Concord Nitrogen Needs
By Luke Haggerty, Dr. Terry Bates, and Dr. Cain Hickey
Nitrogen is the mineral nutrient that is needed most by Concord grapevines. It is also the most widely applied fertilizer. Nitrogen is the backbone of amino acids and, as such, nitrogen is the building block for compounds such as proteins and chlorophyll, the latter, the pigment most responsible for photosynthesis. Nitrogen-deficient vines are often characterized by reduced vigor, crop yield, and photosynthesis. However, excessive nitrogen can cause overly vigorous shoot growth that results in shaded fruiting buds and reduced yields. Nitrogen is a very important nutrient for grapevines, and calculating nitrogen needs is a complicated decision. As such, nitrogen studies have been conducted in Concord for many years.
Evaluation of Concord nitrogen needs started with Dr. Nelson Shaulis in the “West Tier” experiment. In this experiment, three nitrogen rates (0, 50, and 100 pounds of actual nitrogen per acre) and several viticultural practices were evaluated for their impact on vine size and crop yield potential. Vines with high nutrient and water uptake had increased vine size, pruning weight and yield potential (Figure 1). It was shown that 50 pounds of nitrogen per acre increased pruning weights (vines size) and crop yield, but adding an additional 50 pounds of nitrogen per acre offered no further benefit (Figure 2). Concord vines in the Lake Erie region typically have less than three pounds of pruning weight, and average less than eight tons per acre. Therefore, Dr. Shaulis concluded that healthy Concord vines required somewhere between 0 and 50 pounds of nitrogen per acre to increase and maintain vine growth and crop yield. (Bates et al. 2002)
The West Tier experiment was conducted on well drained gravel loam soils with low organic matter. Years later, Dr. Bates conducted the same study, but on heavy clay soils with relatively high organic matter; this experiment was called the “Betts’ Nitrogen Trial”. Vines were again given 0, 50, and 100 pounds of nitrogen per acre over seven consecutive years. Similar to the West Tier experiment, it was observed that crop yield was greater when 50 compared to 0 pounds of nitrogen per acre was applied, but that applying 100 pounds per acre did not further increase crop yield. Figure 3). It was concluded that soils with high organic matter release more nitrogen and, thus, vines needed less supplemental nitrogen.
In other works, Dr. Lailiang Cheng and Dr. Bates investigated the ability of Concord vines to take up nitrogen, as well as their annual nitrogen demand. It was found that only 24% (24 pounds for every 100 applied) of 50 applied pounds of nitrogen was incorporated into the vine. In other words, vines were only able to uptake ¼ of the applied nitrogen. It was also found that about 50 pounds of nitrogen per acre were required in Concord vineyards. Other research found that only about 10 of 100 applied pounds of nitrogen per acre were incorporated into the vines. (Randall et al. 2004)
So where does the excess nitrogen go? Some nitrogen is used by macro and microorganisms in the soil, such as weeds, worms, and bacteria. This nitrogen enters the living portion of organic matter and can eventually be used by the vines in the future (remember from above that nitrogen can be supplied by soil organic matter) A very small amount of nitrogen is absorbed into soil particles by cation and anion exchange. The rest of the nitrogen can be lost through leaching, erosion, and denitrification. Nitrogen loss, especially leaching, is getting more attention these days because of its documented negative impact on the environment, such as algal blooms in bodies of water.
