HIGHLIGHTS
- Horticultural oil sprays can eradicate mildew and also reduce leafhopper and mite populations.
- As an alternative to sulfur dust, oil avoids sulfur’s particulate air contamination concerns.
- Earlier studies in eastern North America found oil sprays could delay ripening and reduce yields.
- Two recent studies with oil sprays at effective levels in California did not find delayed ripening or reduced yields.
Using horticultural oils to control powdery mildew and suppress grape leafhopper, mite, scale and mealybug is becoming a popular tool of integrated pest management (IPM) in vineyards, but Laura Breyer of Breyer’s Vineyard IPM Service in Windsor says that questions have been raised about the oil’s impact on grape ripening, yield and quality. “Some winemakers have become reluctant to allow their growers to use oils because of these concerns,” she says. A recent report from the Central Coast—and another from the North Coast, in which she participated—seem to dispute those fears.
Horticultural oil such as Stylet Oil is an attractive treatment because it can eradicate mildew and also reduce leafhopper and mite populations. Commonly applied sulfur dust can prevent but not eradicate mildew, and it actually encourages leafhoppers and mites. Other attractions include a low probability of the pests developing resistance to oil, and the fact that many people remain concerned about the role of sulfur dust as a particulate air contaminant.
However, some studies in eastern states found that oil could delay ripening and reduce yields. Those studies, which included high levels of oil application, seem not to apply in California’s climate, according to the studies performed in Alexander Valley and Paso Robles vineyards.
Oil is generally alternated with lower level applications of sulfur, and it is also used with pesticides of longer residual duration such as sterol inhibitors (SI) or strobilurin fungicides. This costs more and can take more time than sulfur dust, but it is effective for 14 to 21 days compared to 7 to 10 days with sulfur dust.
Oil can damage the canopy during hot weather, however, particularly if applied too soon after sulfur. It affects the waxy bloom on berries, superficially and temporarily altering fruit appearance.
Virginia Tech professor Tony Wolf published findings in the 2002 American Journal of Enology and Viticulture, “Effects of Horticultural Oils on the Photosynthesis, Fruit Maturity and Crop Yield of Winegrapes,” in which he found delayed ripening and lower yields with some of the treatments. His trials used levels of oil from 3 gallons per acre to 30 gpa.
However, according to Breyer, a typical North Coast grower uses a much lower level of oil in IPM programs—typically 3 gallons of oil or less during a season.
Breyer participated in a study conducted by Jim Cuneo, who manages the Robert Young Estate Winery in Alexander Valley, and has been relying on JMS Stylet Oil as the key component in his IPM program for the past five years, significantly reducing mite, leafhopper, scale and mealybug treatments.
Cuneo estimates a 90% reduction in insecticide and miticide treatments since integrating JMS Stylet Oil into his mildew program. The lower costs from fewer treatments also encourage this use of the oil.
Cuneo set up a replicated but non-randomized trial in three blocks of Chardonnay using his regular 1% JMS Stylet Oil plus standard mildew products vs. the same program without the oil in alternating sets of eight rows.
He applied the Stylet Oil every two weeks from May 29 to July 11, from after bloom until veraison, using 0.29 gallons of oil per acre each time, for a total of less than 1.2 gallons.
Breyer coordinated support from the Sonoma County Winegrape Commission and JMS Flower Farms for data collection and analyses. University of California Cooperative Extension viticulture advisor Rhonda Smith consulted on data collection protocols. Vinquiry in Windsor analyzed the samples without knowledge of which treatment was being processed.
From veraison to harvest, samples were taken from each of the six treated and six untreated sets of rows. A minimum of 300 berries per sample were taken and counted, allowing for gram per berry—and thus sugar per berry—to be calculated along with Brix, pH and TA.
Harvest was not delayed for the oil-treated areas, and these results lend credence to the observation of growers in the North Coast who have been using oils that there is no noticeable impact on ripening.
Of course, Alexander Valley Chardonnay is harvested relatively early, and there are adequate heat units and sunlight to ensure this every year. In other areas such as the East, or with other varieties or different oil programs, more noticeable differences could be possible.
Mark Battany, UC Cooperative Extension’s Central Coast farm advisor, also conducted trials evaluating horticultural mineral oil sprays on the ripening of winegrapes in 2009, using Cabernet Sauvignon vines at two vineyards in San Luis Obispo County. A third trial in Santa Barbara County was abandoned due to unacceptable variability in vine vigor.
At the first site east of Paso Robles, seven applications of a 1% oil solution (JMS Stylet Oil) were applied using a commercial hydraulic sprayer at two-week intervals. The mature Cabernet Sauvignon vines were trained as a California sprawl. Treatments were compared to a control that did not receive any oil sprays. Plots were six rows wide by 35 vines long, with three replications.
At the second site east of San Miguel, treatments consisted of zero, two, four, six and eight sprays of a 1% oil solution. All sprays were applied with a backpack mist blower (Solo 444) equipped with a precision flow meter. The mature Cabernet Sauvignon vines were trained to a modified VSP (sprawling shoots on the west side, vertically positioned shoots on the east side). Each plot consisted of five vines, replicated five times.
Researchers evaluated leaf stomatal conductance weekly during the summer at both sites using a Decagon porometer, and also took berry samples weekly from veraison to harvest. They measured berry weights, juice soluble solids, juice pH and juice titratable acidity, and they measured the fruit weight and number of clusters per vine at harvest.
At both sites, Battany found no significant differences in yield, number of clusters per vine, cluster weight, berry weight, juice pH, juice titratable acidity, total sugar per berry or total sugar per vine.
He did find that there were, in some cases, significant differences in stomatal conductance and juice soluble solids. It’s not clear what, if any, impact these differences might have on wine made from the grapes.
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