September 2017 Issue of Wines & Vines

Researchers: Disease Growing From Maryland to Missouri

by Linda Jones McKee

Charlottesville, Va.—A pair of wine grape growers from different parts of the United States independently discovered grape vine symptoms they could not explain and ended up identifying a fungus that affects vinifera and hybrid grape cultivars. 

Lucie Morton, a well-known viticultural consultant based in Charlottesville, Va., and Dr. Dean S. Volenberg, viticulture and winery operations extension specialist at the Grape and Wine Institute of the University of Missouri, each found inexplicable symptoms on grapevines, leaves and fruit. Volenberg identified the fungus Pestalotiopsis in Norton berry clusters and also in a canker on a single Norton vine (which caused trunk die-back), and began writing about his findings in his weekly newsletter Vinews (Viticulture Information News) in 2015. Morton first found Pestalotiopsis in 2009, and when she intensified her investigation of the fungus in 2016, she found Volenberg’s newsletters online.

The two began to work together, as both of them realized Pestalotiopsis was causing more problems. Volenberg found symptoms early in the season in Norton and Chambourcin in Missouri, and Morton saw evidence during or after véraison in Cabernet Sauvignon and other vinifera and hybrids in Pennsylvania, Maryland and Virginia. 

Morton and Volenberg displayed two posters that summarized their evidence that Pestalotiopsis should be recognized as a fungal disease at the American Society for Enology and Viticulture-Eastern Section meeting in Charlottesville, Va., in July.

Morton’s early work 
Morton identified Pestalotiopsis in a Pennsylvania vinifera vineyard in 2009. “I saw some canes that had bleaching and little black dots, but it wasn’t phomopsis,” Morton told Wines & Vines. When the samples she collected began to grow what looked like a fungus, Morton looked at it under a microscope and saw five-celled conidia, the asexual spores of a fungus. In searching through a fungal key, she found one that looked like what she was observing. However that fungus, Pestalotiopsis, was not found in the Compendium of Grape Diseases edited by Roger C. Pearson. 

At the time, Morton was working on grapevine red blotch-associated virus and didn’t follow up on Pestalotiopsis. However, in 2014 one of her “high-end vineyard” clients in Maryland started to have berry shrivel and cluster stem wilt. It was late in the season, and Morton noted that a number of pathogens can cause those symptoms. She also recalled the grower had stopped using Pristine, a common strobilurin fungicide, earlier in the season.

The following year, the vineyard manager told Morton he had identified what he called “the coral rachis disease.” He could see the red rachis on a grapevine from the tractor and, from his experience, knew that was a sign the cluster would shrivel and the berries start to rot. She collected and then incubated samples, and what grew were the five-cell conidia of Pestalotiopsis. Her next step was to look at other fruits, and she found cluster stem wilt in blueberries that was similar to the Pestalotiopsis problems she had seen in grapes. In 2016, she learned about Volenberg’s discoveries in Missouri. 

Trunk disease, then fruit rot: What’s next? 
Volenberg first saw and then identified Pestalotiopsis in a Norton vine with trunk disease near Hermann, Mo., in June 2015. A few weeks later he visited another vineyard where a grower was seeing rot on clusters after lots of rain. “The grower didn’t think it was black rot,” Volenberg said. “It looked like bruised fruit.” After reviewing the grower’s spray program, he suggested including use of Pristine. After it was applied, the affected berries dried up and dropped off. He later confirmed the culprit was indeed Pestalotiopsis.

The extension specialist did some research and found that Pestalotiopsis had been identified in table grapes in China. He also located a paper, “Characterization of Fungal Pathogens Associated with Grapevine Trunk Diseases in Arkansas and Missouri,” by J.R. Urbez-Torres et al. (2012), that identified Pestalotiopsis as the second most prevalent fungus in vineyards in those states. 

In 2016, Volenberg received reports of leaf discoloration on young leaves of both Norton and Chambourcin in mid-May. In his newsletter in mid-June, Volenberg recommended Missouri growers include strobilurins such as Pristine, Abound and Flint, and also mancozeb in their spray programs because Pestalotiopsis can cause fruit rots as well as trunk and leaf problems.

Teamwork in 2017 
Morton and Volenberg decided to work together to develop a better understanding of this apparently ubiquitous fungus. In both regions, the fungus seemed to require rain to get sporulation, and as a result, Pestalotiopsis probably will not be a problem in dry climates such as California. 

Morton and Volenberg did laboratory testing to evaluate the growth of Pestalotiopsis on potato dextrose agar and also the growth of Pestalotiopsis in the presence of mancozeb and pyraclostrobin in Petri plates. The results indicated that growth of Pestalotiopsis was substantially inhibited, even with low levels of the fungicides. 

Volenberg’s work with Norton that suggests that the symptoms of Pestalotiopsis are transient and may succumb to the heat in summer. “It may be a weak pathogen,” he said. “It hangs around until the conditions are right.” 

According to Morton, many questions remain about Pestalotiopsis. “We should look at cultural practices,” Morton said. “Trunk disease may result from mechanization. Is that making diseases worse or more significant?” Growers need to remember that not every black dot is phomopsis, and not every rot is black rot. They should watch out for Pestalotiopsis, especially if they want the vineyard to be organic.

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