Renewing Grapevines from Suckers Following Severe Winter Injury

Annie Klodd, University of Minnesota Extension, [email protected]

Grapevine suckers with old trunk removed
Many vineyards across Minnesota have been severely affected by winter injury, while others survived the winter well. For the vineyards reporting winter damage, many are reporting partial or entire blocks of one variety to have died to the ground, often leaving behind multiple suckers per vine. So far, more damage has been reported on lower sites with higher water tables or more wet, humid environments; while sites on higher ground seem to have faired better overall. Damage also varies by variety. It is also likely that vines that were less healthy going into last winter suffered more extensive damage, although more data must be collected in order to make that conclusion. A formal multi-state survey will be conducted to determine which varieties, site types, and geographic areas were most affected.

Rejuvenating Grapevines Following Severe Winter Damage

If a vine is dead to the ground but has several healthy suckers growing from the base of the trunk, leave roughly five suckers on the vine and select one to train up to become a replacement trunk. If the vine has more than about five suckers, trim the rest off; or if you have less than five, leave as many as you can. Choose a healthy, strong sucker to be your new trunk, but avoid selecting a bull cane (a bull cane is more than pinky-finger diameter, with wide bud spacing). The reason to leave extra suckers is to distribute some of the excess energy that is in the vine's root system and reduce the risk of the vine developing bull canes. Once you have chosen your new "trunk" shoot, use tapener tape or other fasteners to secure it vertically upward onto a fresh bamboo pole or rebar.

In order to spray herbicides, treat these as you would new vines; apply grow tubes so that you can safely spray herbicide around the vines without injuring the suckers. Try to maneuver the suckers into the grow tube. If the suckers are running along the ground and are very thick, it may be too late to get them all into the grow tube without breaking some. At that point, you may consider leaving the weeds, weed-whacking around the vines instead of spraying, or removing the shoots that are causing problems. Leaving the weeds may take away excess vigor from the vines, if that is an issue on the particular site. Therefore, these decisions depend on the needs and goals of the particular vineyard to some extent.

Remove the grow tubes no later than October, or whenever you are done with your last herbicide spray of the season. Leaving them on past mid-fall could lead to poor winter acclimation and further winter damage the next year. During winter pruning, prune off the excess suckers and consider training up one of them as a second "insurance" trunk.

A vineyard with Frontenac blanc on the left and Marquette on the right, following winter injury

When to remove the dead trunk

You can remove the dead trunk now (June) if it is not producing anything, using loppers or a saw. If the trunk is partially alive and producing a few clusters, it is acceptable to leave it there for the season and cut it down during winter pruning. The fruit may not be high quality if the vine is unhealthy, but it will help distribute some of the excess energy in the roots. 

Is Grapevine Trunk Disease Contributing to Vineyard Die-back?

There has been much discussion about the role of grapevine trunk disease (GTD) in vine die-back in the Midwest. At this point, it is not known how much GTD is contributing to winter die-back in the Midwest. Laboratory analyses are currently underway at the University of Minnesota to help determine this, but at this point it is not possible to say how much it is contributing, and it likely varies depending on the vineyard and on the overall health of the vines. Dispite contradictory claims online, trunk disease species cannot be reliably diagnosed without the help of trained plant pathologists, of whom we are currently working with on this issue. 

As a precaution, when growers cut down trunks to re-train new trunks, they may spray each pruning cut with Rally or Topsin, or apply latex paint. I recommend bringing a small backpack sprayer or handheld sprayer with these products while cutting down trunks in order to make this process more efficient. 

Photos: (top) A vine that died to the ground, with the trunk removed. One sucker should be chosen as the new trunk and trained up on a pole. (bottom) Healthy Frontenac blanc on the left and a dead block of Marquette on the right (photo credit Annie Klodd, 6/12/2019).

Tips for Reducing Grapevine Trunk Disease During Pruning

Many growers have been inquiring about what they can do during pruning to manage grapevine trunk disease (GTD). Below are a few general tips to reducing the risk or prevalence of grapevine trunk disease. They can be adapted to the individual vineyard based on what is feasible within your management system.
Grapevine wood with trunk disease staining
1) Remove and replace any cordons that are not productive. If the cordon is as thick as the trunk, on a mature vine, and it has been declining in productivity, it is time for it to go; it will only continue to decline. Furthermore, if more than 1/2 of the length of the cordon is not producing spurs, it can be replaced. Lack of productivity on a cordon can be a sign of either trunk disease or winter injury, or both. But either way, the cordon is not as productive as it could be and may be harboring disease.
 
To replace a cordon, tie down a healthy replacement cane over it and remove the old one either this year or next year, depending on its level of die-back. For instance, if it is mostly not producing, there is no need to keep it; cut it out this year. However if you wish to harvest some fruit off of it this year, keep it and remove it next year. This method will help remove a good amount of diseased wood if it is present, and will make sure disease does not spread from the cordon to the trunk.
 
2) Remove pruning cuttings from the field, rather than leaving them on the vineyard floor. This is especially if the cuttings have visible disease symptoms on them, like discolored interior wood or black lesions. This can, and is, done successfully in Minnesota. It can be done with a bobcat, or with a trailer behind a 4-wheeler. Once removed, cuttings can be burned in a pile.
 
3) When cutting down a trunk or a cordon that has died, dab that wound with latex paint, and/or spot spray it with a fungicide that has some surface-level efficacy on trunk disease spores such as Topsin or Rally. This can be done with a handheld or backpack sprayer. Then, come back again a few days later and dab the wound with latex paint to seal it up from incoming spores. This is only effective on the surface of the wound - it will not effect fungal infection that is already established in the wood. This tip is based on research in warmer areas, and has not yet been researched in our Minnesota climate. 
 