Concord vines rely mostly on stored starches and nutrients during the transition from dormancy to bloom. Approximately 80% of the reserved starches and nutrients are used for pre-bloom shoot and root growth. Relatively little nitrogen (around 5%) is taken up prior to bloom when compared to what is taken up after bloom. However, this 5% has potential to be important if weak vines have inadequate nitrogen storage reserves. The 5% is less important in healthy vineyards where nitrogen reserves support strong early season growth; this includes new root growth that absorbs additional soil nitrogen. Reserves are depleted after bloom. Thereafter, Concord vines rely on nutrient uptake from the roots, and assimilated carbon via photosynthesis. The amount of time it takes for supplemental nitrogen to reach the rooting zone is dependent on several factors, including rainfall and, consequently, soil moisture. Thus, it is typical for nitrogen applications to occur around or before bloom, when soil moisture is relatively high. Nitrogen should be applied in split applications in vineyards needing more than 40 pounds of nitrogen per acre; these sites may be characterized by large vines and/or low soil organic matter. The general rule for split application timing is at two weeks pre-bloom, and at two weeks post-bloom.
There are basically three nitrogen sources for grapevine growth: mineralization of nitrogen from soil organic matter, remobilization of stored nitrogen from perennial tissues, and inorganic nitrogen fertilizers. By far, the largest pool of N for grapevine growth comes from the mineralization of soil organic matter. While there are many biological, chemical, and environmental processes working in concert, the basic rule of thumb is that about 15-20 pounds of nitrogen is released for every 1% of soil organic matter in your vineyard (table 1). Storage nitrogen reserves are likely the most easily used by vines because it is already in the vine – it just needs to be converted and remobilized in the spring. Inorganic nitrogen fertilizers are primarily used to supplement nitrogen remobilization and mineralization sources during periods of peak nitrogen demand. Peak vine nitrogen demand occurs during rapid canopy and fruit development, which starts a few weeks before bloom, and lasts until about 40 days after bloom. During this timeframe, the release of nitrogen from organic sources may not be enough to keep up with vine demand. Therefore, supplemental nitrogen fertilizers should be applied just before rapid vine growth, even in vineyards with relatively high organic matter soils. In the Lake Erie region, this critical period of nitrogen fertilization is typically in late May or early June.
Much of the information used for this article has not yet been published.
Bates, Terence R., Richard M. Dunst, and Paula Joy. “Seasonal dry matter, starch, and nutrient distribution in’Concord’grapevine roots.” HortScience 37.2 (2002): 313-316.
Randall J., Thomas J. Zabadal, and Eric J. Hanson. “Effect of nitrogen application timing on N uptake by Vitis labrusca in a short-season region.” American journal of enology and viticulture 55.3 (2004): 246-252.
Soil and Petiole Testing
Why is it important?
Just as you would never medicate your children without knowing whether or not they actually have an infection,you need to be able to diagnose nutrient deficiencies in your vineyards. How to do it? Soil and petiole testing can provide a clear picture of what is going on in your vineyard. The soil tests will determine what nutrients are available to be taken up by the vines’ roots, and the petiole tests will show whether or not the roots are actually absorbing those nutrients. Once you have the soil tests, you have one side of the story, and petiole tests will give you the other side of the vine nutrient story. A soil test can indicate that the soil pH, K, Mg, and N levels are all adequate, but if the vines are still puny, something else might be at work. This is where a petiole test comes in handy. Most likely, however, puny vines could be due to too much or too little water in the vineyard, and a large crop size will also affect overall vine size.
Regarding nitrogen application:
In reference to growers traditionally applying 100+ lbs. actual N/acre, one grower recently commented, “At $600 and up per ton, there is no such thing as tradition.” He’s got a good point. Instead of applying 100lbs actual N to every nook and cranny of your vineyard, just because it’s what you (or your dad or grandpa or great-grandpa) always have done, take the time to get a soil test to determine whether you really need that much N everywhere. Nitrogen availability depends on organic matter in the soil, and each percent organic matter in the soil account for about 20 lbs. actual nitrogen/acre. Hans Walter-Peterson at the Finger Lakes Grape Program developed a worksheet to calculate nitrogen needs based on soil tests results. I recommend you take the time to assess your individual blocks to determine nitrogen needs through soil and petiole testing. You may be surprised by what you may (or may not) need.
