Jun 3, 2019
Midwest cold damage masks vine trunk diseases

Old Man Winter is broadly blamed for damaged grapevines across the Midwest.

But after looking beneath the surface, one Extension specialist discovered that much of the Midwestern damage is due to disease-causing fungi better known to Western growers. The main difference in the Midwest is that winter injury provides entry points for those diseases.

Michael J. White, an Iowa State University (ISU) Extension and Outreach viticulture specialist, spoke at the Southwest Michigan Horticultural Days about his discoveries. Twenty years ago, when he started his work in that field, Iowa had only a couple of wineries. Now there are roughly 100 with about 245 commercial vineyards covering 1,100 acres. ISU created the Midwest Grape and Wine Industry Institute in 2006, which has a food science department for wine testing in addition to a viticulture program. Iowa winegrapes growers are dependent on hybrid varieties rather than Vitis vinifera, White said. During the last polar vortex in 2012, low temperatures ranged from -35˚ F to -40˚.

With that sort of cold weather as the main challenge to growing winegrapes, historically it’s been easy for growers to overlook grapevine trunk disease.

“I knew maybe we had some,” White said. “I couldn’t really put my finger on it. If you look worldwide, it’s probably one of the most important problems we have in the worldwide grape industry.”

The six major diseases worldwide are Esca, Eutypa dieback, Phomopsis dieback, Botryosphaeria dieback, Young Esca or Petri disease and Black Foot. All six are fungal diseases.

Above, Botryosphaeria dieback on an Edelweiss grapevine. Edelweiss is a cold-hardy hybrid variety sometimes used in the Midwest for winegrapes. Photos: Michael J. White/Iowa State University

Iowa State University’s Michael White speaks at the Southwest Michigan Horticultural Days. Photo: Stephen Kloosterman

Viticulturalists know the value of various winegrape cultivars in growing regions, but diseases don’t discriminate. For White, an eye-opening experience was finding out during a California trip that Eutypa had claimed no less a vine than the famed Winkler Vine at the University of California-Davis. The mission cultivar was planted in 1979, named for Albert J. Winkler, and cultured as a large vine, where, White said, fundraising dinners were held in an arbor holding its up its expansive vines.

“It died from trunk disease,” he said. “I actually went up to some of these research buildings and knocked on the door and said, ‘What happened to the vine?’ ‘Oh, it’s out back.’ They wanted me to take it back with me on the plane. I should have.” The Winkler vine had a canker on the trunk that was a sign of Euytpa.

On a separate trip through the Midwest with vineyard consultant Richard Smart, the two diagnosed grapevine trunk disease at several vineyards, including the “mecca” of breeding cold-climate grape hybrids grape breeding, the University of Minnesota.

“They’ve got about 110 acres there … that’s where their winegrape cold varieties are being grown,” White said. At the time, Minnesota wasn’t aware of grapevine trunk diseases there, he said.

He determined the presence of new diseases by sending diseased trunk samples to labs in California in addition to ISU’s own plant science lab.

“All of the sudden, all these diseases we never had in Iowa started popping up,” he said.

It’s a relatively new science. In 2000, White said, only 15 species of fungus were identified as causing grapevine trunk diseases, but today there are 230. Trunk diseases cause an estimated $260 million per year in economic losses, he said.

In many cases, signs of disease such as a discolored trunk or leaves don’t always show until the plant is stressed, he said.

“When you read about trunk diseases in South Africa or California, or Australia, or anyplace in the world … you’ve got them here. And you may have them worse because you have something they don’t have: Winter injury.”

What you can do

Grapevine samples prior to being mailed off to a lab for diagnosis. It’s recommended to wrap the vines in plastic prior to express shipping.

Diagnose. Cut live-cordon sections, wrap them in plastic and send them quickly to a lab. White recommended two California labs – Trical Diagnostics in Gilroy or AL&L Crop Solutions in Vacaville – but more recently, Michigan State University Diagnostic Services has also hired staff with commercial experience testing for grapevine trunk disease.

Sanitation. Some people try cleaning their pruning equipment with Clorox, water, alcohol and even fungicide. White didn’t recommend this.

Double or long pruning. Bot and some other fungi will enter the vine through spores that find pruning scars, White said. It can work its way toward the roots at a rate of 3-6 inches per year and the whole plant will be infected once it reaches the graft union. A final, follow-up pruning in the spring will allow the pruning scars to “bleed” sap, cleaning out fungi, and the scars will heal more quickly in the warm weather, White said. “What that does for you, is those new pruning scars, at that new temperature, will heal over much faster.”

Always have renewal suckers ready to replace old, diseased cordons. A healthy vineyard should yield about 4 tons an acre. “Be renewing that constantly to keep your yields up, and cut out that infected stuff before it gets on the trunk,” he said.

Don’t prune in the rain. Rain increases the production of disease-causing airborne fungal spores.

Wild grapes in the Midwest are often hosts for disease, as shown by this wild grapevine in Indianola, Iowa.

Get rid of pruned limbs. Compost them or burn them, but most importantly get them out of the vineyard. “Most of these grapevine trunk diseases are living on that residue,” White said, and the fungi can send out spores when the weather warms up.

Barrier products. Several different products are available for painting pruning scars or vine trunks, although plain latex paint is also used by some growers. Many include boric acid in a paste or paint. White said fungicide in the paint is also proving effective. “Right now in California, everybody’s painting their pruning scars, or they’re spraying them on,” White said. “What you want to do is apply this within 24 hours after pruning.”

Fungicides. White said he was aware of three fungicides labeled for protection against major grapevine trunk diseases in the U.S.: Topsin-M, Rally and Mettle. Some growers are mixing fungicides in barrier products. A spray dye is recommended to ensure the pruning wounds are covered. Natural fungicides for grapevine trunk disease include Trichoderma and Bacillus subtillis – both of them, he said, are marketed under numerous labels. They are “fungal organisms that cover and colonize the wound preventing further infection,” according to his presentation.

– Stephen Kloosterman, associate editor


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