Unlike the general public, grapegrowers know that disease management does not involve a choice between “chemical” and “non-chemical” approaches, but rather an informed integration of those components to fit each individual’s needs and philosophies. This article’s focus is the new information pertaining to a non-chemical tool for the vineyard, specifically the dramatic effect that sunlight has reducing powdery mildew development—and its implications with respect to disease management.

A subsequent article will review some of the new fungicides now available to help growers control both powdery mildew and downy mildew.

Sunlight exposure and powdery mildew
Growers have long observed that powdery mildew (PM) is most severe in shaded areas of vineyards, but the causes and actual impact of the phenomenon have been poorly understood. We recently completed a five-year study to determine the impact sunlight exposure has on powdery mildew development, which has shown that growers can greatly enhance their disease management programs simply by using common canopy management practices to optimize light interception throughout the canopy and fruiting zone.

An early experiment was conducted in a Chardonnay vineyard near Dresden, N.Y., where a small portion of the easternmost row was bordered by a group of 50-foot pine trees. Despite a spray program that controlled PM adequately on nearly all other vines, we had seen that this disease completely destroyed the clusters on the three panels of vines immediately next to the trees, although these panels were sprayed exactly the same. These panels were shaded during the morning, and it wasn’t until the sun crested over the treetops just before noon each day that the vines received their first direct exposure to sunlight.

The next year, we initiated a trial in which we inoculated leaves on either the outer (exposed) or inner (shaded) portions of vines, which were located either immediately next to or 200 feet away from these trees, thereby providing a total of four levels of natural shade. The resulting disease severity increased substantially with each increasing level of shade, becoming eight to 40 times more severe on the most heavily shaded leaves (interior vines next to the trees) compared to the sunniest leaves (exterior vines away from the trees). (See Figure 1.)

Although shading could potentially change air temperature or relative humidity within the vine canopy, our measurements did not show this. However, they did show that UV radiation levels and leaf temperatures were dramatically different among the different treatments. As one would expect, within the shaded regions UV levels were a mere fraction of those in the sun, and 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 effects of sunlight on PM development.

Sunlight characteristics influence powdery mildew development
UV radiation from the sun can damage the cellular structure of virtually 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 organs, 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.

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 Center in Prosser, Wash., with the assistance of Dr. Gary Grove and his staff. Plexiglas blocks UV radiation but permits passage of the sunlight wavelengths that elevate surface temperature.

At the Chancellor vineyard in Geneva we also suspended shade cloths 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 with the Plexiglas filter increased disease severity on fruit by 50%-500% for both varieties and locations. 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 New York 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 provided more shoots per linear foot of row than VSP, hence more potential for canopy shading in the fruit zone.

Within each training system, we removed basal leaves at two dates: Two weeks post-bloom (fruit set) and five weeks post-bloom. We inoculated clusters with powdery mildew spores at bloom and rated PM severity in each treatment. We found that both factors affected PM severity. (See 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 are likely due to the oft-repeated fact that most serious berry infections occur during the first few weeks following the start of bloom. This is when growers should hit the fungus with both barrels—use the best fungicides and the cultural control tools available—rather than wait until significant damage has occurred before employing them.

The most important result was that simply by utilizing a VSP training system and basal leaf removal at fruit set, we were able to reduce fruit disease severity by 35% relative to UK-trained vines with no leaf removal.

Exposure of fruit to sunlight and pesticides
Canopy management practices that increase sunlight penetration into the fruiting zone should also increase the penetration of sprays applied to control pests and diseases. With the assistance of Dr. Andrew Landers at Cornell University, we were able to quantify the effect that canopy density can have on spray coverage.

Vines in our Chardonnay planting subjected to the above canopy manipulations were sprayed with a conventional air blast unit, and deposition on clusters from each vine was assessed in the lab. As expected, we found a direct relationship between the quantity of spray deposited on each cluster and the sunlight exposure level (Figure 4), with well-exposed clusters receiving approximately twice the deposition as those with poor exposure.

Management implications
In all vineyards, in all seasons, for all experiments at all locations, increasing sunlight exposure on leaves or fruit reduced the severity of powdery mildew on those tissues—independent of spray coverage. And when improved spray coverage is factored in, the benefit of canopy management for PM control is not only compounded but extends to other diseases as well. However, a central concept associated with quality viticulture is “balance.” Zero sunlight exposure might lead to diseased berries, but absolute maximum exposure can lead to sunburned berries. It’s all about balance.

Wayne F. Wilcox is a professor in the Department of Plant Pathology and Plant-Microbe Biology at Cornell University’s New York State Agricultural Experiment Station at Geneva, N.Y. Craig N. Austin is a graduate student in the same department.