Directions for Collecting Petiole Samples
Selecting the Vines:
Select an area containing at least 30-50 vines of the variety to be sampled if possible. These 30 vines should be representative of: (a) a problem area, or (b) the average of the vineyard. The final fertilizer suggestions will apply ONLY to the area represented by the selected vines.
Select 30-50 Vines to represent the sample. If more than one area or block is being sampled, give each area a REFERENCE NUMBER and record this number for future reference. If you have the fields identified with either a number or a letter, this may be used for the reference number. When the diagnosis sheet is returned, it will refer to this field number/name.
Be sure to maintain thorough records of your sampling dates, techniques, and locations. Maintaining proper records will enable you to observe patterns over time and to treat specific areas in a timely and efficient manner.
Remember: Soil analysis in addition to petiole testing will provide the most accurate picture of what’s going on in your vineyards. A soil or petiole test alone will not necessarily indicate whether a vine requires a specific nutrient or if the soil requires a change in the pH.
Collecting Petiole Samples:
Time Of Collection. Collect petiole samples at bloom or 70 – 100 days after bloom (late August or early September). Samples should not be taken after harvest.
Materials Needed: A 2 or 3 brown paper lunch bag.
At BLOOM select a leaf opposite a cluster. At 70-100 days AFTER BLOOM, select the youngest mature leaf on a shoot bearing a cluster. Leaves should be well exposed to light and free from injury and disease. The petiole is the slender stem that attaches the leaf blade to the shoot.
Remove and discard the leaf blade and keep only the petiole. The 60-100 petioles constitute the sample. Place all 60-100 petioles in the paper bag and mark the identification number on the bag.
Collect no more than 2 leaves from each vine. Be sure to collect petioles throughout the vineyard to obtain a representative sample of the block.
Wash The Petioles before they wilt to remove spray residue and dust. This may be done by dipping the petioles in a weak detergent solution (a couple of drops of Tide, etc., in 2-3 cups water) and then rinse quickly and thoroughly with clean water. Do not allow the petioles to remain in the detergent or rinse water for more than one minute. Blot the petioles dry on a paper towel or clean dish towel then place them loosely in the bag. Allow the petioles to dry at room temperature until they become crisp, or for a faster drying time, place bags in oven at 200°F for 30 minutes.
Basic Soil Sampling Instructions: WATCH SOIL TESTING VIDEO
Spade and trowel or soil probe
Sampling bags (in kit)
Sampling sheet (in kit)
Pencil or pen and permanent marker
Plastic re-sealable bag
*Sampling techniques may vary slightly, depending on where they are submitted, so be sure to follow the proper instructions. This set of methods is based on the Cornell Nutrient Analysis Laboratory’s (CNAL) protocol.
- Timing of soil sampling: Although soil sampling can be completed at any time of year, it is best to sample when soil is not too wet or to dry.
- With a spade, dig a hole about 12 inches deep. Use the trowel to scrape along the side of the hole from about 0 to 8 inches below the surface into a container. Use this same technique for the subsurface sample (8 to 24 inches). Be sure to label separate samples.
- A soil probe can be used to collect soil from 0-8 inch and 8-24 inch depths, and be sure to keep surface and sub-surface samples separate.
- Place 1.5 cups dry soil into a labeled plastic bag and fill out forms completely for more accurate recommendations.
- (For CNAL) Seal the form in the envelope provided and seal the plastic bag with the soil sample in the mail bag. Be sure to keep the mail bag attached to the envelope, which should be filled out properly.
Note: For more accurate results in vineyards that are not uniform, select the most uniform blocks from which to sample and combine 5 to 6 samples from within a block, mix them thoroughly, then submit 1.5 cups of that as one sample. For instance, a vineyard with sandy loam soil at one end and more clay at the other, submit two samples for the two blocks within that vineyard.
Additionally, to determine your soil type, you can obtain a soil survey map from your county extension office, or you can use the USDA’s Web Soil Survey page (http://websoilsurvey.nrcs.usda.gov/app/) to zone in on your land and determine soil type.
