Vinews

No. 14 — July 1, 2019

Grapevine Trunk Diseases: What Richard Smart Found in Missouri

Chardonel grapevines graft union, showing discolored tissue of the rootstock area
Figure 1. The graft union of two unplanted Chardonel grapevines show the telltale signs of grapevine trunk disease. Note the rootstock would be in the lower portion of the picture. The discolored tissue of the rootstock area suggests the infection began with the rootstock, according to Dr. Richard Smart. Photo credit: D. Volenberg.

The flying vine doctor, Richard Smart, was in Missouri last week to provide education, consulting and wisdom on canopy management and grapevine trunk disease. I was fortunate to spend more than 30 hours with Dr. Smart. Over that period of time, we covered the gamut on canopy management and trunk diseases. For those of you unfamiliar with Dr. Smart’s contributions to viticulture, his contributions are immense and he is very well known for his book “Sunlight into Wine." Over the last two years, Dr. Smart has been traveling among eastern USA vineyards and diagnosing grapevine trunk diseases (GTD). In 2018, Dr. Smart and Mike White from Iowa State University visited Minnesota, Iowa, Nebraska and Illinois. In 2019, Dr. Smart visited New York, Quebec Canada, Michigan, Iowa, Kansas, Nebraska and Missouri. What did Dr. Smart find in common in all of these states?

Grapevine trunk diseases were prevalent in all these eastern viticulture states as well as in Quebec, Canada. A common theme occurred across this wide geographical area. Many of the vineyards that are experiencing vine decline where vines have dead spur positions, blank wood along the cordon, etc. In most cases, vineyard managers were pointing their fingers at winter cold damage. Certainly the polar vortex of 2013 and 2014 did some damage. However, as Dr. Smart demonstrated, he has observed vines in warm climates such as Australia or South Africa that have the same type of symptomology as he observed in declining vines here in Missouri and other eastern viticulture states.

Grapevine trunk diseases are very different compared to the foliar diseases of grapevines. Trunk disease microorganisms are epiphytic. The grapevine and the epiphytic microorganism can live in harmony together and the epiphytic microorganism does no harm to the grapevine initially.

The epiphytic microorganism at some point becomes a pathogen and starts to cause harm to the semi-permanent wood (trunk, cordon and spurs). The change from friend to foe by the epiphytic microorganism is believed to result from vine stress(es). Dr. Smart pointed out that these stresses can include saturated soils, drought and cold winter temperatures. Missouri being a continental climate experiences cold temperatures, drought and recently this spring saturated soil conditions.

One of the more common grapevine trunk diseases is Phomopsis (Phomopsis viticola). Most of you are familiar with Phomopsis as foliar or shoot pathogen. Phomopsis infects grapevines early in the season during cool wet conditions. In 2018, Mike White sent several grapevine trunk disease samples to laboratories for identification. Many of these samples came back positive for Phomopsis. Dr. Smart pointed out that Phomopsis was first reported in a paper from New York 108 years ago and that Phomopsis has been largely ignored as a problem. However, I have been advocating for years that if you want to prevent Phomopsis infections, it is critical that early season protective sprays need to be applied at 0.5- to 1-inch shoots, especially when environmental conditions are wet and cool.

Dr. Smart and I collected more than 25 samples (Figure 2 shows a small sampling of the symptomatic wood) of grapevine trunk disease from Missouri vineyards. I am hoping to have all these samples evaluated by a laboratory specializing in grapevine trunk diseases. This preliminary survey will provide information on prevalent grapevine trunk diseases here in Missouri. The last information on grapevine trunk diseases in Missouri was published by José Ramón Úrbez Torres who spoke at the Missouri Grape and Wine Conference and Symposium in March 2019. Further research is being conducted by graduate student Emily Serra in the laboratory of Dr. Megan Hall of the Grape and Wine Institute. Hopefully, through research and educational outreach through MU Extension, more answers can be found in managing grapevine trunk diseases.

Current management practices for grapevine trunk diseases includes preventative practices to reduce infections, trunk renewal and sourcing grapevine trunk disease free planting material. Grapevine trunk disease microorganisms can enter a grapevine through wounds. Some of the most prevalent wounds are pruning wounds in spur pruned grapevines. Dr. Smart recommends treating these pruning wounds with labelled fungicides or other materials that block the entry of grapevine trunk diseases from entering the pruning wound. Prior to suggesting prophylactic protection with fungicides, I would like to know when the grapevine trunk diseases are releasing infective spores. In addition, I would like to know what are the most destructive grapevine trunk diseases we are battling here in Missouri. Trunk renewal is a method to eliminate trunk disease in an infected vine. Dr. Smart suggests that trunk renewal should be initiated when a vine starts to show decline. In some cases this renewal may be on what most would consider very young vines (3 years). However delaying trunk renewal on a grapevine disease infected plant may preclude trunk renewal as an option. If the grapevine trunk disease moves into the soil/shoot junction then trunk renewal cannot occur. Dr. Smart recommends that uninfected wood should be 10 to 12 inches away from the infected wood. Planting grapevine disease free planting material is another way to manage grapevine trunk diseases.

Richard Smart standing between grapevines
Dr. Richard Smart standing with pre-Civil War planted Norton grapevines near Hermann, Mo., June 28, 2019. Photo credit: D. Volenberg.

During Dr. Smart's visit to Missouri, he had the opportunity to dissect some grafted chardonel vines (Figure 1) that had not been planted in the field and some 2-year-old, field planted, grafted Norton grapevines. In all cases, the graft union of these grapevines was infected with grapevine trunk diseases. It is highly likely that the grapevine trunk disease is traveling from the infected rootstock to the scion. One option to reduce the incidence of infected planting material is to plant grapevines that are own rooted. As an industry, we need to work with nursery plant suppliers to reduce or eliminate grapevine trunk diseases from planting stock.

Other preventative practices should be undertaken to reduce grapevine trunk disease infections. These include vineyard sanitation and removing wild grapevines in the vicinity of the vineyard. Vineyard sanitation involves removing and destroying all dormant prunings from the vineyard floor. Wild grapevines can also be a source of grapevine trunk disease as evidenced from dissections of wild grapevines near vineyards in Missouri (Figure 2.F). How big a role wild grapevines are having in causing infections of grapevine trunk diseases is unknown at this time. This is one of the reasons why the samples collected during Dr. Smarts vineyard visits are important. Maybe the results will show that wild grapevines have a different complex of grapevine trunk diseases than cultivated grapevines. This result would suggest that wild grapevines infected with grapevine trunk diseases are likely not a great threat to cultivated grapevines as far as trunk diseases are concerned. Time will tell and I look forward to sharing the results with all of you.

I would like to thank Dr. Smart for his visit to Missouri. In addition, I would like to thank the Missouri Wine Research Marketing Council for providing funding to bring Dr. Smart to Missouri. Also thanks to the Missouri Grape Growers Association and the Missouri Vintners Association for hosting the field day with Dr. Richard Smart. Many thanks to the grape growers that participated in the field day and hosting Dr. Richard Smart at their vineyards.

close-up of cross-section of Valvin muscat branch close-up of cross-section of Concord branch close-up of cross-section of Vidal blanc branch close-up of cross-section of Norton branch close-up of cross-section of Chambourcin branch close-up of cross-section of a wild grapevine branch
Figure 2. Grapevine trunk diseases identified in Valvin muscat (A), Concord (B), Vidal blanc (C), Norton (D), Chambourcin (E), and a wild grapevine (F) by Dr. Richard Smart on his visit June 26-28, 2019. Photo credit: D. Volenberg.

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