I am writing again about Grapevine Trunk Diseases (GTD) in the Midwest, following the article with Mike White (ISU extension viticulturist) published in this magazine in September-October 2018. In that article we raised the question as to whether the commonly seen “winter kill” symptoms of dead cordons and spurs, and poor budbreak, may be mis-diagnosed in the region, as they also correspond to common GTD fungi symptoms. I raised this with some “old hands” in the industry, and they laughed at the suggestion. I hope this article might cause them to reconsider (but probably not).
We have two developments to report. Firstly, there has been quite some activity in testing of samples, much of it by Mike. Initially this was by local diagnostic laboratories, but there was concern with apparently inconsistent and negative results which caused us to send samples further afield. We have been concerned about test results for some samples which may lead growers to believe that there is no problem. However, as we have discovered some testing procedures are more reliable than others. This problem is not unique to samples from the Midwest, as Richard has also had problems with some samples from California tested in different laboratories.
GTD Fungi Isolated in Iowa
The following is a list of fungi and one bacterium which have been diagnosed in samples from Iowa, provided by Mike:
- Botryosphaeria dothidea (“Bot”)
- Crown Gall (Agrobacterium vitis, a bacterium)
- Cytospora viticola (associated with trunk disease)
- Diatrylpella species. (associated with Eutypa, a major trunk disease)
- Eutypa species including Eutypa lata
- Phaeoacremonium species. (associated with Young Esca (Petri Disease)
- Phomopsis species (associated with foliar, cane, fruit and trunk disease)
- Seimatosporium species. (associated with Dead Arm Disease)
- Pestalatiopsis (foliage, fruit and trunk disease)
- Phaeomoniella species. (associated with Young Esca – Petri Disease)
- Diaporthe species (Phomopsis)
- Stereum species (typically saprophytic feeding on dead wood)
In fact, as more samples are properly tested, it is likely there will be found other fungi. It would appear from this limited experience that Polymerase Chain Reaction (PCR) testing as is carried out by some Californian laboratories is the more reliable method. Many samples will have multiple infections.
The attached figures show the vine appearance of three samples analysed in California. Concord samples which showed internal streaking and necrosis (dead tissue), centered on the pith, and necrosis associated with cankers. The sample contained four fungi, Eutypa spp., Phomopsis spp., Phaeoachremonium minimum and Phaeomoniella chlamydospore, see Figure 1.
Similarly Marechal Foch yielded Eutypa spp.,Diatrypella spp. and Phaeomoniella minimum, see Figure 2.
The last sample to be discussed was especially interesting, being Vitis riparia from the wild, harvested at Indianola, Iowa, from a creek bottom. Mike and I on our travels together had found suspicious staining in several wild vines, here was some confirmation. The sample showed necrosis and streaking, and some dark tar-like deposits, and white rot in the center. The diagnosis was for Phaeomoniella minima, Phaeoachremonium chlamydospora and Botryosphaeria dothidea, see Figure 3. There may be some implications for new varieties bred from Riparia parents re GTD susceptibility.
The Riparia was to be the first confirmation of Botryosphaeria in Iowa. In my tour with Mike last summer, I often spoke of “Bot” in a general sense as being the likely cause of some symptoms we saw. I should have been more careful. Botryosphaeria symptoms are like those of other GTD, but in particular those of Phomopsis. This finding in Riparia was interesting from other points of view. Was Botryosphaeria present in wild vines before present grapevine plantings were introduced? Whatever the answer to that question, wild vines may constitute a reservoir of spores for infection of adjacent vineyards. Of course, other native plant species may be infected is as well.