Just as in humans, nutritional balance within the vine is essential to consistent fruit quality and quantity. How do you balance nutrition in the vineyard? While the majority of nutritional management can be dealt with prior to planting, such as adjusting soil pH and organic matter, vineyard soil management is a continual process and requires monitoring to ensure healthy, productive vines. It’s better to maintain proper soil health practices than to try to correct problems when leaf symptoms are visible. How can you keep an eye on vineyard nutrition? Soil and petiole testing will help you monitor nutrients available in the soil and in the plants. We recommend doing BOTH soil and petiole testing to verify that what you have available in the soil is actually getting into the vines. If a petiole test result indicates a deficiency, but soil test results indicate adequate levels of a nutrient, an alternative problem, such as improper soil pH or inadequate water drainage, could be the culprit.
The ability for a vine to withstand cold temperatures is limited by the genetic potential and the environment in which it grows. Concord grapevines are relatively hardy compared to other grape varieties grown in the eastern US, which is the primary reason why the Concord industry has survived for so long in Western NY. The subzero temperatures the Lake Erie region received on February 16, 2015 tested and exceeded the cold hardiness limit of many area vineyards and impacted all vineyards in the belt to some extent. The bud damage assessment following the Feb 16th cold event showed an average of 50% bud mortality across the belt.
Although temperature had a relationship with the amount of damage found, there was still a wide variation between vineyard blocks and within each block. There are many contributing factors to this variation such as the previous year’s weather, time of harvest, disease (see picture), vine health, and when the vine began the acclimation process for the dormant season.
Seasonal differences will impact all grape cultivar’s cold tolerance, and maximum hardiness will vary for year to year. The period from veraison to harvest has the most influence on vine acclimation leading to the cold hardiness. Water status of the buds, vascular system, and trunk will delay or induce vine acclimation. Wet conditions promote continued growth and slow down acclimation. In contrast, dry conditions help the vine acclimate by allowing excess water to leave various cell tissues.
Cropping levels can have a high impact on vine health and effect vine cold hardiness. High crop levels slow down the acclimation process and can lead to lower cold hardiness. This is where vine balance factors in. Average yield on a favorable site can be higher than a less favorable site. If there is vine balance at both sites they could have similar cold hardiness, but if either site is overcropped it can throw off vine balance and become more susceptible to winter injury.
Cold hardiness is made up of many complex mechanisms and processes that a vine goes through to survive exposure to low temperatures. Here are a few take away points.
Take Home Points:
- Weather during the previous season has a great effect on cold hardiness.
- Vines need to go through an acclimation process where they ‘harden off’ and develop cold hardiness/cold tolerance. This process begins in late summer when the shoots stop growing and become woody and ends as the vines go into dormancy.
- Achieving vine balance and good vine health can optimize cold hardiness.
- Crop level has an effect on bud hardiness. Over-cropping leads to decreased hardiness.
Management practices that reduce winter injury:
- Grow balanced vines. Adequate vine size and crop level promote vine health.
- Know soil health and properly adjust soil nutrients.
- Increase soil drainage on sites that need it (tiling, cover crops, ripping etc.). The vines need to get rid of excess water after veraison, which can be difficult for vines on wet sites.
- Grow multiple trunks and renew trunks on a regular basis.
- When possible harvest sites when they reach target sugar. Sample sites and harvest accordingly. Do not follow the previous year’s harvest schedule… rotate when possible.
- Vine diseases and pests that bring down vine health also bring down cold hardiness/tolerance. Take care of them.
How to Assess Bud Damage
Luke Haggerty, Viticulture Extension Associate
Lake Erie Regional Grape Program
Cutting buds is necessary to evaluate bud damage after a freeze event. After practice this process is as simple as making a single cut and scoring dead or alive on a sheet of paper. The most important part of accurate bud assessment is cutting the bud at the correct depth. When cuts are made too shallow live buds may appear dead and if you cut too deep dead buds will appear live when they are actually dead. The illustrations in this article show how to accurately cut buds for evaluation.
