Winter Kill, Trunk Disease, or Both….the Situation in the Midwest, USA
[PHOTO CAPTION: DR. RICHARD SMART]
We eagerly anticipated my second visit to the Midwest in June of this year (This article is written in the first person illustrating Richard Smart’s experiences). The prime purpose was to present at the Grapevine Midwest Viniculture Expo at Davenport, Iowa. The topic of my presentation was “Canopy management and how it can affect vineyard profitability and wine quality”. I intended to critically discuss the present tendency towards use of a single high wire cordon.
Mike White, the extension viticulture Specialist at Iowa State most kindly offered to escort me on a three-state tour of vineyards, including Iowa, Nebraska and Minnesota. This gave me a chance to see many vineyards and meet many growers, observe their vineyards and to discuss their problems first hand.
[PHOTO CAPTION: Cordon dead on one side of the vine. (Mike White, Extension Viticulture Specialist with Iowa State University]
I stand rightly accused of introducing another viticultural consideration into these visits, some would say a “real curve ball”. Imagine my surprise in the first block of the first vineyard which I visited to be presented with a dilemma. What I saw and described as clear-cut symptoms of Grapevine Trunk Diseases (GTD) were uniformly labelled by the locals as “winter injury”.
[PHOTO CAPTION: Not yet dead cordon, but short shoots very evident]
[PHOTO CAPTION: Dead cordon, with some short shoots]
I therefore checked for internal staining as is caused by GTD fungi, easy enough to do by cutting the spurs, cordon and or trunk tranversally. The presence of fungi is indicated by black or brown staining in the xylem surrounding the pith. With established infections this can lead to dark brown staining running longitudinally in the cordons and or trunks. So far as I am aware winter injury symptoms do not include internal xylem staining. How could this be? What is widely regarded in the Midwest as winter injury symptoms were identical to those published for some grapevine trunk disease fungi, especially Botryosphaeria and Phomopsis. I have seen such symptoms in many parts of the world, and in many regions not subject to winter injury. I mention these two fungi because they are different from others like Esca in Europe and Eutypa in California/Australia in having no distinctive leaf symptoms; however vines do show symptoms of internal staining similar to what I was seeing in the Midwest.
Grapevine trunk diseases are a current interest of mine. This is because for the last eight years or so I have been confronted by their impact on clients’ vineyards I visit irrespective of location in the world. Some say GTD is presently a larger problem than phylloxera because it is so widespread, and is being currently spread by grafted plants from nurseries. It is also generally unrecognised and untreated by most growers in most regions.
Because of this general situation I had published a column in The Grapevine Magazine in the May/June 2018 edition on page 60 titled ”Don’t let trunk disease ruin your day… and your vineyard”. In that article I raised the possible confusion between winter injury and trunk disease symptoms, as I had seen in Ontario Province, Canada. However, it was still a shock to me and Mike to see so many trunk disease symptoms in the Midwest, in every vineyard we visited.
Of interest was to recently learn that Phomopsis dieback was one of the earliest trunk diseases to be described in the scientific literature, and that this description arose from studies at Geneva Experiment Station, NY, published in 1908! I do not know the extent to which Phomopsis symptoms may have been similar to those of winter injury, as I have not seen the original paper. One of the wineries visited, Soaring Wings Vineyard and Winery in Nebraska had some vines analysed at University of Nebraska at Lincoln lab, indicating the presence of Phomopsis and Botryosphaeria which are both GTD fungi.
Other Symptoms of Trunk Diseases
Sometimes the first symptoms will be a vine with slightly yellow foliage, or maybe magnesium deficiency, showing pre-harvest for single, isolated vines. Another symptom can be a vine looking healthy, but with few and small bunches, often associated with poor bud break.
In spring the first conspicuous symptoms will show, a few spurs with poor or no bud break. In bad cases this might affect many spur positions on a cordon, and even one cordon on half of one vine. Also, there can be obvious patches of “short shoots”, a term we learned in Nevada! I have subsequently learned that “short shoots” for part of or a whole vine is often a clear symptom of developing trunk disease. Also, as the fungi invade the trunk, the vine is more likely to throw out suckers at the base.
