A Dairy Solution to Mildew Woes

Milk may prove the savior of some organic wines.

New field trials show that it kills a powdery mildew that would otherwise damage vine leaves and destroy grapes.

Many urban dwellers don’t realize that mildew is more than the unsightly bane of shower stalls. On farms around the globe, it causes extensive plant infections that can exact an enormous economic toll. Though a variety of synthetic agricultural chemicals have been developed to counter mildew, such agents must not be used if crops are to be certified as having been grown organically–at least for those sold from 2007 onwards within the European Union (EU).

So, Peter Crisp of the University of Adelaide, Australia, has been experimenting with natural alternatives that will quash powdery mildew on grapes and other plants, including rose bushes. One promising alternative may be as close as your refrigerator.

For the past 2 years, Crisp has been spraying ordinary milk–diluted with water to a 10-percent solution–on grape vines at two commercial vineyards. He finds that in most cases, milk performs as well as the leading nonorganic powdery-mildew fighters–sulfur and a synthetic chemical known as Topas. He also has achieved about the same success with diluted sprays of liquid whey, a waste byproduct of cheese production.

Though more expensive than sulfur, milk and whey sprays cost less than synthetic fungicides. They’re part of a battery of new agents that may permit organic vintners to continue exporting their wines into European and U.S. markets.

‘A tough fungus’

Worldwide, powdery mildew is “the most serious grapevine fungal disease,” observes David Bruer. Formerly a professor of enology–the science of winemaking–at Roseworthy College (now part of the University of Adelaide), Bruer and his wife Barbara Bruer, who are both chemists, now own a 67-acre organic vineyard. Most years, powdery mildew strikes at least some of their vines. And although not tremendously invasive, he says, “it is a tough fungus–hard to kill.”

The bottom line: If this fungus is ignored, he says, “you won’t pick any fruit.” So, he and his fellow vintners remain vigilant and spray vines at the first sign of infection.

Until recently, Bruer relied on sulfur or synthetic mildicides. Then Crisp recruited Temple Bruer Wines and a couple other organic winemakers to test alternatives on their vines.

With different varieties of grapes–and their inherent differences in susceptibility to mildew–”we get different degrees of control,” Bruer told Science News Online. But overall, he maintains, the results “are marvelous.”

For example, he used the new sprays on Grenache grapes, which are used to make a red wine and are moderately susceptible to mildew. Bruer says, “we got zero infection.” In contrast, his neighbor’s especially susceptible Chardonnay grapes “are quite difficult to protect.”

It started with zucchinis

Crisp says that the idea for using milk came from a 1999 research paper by Wagner Bettiol. A researcher in Brazil, Bettiol reported using various dilutions of fresh cow’s milk to control powdery mildew on zucchini squash growing in greenhouses.

That particular fungal species was different than the one that ravages grapes, but Crisp decided to give milk a try. And it worked. He has since applied it to some other plants, such as roses, and shown that it also kills the mildew that plagues them.

The Bettiol report and others described additional organic agents that appeared to control powdery mildew on plants, such as canola oil, clay, seaweed extracts, and bicarbonate salts. So, Crisp added them to his test arsenal for grape protection.

Indeed, Bruer’s preferred new regimen consists of spraying with emulsified canola oil and bicarbonate once a week and then moving to a spray of whey and seaweed extract the next cycle.

“We’ve been using whey instead of milk, Bruer says, because “the supply of whey far exceeds the demand.” It’s therefore quite inexpensive and solves a major waste problem for the dairy industry.

Although spraying milk or whey every 2 weeks appears to work well at controlling the fungal blight, Bruer says, “we are very reluctant to use one fungicide repeatedly.” That might make it easy for the mildew to develop resistance to control. So, even if there was a great difference in efficacy between the alternatives that Crisp identified–”which there isn’t,” Bruer says–”we probably wouldn’t use the most effective one every time. We prefer to mix up fungicides, no matter what we use.”

The different organic agents also appear to offer different modes of action in killing mildew, Crisp says. In the presence of sunlight, milk and whey, for instance, appear to foster the production of biologically damaging free radicals, such as hydrogen peroxide and superoxide radical.

Bruer notes that in whey, “a protein, ferroglobulin, under the influence of ultraviolet light produces an oxygen radical that is extraordinarily toxic.” So, any fungus that encounters it “will be in big trouble,” he explains. Why doesn’t it hurt the plant? Grape leaves have a thin layer of surface wax that the water-soluble radical can’t penetrate.

But this mechanism also explains why the dairy treatments don’t work well on heavily overcast days, Bruer adds. They need the sun’s light to kick-start their mildicidal action.

Hoping for a sulfur reprieve

Over the next few years, Crisp hopes to not only prove the utility of the organic fungicidal regimens–under both strong and weak infestations–but also to confirm that they have no deleterious impact on a wine’s flavor.

Head-to-head taste comparisons of wines made from grapes treated with the conventional mildicides and the new alternatives are scheduled for vintages bottled using the coming season’s grapes.

Even if the organics pass with flying colors, Bruer argues against the EU’s phase-out of sulfur and copper treatments on organic grapes. It’s not that he prefers the old-guard fungicides, but he thinks they should be available as a second-string defensive line to step in when the organic agents aren’t sufficient to knock out a particularly heavy blight.

If permitted on a restricted basis, Bruer says, they would be available for use “when you could demonstrate you would have an economic loss if you didn’t use them.”

And why does an Australian winemaker even care what the EU organic-produce rules prohibit? For starters, he notes, many of Temple Bruer’s 15,000 cases of wine each year are exported to Europe. But even for those bottles exported elsewhere, he notes, countries that support certified-organic labels for foods tend to follow the EU. “It’s a fact,” he contends, “that the EU has an absolute hegemony over organics at this stage.”

Meanwhile, anyone can test the milky solution to mildew in his or her backyard garden. Crisp recommends using dry-milk powder–15 grams for every liter of water. So far, he finds, this formulation appears to work on all surface mildews.

Best of all, if you get thirsty while spraying, you can just sip the milky concoction. Though it makes heads turn, he concedes, “I’ve been known to do that.”