Collecting canes for sampling:
Collect healthy canes that should have viable buds (canes that would be saved when pruning). There is no need to collect diseased canes, laterals, or bull wood as these types of canes are less hardy and are not expected to have viable buds. Collect samples from areas in the vineyard that will give you the best representation of that area. Collect approximately 10-15 canes (100 buds) from different areas within a vineyard block.
Handling and Timing: If you are assessing your buds after a recent freeze make sure to give the buds enough time to oxidize (turn brown and black) and show the damage they have received. It is best if the canes are left in room temperature for 24-48 hours. From my experience the longer you wait after the cold event the better the damage shows up.
Making the Cut: The following examples show the cutting of a healthy Concord bud (figure 1-4). Remember cutting down to the correct depth is very important. Because depth is important it is best to make a series of shallow cuts when first starting out. Using a razor blade, make a cut that is parallel to the cane taking the top third of the bud off.
The cut in figure 1 is too shallow to get an accurate assessment if the bud is alive or dead, but it is a good look at the top portion of a healthy bud. The light brown material surrounding green tissue is normal and is not to be mistaken for damaged tissue.
Figure 2 is an example of the correct depth needed to determine if the primary bud is alive or dead. Bright green tissue signifies if the primary bud is viable/healthy and should be scored as an alive bud. The secondary in the figure 2 needs to be cut deeper for assessment.
Figure 3 shows the secondary bud cud at the correct depth for assessment. Be careful not to cut too deep. Figure 3 is the lowest I would recommend cutting for evaluation.
Figure 4 shows a cut that is too deep for a good assessment. The green tissue you see in this picture is the bud cushion. In most cases of freeze damage the bud cushion will still be green and is the reason it is important not to cut too deep.
Tips: Make several cuts when you first start evaluating buds. Take time to use some sort of magnification (reading glasses or magnifying glass) to get a good look. Good lighting can make a world of difference. The light brown material surrounding the bud tissue in figures 1-3 are okay and are examples of healthy buds. Key color indicators of freeze damage are gray, brown/green tissues, dark green, dark brown, and black (see examples in figure 6). Practice makes perfect. Cut 200 buds before you start scoring the damage… or as many as it takes for you to feel comfortable with what you are seeing.
All pictures in this article were taken by Luke Haggerty
For more information on how to assess winter injury to buds or to see a video tutorial visit: http://www.fruit.cornell.edu/grape/pool/winterinjurybuds.html
Lake Erie Regional Grape Program
Concord in full bloom: 6/14/2017
This season, the staff at CLEREL recorded trace bloom in the standard phenology vines on 6/11/2017 and official bloom (50% cap fall) on 6/12/2017. The Concord flower cluster in this image is at over 90% full bloom on June 14th.
Concord Flower Anatomy: 6/14/2017
Most of the wild grapes you see on the roadside or in the woods have either all male or all female flowers. However, most of the cultivated grape varieties we grow have “perfect” or “hermaphrodite” flowers. Interestingly, cultivated grapes are also highly self-pollinated because the pollen will go from the anther to the stigma before the cap pops off.
The anthers typically release pollen prior to cap fall so pollination likely started on Monday (6/12) when we recorded trace bloom. To catch and rehydrate the pollen, the stigma produces a sap (seen at the tip of the stigma arrow in the picture). Rehydration of pollen takes about 30 minutes and then the pollen uses stored starch in the pollen grain to grow down the style. The speed of pollen tube growth and the time it takes to reach the ovule is related to temperature (roughly 48 hours at 60 degrees F, 24 hours at 70 degrees, and 12 hours at 80 degrees). The colder it is, the slower the pollen tubes grow. Since the ovules are only receptive for a short time, cool weather during bloom can cause the pollen to miss the window and lead to poor fruit set. Fortunately for 2017, it has been sunny and warm (between 60-80) for the past three days so Concord fruit set in the Lake Erie Region should be in decent shape.
