Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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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 86F, and subjected to outdoor levels of UV for 6 hours/day x 4 days. At both 68 and 77F, 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 86F (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,
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
(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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