
leaves around clusters to provide different
levels of light exposure in the
fruiting zone.
UK-trained vines provided more
shoots per linear foot of vine row
than VSP, hence more potential for
canopy shading in the fruit zone.
Within each training system, basal
leaves were removed on two dates:
two weeks post-bloom (fruit set) and
five weeks post-bloom. We inoculated
clusters with PM spores at bloom and
rated disease severity in each treatment.
We found that both factors affected
PM severity (Figure 3). First, powdery
mildew severity was lower in the
VSP than in the UK training system,
regardless of leaf pulling treatment.
Second, leaf removal at fruit set significantly
reduced the amount of disease
in both training systems, but leaf
removal five weeks after bloom had
no effect.
The benefits of the early (versus
late) leaf removal once again illustrate
the critical nature of those first few
weeks following the start of bloom.
This is when the fungus either does
or does not get established on the
clusters, and you want to hit it not
only with your best spray materials
but also with the cultural control
tools you have available.
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