Concord Pre-Fruit Set: 6/16/2017
Just after flowering, the pollinated pistils on the grape clusters will start to develop but not all of the ovaries will successfully develop into fruit. This Concord cluster has about 100 developing ovaries but will only retain and develop 25-30 fruit, on average. Expanding pollen must first fertilize at least one of four ovules while they are receptive. Successful fertilization induces the production of certain plant growth hormones for cell division (auxin) and cell expansion (gibberellin) in different tissue layers. The balance of these hormones is important for the successful retention of the developing ovary. Percent total fruit set is influenced by cultivar and certain management practices, such as pruning level. A variety of environmental stresses (light, temperature, carbohydrate, nutrient, and water stress) can also reduce fruit set. Many of our management recommendations, such as for weed control and mineral nutrition, aim to eliminate any vine stress during the fruit set and berry cell division phase in the 3-4 weeks after bloom. Unfertilized or stressed ovaries will eventually abscise or “shatter.”
Concord Mid-Shatter: 6/19/2017
Just one week after the start of bloom, Concord clusters are setting fertilized berries and dropping others. This cluster has dropped about 40% of the pistils which were originally pollinated but not successfully fertilized. A corky abscission scar can be seen where the pedicel of aborted flowers have separated from the rachis (cluster stem).
Why is it important to track fruit set?
Current research objectives aim to improve mid-season crop estimation. Grape yield is a function of shoots/vine, clusters/shoot, berries/cluster, and final berry weight. These “yield components” can be influenced by biological factors such as vine size and vine water status, management factors such as pruning level, or environmental factors such as temperature during fruit set. Spatial data from the Efficient Vineyard project illustrate how yield components can vary from vineyard to vineyard as well as within a vineyard. We are testing the use of the Carnegie Mellon Image sensor to directly detect and count certain yield components – such as shoot number and berry number across a vineyard. We are also combining this information with other spatial data to direct vineyard sampling during the middle of the growing season to predict final crop size across whole vineyard blocks.
Lake Erie Regional Grape Program
Concord Berry: 30 Days After Bloom
A cross-section of a Concord berry at 30 days post-bloom. In addition to seed development, the cell division and expansion of the mesocarp tissue has a considerable influence on fresh berry weight. Cell number will double two times during stage I through cell division. In addition, cell volume will increase through cell wall loosening and the expansion of cell vacuole volume. Presumably, the cell division hormone, cytokinin, diminishes through stage I, which slows cell division as the berries enter the lag phase of berry growth (stage II).
Concord Berry: 45 Days After Bloom
At 45 days after bloom, Concord berries are between 60-65% final fresh weight and are entering stage II of berry development. This stage is also referred to as the lag phase and will last until veraison (approximately 69 days after bloom in Lake Erie Concord). Lag phase is dominated by seed development and maturation. By veraison, the seeds will reach their final size and lignify in preparation for dispersal. The growth of the mesocarp and exocarp slows during the lag phase. Again, it is the balance of plant hormones, such as auxin, cytokinin, and abscisic acid, which controls the cell division and expansion in berry tissues and prevents the seeds from germinating too early.
Looking at Trunk Damage
Lake Erie Regional Grape Program
The rising temperatures have helped kick-start sap movement and allowed us to get the first good chance to inspect for damage. Over the past week I have been out cutting the outer layer of trunks and cordons and looking for freeze damage. Much like the earlier bud cuttings the damage varies from cultivar, location, and the low temp at vineyard site.
