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WineEast temperatures of leaves in the sun were often 10°-30°F higher than those of leaves in the shade. As we later found, both elevated leaf temperature and UV radiation are responsible for the inhibitory ef- fects of sunlight on PM development. Sunlight characteristics influence powdery mildew development UV radiation from the sun can damage the cellular structure of virtu- ally all forms of life. However, powdery mildew is uniquely vulnerable to such damage. Unlike almost all other organisms that cause grape diseases, the PM fungus lives primarily on the outside of infected tissues. Most other pathogens live almost entirely within infected or- gans, where they are protected from UV. Furthermore, the PM fungus is white and has no pigment to protect against this radiation. Direct sunlight heats up exposed leaf surfaces—or anything else for that matter, as we all know from the difference between standing in the sun and taking two steps into the shade. This additional heat can suppress or even kill PM colonies on sun-exposed leaves and berries. Recall that powdery mildew grows best at temperatures near 80°F, but it stops growing at temperatures above 90°F and will start to die at temperatures above 95°F, depending on how hot it is and for how long. On a hypothetical summer day, when temperatures are in the 80°s, shaded leaves and clusters will remain near the air temperature—that is, at or near optimal for PM development. However, nearby vines or portions that are exposed to sunlight can often have temperatures elevated to a point where PM growth will stop or even retreat. 80 60 40 20 Grapegrowing Disease Area and Sunlight Sources of Shade No Shade Trees Canopy Trees and Canopy 2005 2006 Figure 1: Percentage of Chardonnay leaf area diseased on leaves re- ceiving full solar radiation on the outer edge of vines away from trees (no shade), morning shade from adjacent pine trees but otherwise exposed to the sun (trees), shade provided by the vine itself (canopy) or both tree and the internal canopy shading (tree and canopy). Surface temperature and UV: field experiments In order to separate these two specific sunlight components, we suspended a Plexiglas "roof" over Chancellor and Chardonnay vines in Geneva, N.Y., and Chardonnay vines in a vineyard at Washington State University's Irrigated Agriculture Research and Extension You make the best wines... as ask for or the be esst eq pmenequuipment Tanks Variable capacity, closed top, combined tanks built to your requirements Equipment • Presses • Crusher destemmer • Pumps and more. plus a variety of tanks in stock. Get a quote online now at www.raynoxinc.com 1148 St-Michel street, St-Cesaire, QC, Canada, J0L 1T0 Phone (450) 469-4822 Toll Free 1 866 469-4181 52 Wines & Vines MARCH 201 1 Foliar Disease Severity %