Outstanding Webinar on GTD by World Expert Dr. José Ramón Úrbez Torres of British Columbia
This webinar was held Tuesday, December 11, and organized by Dr. Tim Martinson of Cornell University. Around 300 persons registered for the event, indicating a heightened awareness and interest, especially in cold winter climate regions. The webinar lasting a little over an hour is now posted on the Northern Grapes Project website for viewing. If you have unexplained dead or unhealthy vines in your vineyard, we suggest that you watch it here: https://northerngrapesproject.org/
The seminar was very comprehensive. It began by describing the many species of fungi now known to be involved in grapevine trunk disease, and described how they are spread, both between and in vineyards. The seminar concluded by presenting a range of methods of protecting vineyards from GTD.
GTDs are caused by a group of fungi of different genera and species, and they often occur as mixed infections. The fungi have in common a typically insidious nature when pathogenic on grapevines. For some diseases, there are no conspicuous foliar symptoms, and the first a grower may know of the problem is spur then cordon then vine death. Unfortunately, by the time the first vine dies, many others can be already infected and they will gradually die unless the problem is treated. But treatment must commence early if the vines are to be saved. So, it is very important to recognise early disease symptoms in just a few vines where they exist.
Grapevine trunk diseases can affect and kill vineyards of many ages. Some deaths may even be recorded in the year of planting, and, typically as the vineyard ages, more and more vines die. For growers concerned with vineyard profitability this is an issue; some vineyards in some parts of the world are being replanted at 20 years of age due to trunk disease. Unfortunately, vines delivered to growers from nurseries may be already infected with grapevine trunk diseases. This is a worldwide problem, and research shows that it can be involved with the bench grafting process, but even own rooted vines may be infected, as can occur in the Midwest.
Vigilant growers should inspect nursery stock for staining within the stem. It is normally particularly evident at the base of the rootstock cutting and in the graft union. The webinar was excellent for showing a full range of symptoms in both young and old vines, including many sections across the trunk. Sometimes fungi will be resident within young vines without causing problems; however, it is known that when the vines experience any stress that these fungi will become pathogenic and will affect the growth of the vine and eventually kill it. Severe winter temperatures are known as an important stress. In fact, as was emphasized in the article referred to above, the common death of cordon and spurs which is typically called “winter injury” may in fact sometimes be due to trunk disease.
There has been much more concern globally about GTD since 2000 even though there are reports that the diseases had been present for a long time, and some of the early research has been forgotten. For example, the roles of Phomopsis in trunk disease was shown by early research at Geneva New York in 1909 and again more recently with Eutypa in several Eastern states from the 1970s onwards. In France it has been declared that “wood diseases are a national crisis responsible for 12% of French vineyards being non-productive”. So, while some might think trunk diseases are new, they are not, but there is clear evidence that they are becoming more of a problem worldwide.
What to do about trunk disease? They can be controlled using a range of management techniques. In regions of cold winters there is evidence that early pruning may be more effective, whereas the opposite is the case in California for example where late pruning in the dry part of the spring helps to prevent spread.
It is imperative to protect pruning wounds, and fortunately a range of fungicides are available in the USA although not in Canada. There is certainly a need for local research in places with very cold winters as to how these may be applied in freezing temperatures. For larger vineyards in other regions using a modified vineyard sprayer is becoming popular. Protection of pruning and other wounds is the first line of defence against GTD, and likely in the majority of Midwest and Eastern vineyards this is not now practiced, which may change if GTDs are found to be a widespread problem.
The second line of defence against trunk disease is trunk renewal. This is now practiced of course to overcome “winter injury”, and thankfully at the same time can be protective against trunk disease. Provided the trunk is renewed from below the level of staining in the trunk, then the vine may be rejuvenated and is free of disease.
Growers in the Midwest and Eastern states have not been concerned as they might about trunk diseases, probably because some symptoms have been thought due to winter injury. Yet as was declared in the webinar, GTD occur everywhere in the world where grapes are grown! At the earliest signs of disease/poor health we encourage growers to take sections of the trunk and look for staining. If any is found, a lab confirmation should be sought.
I urge all growers, advisers and nurserymen to watch this webinar. It is an hour very well spent and may give you a new perspective on your vineyard and its health and profitability.