To assess trunk and cane damage, make a shallow cut into the trunk or cane deep enough to expose the outer layer of the vascular tissues (about 1/8 inch deep). Examine the vascular tissue for freeze damage by the amount of oxidation (browning) in the various tissues. Trunk damage occurs in this order; phloem, xylem, and then cambium. Damage to the phloem (A-E outer ring) will prevent the flow of carbohydrates needed for shoot development, but will most likely not kill the trunk. Damage to xylem will restrict flow from the roots to the canopy resulting in stunted, chlorotic (yellowed), or dead shoots and can cause vines to collapse under stress. To learn more on how to evaluate trunk damage see the 2015 March Newsletter http://lergp.cce.cornell.edu/newsletter.php.
As you evaluate trunk damage keep in mind the severity and location of the damage. This will help you plan herbicide treatments in specific areas where trunk renewal is needed. Suckers should be retained with the purpose of vine or trunk renewal when trunk damage looks like B, C, and D in the figure below. Severe trunk damage in figure A shows the trunk has no sap flow signaling it is dead. Although the trunk in figure B has sap flow the damage is severe enough that the vine will not be able to function and will most likely die to the ground. Figure E shows minimal damage and should be able to grow out of the minor vascular damage. Both F.1 and F.2 are examples of healthy trunks with no vascular damage.
Renewing Damaged Trunks
Luke Haggerty, LERGP Viticulture Extension Associate
With temperatures dipping down to -17 to -30oF throughout the region, winter injury was inevitable. The winter’s extremely cold temperatures have left most of the Lake Erie grape region in a varied state of damage. V. vinifera cultivars suffered the majority of the loss/damage with many ‘hybrid’ and ‘native’ cultivars also suffering winter injuries. As the growing season progresses, the apparent bud and vine vascular damage can be seen by dead or stunted shoots on injured vines. Depending on the severity of the damage, growers are taking action by replanting dead vines or renewing vines/trunks that have suffered damage.
With bloom closely approaching all trunk and cordon vascular tissues (phloem, vascular cambium, and xylem) should be functioning in full force. Trunk damage occurs from the outside in; making the phloem first tissue to show sign of injury. Damage to the phloem (Fig. 1A) will prevent the flow of carbohydrates needed for shoot development. Damage to xylem (Fig. 1C) will restrict flow from the roots to the canopy resulting in stunted, coloristic (yellowed), or dead shoots (Fig. 1B). Phloem and xylem damage can be assessed by cutting a shallow strip off of the trunk, cordon, or canes and examining the amount of browning.
In cases of severe trunk damage, the xylem and phloem no longer function and the vine can collapse (Fig. 3A). Vine collapse occurs when expanding leaf size and overall canopy size demand more water than the trunk can supply. Timing of vine collapse is unpredictable and can even happen the following growing season. Depending on the location of the vascular damage, ‘partial vine kill’ can occur on one side of the cordon or select canes. In cases of partial vine kill, vines can be managed by pruning out the affected areas. However, when there is obvious or suspected trunk or cordon damage, suckers should be retained with the purpose of vine or trunk renewal. The overall goal of trunk renewal is to balance the amount of living tissues above ground with the potential of the roots below ground.
Trunks should be renewed on any vines that are suspected of trunk injury. Protected below the soil line, root systems are generally unharmed by winter injuries and readily supply carbohydrates to the awaiting plant tissues above ground. When there is trunk damage, hidden buds at the trunk base awake from dormancy and produce ‘suckers’. A vigorous eruption of suckers has long been a sign of trunk damage, and the typical response from most growers is to save the sucker to replace the existing trunk which renews the vine. The amount of sucker and fullness of canopy are cues for guiding decisions on how to balance the vine. If viable, 4 to 6 suckers should be retained to balance the root support when the canopy is severely stunted and or showing visible nutrient deficiency. In vines that have full canopies and produce large vigorous suckers, only 2 to 4 suckers should be retained to obtain balance. Although balance is difficult to put into words, a different decision will need to be made for every vine. The goal of vine renewal is to manage the existing root structure with the amount of living plant material above ground.