4) Next year, consider pruning earlier in the season (Feb-March), rather than starting in late March or April. This is easier said that done in years when there are 2.5 feet of snow on the ground, like this year. But there is good, consistent data out of several areas of the US showing that if temperatures remain below freezing for several weeks after pruning, it drastically reduces the risk of GTD to infect the pruning wounds. This is because the wounds have an opportunity to dry and harden before the fungal spores become active in the spring, therefore they cannot infect the dried up wounds.
 
5) Check your cordons for interior disease symptoms when pruning, and save those samples so that our team can analyze them for trunk disease. Grapevine trunk disease appears as brown discoloration in the wood. On green wood, it more often appears as black discoloration.
Grapevine wood with trunk disease
I know of at least two Minnesota growers who plan to try spraying their entire vineyard with Rally or Topsin to control GTD, based on recommendations from warmer wine regions. If you do this, be aware that you are doing it experimentally; we do not yet know the best timing or effectiveness of this treatment in Minnesota, when there is often a long time period between when we prune and when we can spray. The recommendations from other regions say to spray immediately after pruning if you are pruning when the spores are active (above 35 degrees consistently), and then repeat the spray one or two times during pruning and bud break. If the ground is saturated or snow-covered and you cannot get the tractor into the field, you may miss this critical time period. As part of our 2019-2021 research into grapevine trunk disease, we hope to determine the best time frames for spraying based on when the spores become active in Minnesota.
 
For questions about grapevine trunk disease in Minnesota, contact Annie Klodd at [email protected]

Statement on Botryosphaeria in Minnesota Grapevines

Authors: Annie Klodd, Matt Clark, and Mike White. With input from John Thull, Jenny Thull, Mizuho Nita, and Tim Martinson. 

During Dr. Richard Smart’s visit to the Horticultural Research Center on Monday, he discussed the common trunk disease botryosphaeria (“Bot canker”) and demonstrated it in several native and cultivated vines here. This fungal disease enters the vine through pruning cuts and injury, and gradually spreads through the vine over a number of years if not managed. It appears as brown streaks and cankers on the inside of woody tissue (xylem) (see photo).

Botrytosphaeria \

Signs of the disease are not visible in the current year’s growth. Additional pathological testing is required to determine which fungal organism(s) are present and causing symptoms as there are numerous Botryosphaeria species and other trunk diseases like Phomoposis and Eutypa that can have similar symptoms. This includes the classic wedge-like canker in mature vines.

Impact on Minnesota Vines 

Two important things to emphasize: 1) This trunk disease is not reason to panic, and grape growers around the world have been coexisting with it for many years. It is manageable, and does not warrant pulling out productive vines or cancelling orders. Energy should instead be focused on management. 2) It is unknown what percentage of vines Bot canker is present in, or what varieties are most susceptible. Research will be needed to determine the causal relationship between the presence of Bot canker and vine decline, including winter injury.

Bot canker is manageable through good sanitation after pruning, cordon replacement, and proper site selection. Bot that is not managed will eventually cause reduced yields, trunk die-back, and a shorter lifespan. These symptoms look similar to winter injury, and the two factors may be combining to lead to vine decline.

Botryosphaeria is common throughout the world, and grape industries in California, France, Italy, New Zealand, and others have successfully coexisted with it for quite some time. Now that we have a heightened awareness of this disease in Minnesota, we are able to improve our management practices in order to minimize its impact on our beautiful Minnesota vineyards.

Recommendations

Dr. Mizuho Nita is the grape pathologist at Virginia Tech who has been researching Botryosphaeria biology and management for many years. He is an authority on the subject. Here are some take-aways from a conversation with him this week:

1) Botryosphaeria is present in the air. It mainly enters vines through pruning wounds that are made when the air is humid, like after a rain. 
2) He recommends pruning during dry conditions, and then spraying Rally or Topsin-M  (fungicides) on the vines once the temperatures warm above freezing but before it rains.
3) Growers should grow up suckers to replace trunks and cordons if die-back occurs.
4) It infects the vine gradually over the course of 10-20 years. If vines are not stressed and they are managed well, they are often able to produce for years before showing decline. 
5) Vine decline due to Botryosphaeria is typically observed on older vines; it can exist in the vines for a long time without causing damage. The two times Dr. Nita has seen decline on 3-4 year old vines were instances when they were pruned right before a rain, in the shade. Those are classic examples of disease conditions.
6) If the vines experience winter injury, the Bot infection may get worse because the vine is less healthy and therefore less able to tolerate infection. 

When growers see vine decline in Minnesota, such as dead spurs, cordons,or trunks, this may be a combination of factors including Botryosphaeria, winter injury, management, and wet conditions. If you suspect Bot canker in your vineyard, the only way to truly confirm it is through lab analysis. Send a sample to the UMN Plant Diagnostic Lab, with a note that you suspect Botryosphaeria. Then if it is confirmed, follow the above recommendations about training up new suckers and replacing cordons as needed. Practice preventative measures including spraying Rally and Thompson after pruning (once temperatures are above freezing), and removing pruning debris  from the field. Do not prune in wet conditions.

In addition, make sure that vines are grown in ways that minimize stress: Prune to the recommended spur counts for the variety, plant cold-hardy and disease-resistant varieties, and plant in well-drained sites (avoid planting in wetland and low-lying flat areas).

Stay tuned for further information. This is a new disease problem that has been brought to our awareness. Awareness is the first step in solving a problem.  Further research and experience will shed more light on dealing with this disease in our cold climate region. 

Please contact us with questions or if would like to discuss recommendations. Extension Educator Annie Klodd can be reached at [email protected].

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