Don’t be afraid to cut into the vine and look for staining. This can be found on spurs often below pruning wounds that are one and two years old. If you are brave you can cut into the cordon, you will likely find in weakened vines staining moving towards the trunk as the vine succumbs to trunk disease.
Remember early detection is the key to early action to stop trunk disease. Hopefully many growers will take up protection of trunk diseases this winter, an important first step. As vine health deteriorates and as the fungi move down the trunk, this is the time to begin trunk renewal.
We had some opportunity to compare apparent sensitivity of different varieties to GTD, based on presence of symptoms and staining. We found Steuben apparently very sensitive, followed by Edelweiss then Marquette and St Croix. More comparisons should be made.
Chicken or Egg Situation
Which comes first, the trunk disease or the winter injury? Or do they present together? It is well-known that trunk disease organisms can live happily inside healthy vines as endophytes and not cause any problems to the plant. However when the plant comes under stress, such as by winter injury, the fungi respond by becoming pathogenic, sometimes releasing toxins and also in producing fruiting bodies and spores for their continuing survival. We do not know whether vines weakened by winter injury are more susceptible to fungal invasion, or, alternatively does the weakening effect of the fungus makes the vines more prone to winter injury? This requires some careful experimentation by researchers for resolution. Seems a nice PhD research topic to us.
[PHOTO CAPTION: Staining in cordons, cordon on left shows canker]
Where might Trunk Diseases be Coming From?
This question is much studied in other vineyard areas. Grafted plants are a common source of infection. It is well-known that most grafted plants are symptomatic for GTD, a fact arising from the nature of bench grafting which has been used since the 1980s. Also, the management of rootstock mother vine vineyards is questioned since the shoots are trained along the ground from very short trunks covered by spurs which are not wound protected. Some trunk disease fungi are also ubiquitous in the environment. Botryosphaeria for example has 200 known alternate host plants which can often be found along the vineyard borders.
In the Midwest it is not necessary to graft vines on rootstock against phylloxera. However this is not to say that young vines are free from trunk disease when sold to growers. There are techniques such as hot water treatment which would greatly reduce trunk disease infection in plants from local nurseries, another subject for local research.
[PHOTO CAPTION: Wedge shaped canker typical Botryosphaeria with light brown staining]
This is an interesting question to ask. It may be that if vines are protected from trunk disease infection then they could be more resistant to winter injury. This would be a simple situation to investigate and may lead to a reduction in winter injury which is presently regarded as an inevitable consequence of growing grapes in the Midwestern environment.
Knowledge of an interaction between winter injury and trunk disease might also put into better perspective some other vineyard conditions which predisposes vineyards to trunk disease damage. One which is especially important is soil water logging, and I wonder if there are observations about this condition and winter injury susceptibility.
Managing Trunk Diseases
There are key elements to controlling trunk disease. First one should try and reduce infection by planting high health vines. Trunk diseases are spread in the vineyards by spore invasion of pruning wounds. Spores are produced in rainy weather which should be avoided as far as possible for winter pruning. Further, pruning wounds can be protected using an application of fungicide and/or a physical barrier material, and this should be commenced in the first year. This practice is becoming more common in California, with application by hand spray bottle, painting or tractor drawn sprayer. Economic studies endorse such actions, especially when symptoms are first noted.
In time cordons and trunks become GTD infected causing in turn dead spur positions and eventually dead vines. Cordons should be replaced periodically and also trunks. The present practice of trunk renewal in response to winter injury undoubtedly helps protect some Midwest and Eastern vineyards from trunk diseases.
Trunk diseases are obviously a neglected area of research in the Midwest. This research should be encouraged to answer some of the questions posed in this article. I am sure growers would appreciate an outcome that minimised winter injury damage to their vineyards.
For the moment there is urgent need for more lab testing of unhealthy vines. I suggest using recently dead spur locations after checking for characteristic staining at the base of the spur.
There is no reason for panic just because this disease possibility has been raised. The situation in the field has not changed, only our understanding of possible causes. Trunk diseases can be managed to minimise impact as is currently being done elsewhere in the world. There is no reason why growers in the Midwest might not learn these skills and use them. Dr Smart would be pleased to receive growers reactions to this article.
Dr Richard Smart is an Australian viticultural scientist and consultant with world-wide clients, currently living in the UK.
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