Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science
Another growing season has started for many Pennsylvania grape growers. Unfortunately, but not surprisingly, we are seeing and hearing of situations of vine winter injury across the State. This past winter, the lowest temperatures occurred at the end of January and during the first two days in February, with values around -5 °F (-20.6 °C) here in State College (central PA) and even lower temperatures were recorded at other locations.The injury seemed to have mainly affected Vitisviniferavarieties with reports of bud kill up to almost 100% for the most cold-sensitive varieties and, in some cases, trunk splitting.Growers also noticed uneven /nonuniform budburst which is typical of winter-injured vines. We ask that more growers share their experiences with us; in particular, we would like to know if growers made any pruning adjustments and what the results are/have been.
Since winter injury is a reoccurring issue for the eastern US, during certain years, we have covered topics related to vine cold hardiness, injury assessment, and pruning techniques for winter-injured vines at Extension meetings. Also, we have posted an announcement that focused on Pruning strategies for cold climate viticultureon the Penn State Viticulture and Enology Facebook page in January 2019, just before the “Arctic Vortex” event hit our region. Please do not hesitate to contact us if you have questions on how to manage cold-injured vines.
We heard from several PA growers in southern and central PA that budburst occurred earlier this year, a week to 10 days is what has been typically reported, than in 2018. This was also true for the hybrid varieties grown at the Penn State research farm at Rock Springs (central PA). I checked the growing degree days (GDD), a widely used index of heat accumulation, data calculated by the Network for Environment and Weather Applications (NEWA Cornell) for weather stations located in North East, Erie (northwestern PA), Biglerville (south-central PA), and Reading (southeast PA). Although historic data are not available, I compared the average GDD accumulated from January 1 to May 15 for 2013-2017 to those accumulated for the same period in 2018 and 2019 (Figures 1, 2 and 3).
Trends across locations/regions
Not surprisingly, it was cooler in Erie compared to south-central and southeastern PA between January to Mid-May, not just in 2019 but for each year analyzed. In 2019, approximately 158 GDD accumulated between January 1 to May 15 in Erie, while GDD were at least double in south-central and southeast PA. Differences in temperatures across regions and locations explain why budburst typically occurs much earlier in southeast PA compared to the northwestern part of the state.
Difference between years
In Erie, the GDD accumulated between January to mid-May 2019 (red line) were slightly lower than those for the same period in 2018 (blue line) and for the 2013-2017 average (black line). Also, note that there was no accumulation of GDD for a few days in May 2019 due to cool temperatures (Figure 1). The trend, however, was opposite in south-central and southeast PA, at least at the locations reported in this post. April was warmer (higher GDD) in 2019 compared to 2018 and the 2013-2017 average. While warmer spring temperatures favor earlier budburst they also increase the chance of freeze injury to green, tender plant tissues (Figure 4).
At several locations across PA, temperatures were below freezing in the early morning of April 29 and some varieties were close to or already passed budburst. Below freezing temperature does not necessarily mean freeze injury as many factors affect the temperature at which the plant tissue is damaged or killed. However, the freeze event on April 29 did cause freeze damage to vines at several locations, while others avoided the damage by using frost protection methods, such as frost dragons. Some of the varieties grown at the Penn State research vineyard at Rock Springs, chiefly Marquette and young LaCrescent vines, sustained freeze injury. It is too early to estimate crop losses, but at least we are seeing some secondary shoot development (Figure 5).
How to recognize a secondary from a primary shoot
A relatively easy way, especially for caned pruned vines, is to check the angle of projection from the cane. Primary shoots typically grow with an angle of 45°, while secondary grow at an angle of 90° (figure 5).
You can learn more about the basics of spring freeze injury and methods of protection at https://extension.psu.edu/understanding-and-preventing-spring-frost-and-freeze-damage
It is almost time for some early season canopy management practice. Please check the following articles if you need information on shoot thinning or early leaf removal:
By Bryan Hed
Since the new year was ushered in we have had several scary moments when Mother Nature unleashed an “excess of personality.” I’m referring to the cold weather events we experienced around January 1, 7, and 14, when temperatures slipped down below zero in many places across Pennsylvania, even in some south central parts of the state. As many of you might remember, the last time we saw below zero temperatures that far south (February from hell, 2015) primary bud damage was widespread and grapevine trunks in vineyards all over Pennsylvania (and certainly other parts of the Northeast) exploded in crown gall the following spring. This generated a two-year trunk renewal process that we’ve only just recovered from. Therefore, this may be a good time to review grapevine winter hardiness and the factors that affect it, as well as how we can prepare for possible remediation pruning and renewal this spring.
Now I don’t want to raise alarm bells just yet, as the conditions we’ve experienced this January haven’t been as horrific as February of 2015. But it’s always good to be prepared for any potential consequences, like bud loss and trunk damage, so we can anticipate altering our winter pruning plans and production practices this season.
Let’s start with a review of the temperature stats available to everyone on the NEWA website (newa.cornell.edu) and see just how cold it got in various places across the state during the first half of January. In the table below, I’ve listed low temperatures for January 1, 7, and 14 for many of the NEWA locations. Starting at northeastern PA and moving counterclockwise to swing back up into northern New Jersey and finally western New York, we get the following data (Table 1).
Areas of southeastern and northwestern Pennsylvania, at opposite corners of the state, appear to have escaped the below-zero temperatures for the most part, but some areas of south central Pennsylvania took a hit (look at York Springs). Areas of southwestern Pennsylvania experienced some of the most extended periods of below-zero weather, and parts of northeastern and central Pennsylvania also got quite cold. The temperature low is the most important bit to consider when sizing up vine bud damage, but the duration of those lows can affect the extent of trunk damage, especially in big old trunks where it may take longer for the core to reach ambient temperatures. Up in the northwestern corner of the state, the buffering effect of Lake Erie probably played a role in our relatively mild temperatures during that period, and we expect little to no damage to most of our vines as our wine industry there is heavily invested in tougher hybrids. The Erie area was also blessed (?) with a heap of snow (10 feet!) before the cold snap that provided added protection to bud unions of grafted vines.
If you’re anticipating primary bud damage, here’s a review of the ranges of temperatures for the LT50 (low temperature at which 50% of primary buds fail to survive) for the cultivars you’re growing. For Vitis vinifera, the LT50 range of the most winter sensitive cultivars falls between 5o and -5oF. This includes cultivars like Merlot and Syrah. But for most cultivars of V. vinifera, LT50 values fall more in the 0o to -8oF range (Chardonnay, Cabernet Sauvignon, Pinot gris, Pinot noir, Gewurztraminer). And finally, there’s the tougher V. vinifera and sensitive hybrids that have buds with LT50 values of -5o to -10oF. This includes cultivars like Riesling, Cabernet franc, Lemberger, and Chambourcin. On the flip side, most hybrids fall into the -10o to -15oF range (which is why Northeastern U.S. vineyards are perhaps still more invested in hybrids than V. vinifera). Then there are the V. labrusca (Concord) and the Minnesota hybrids that range from -15o down to -30oF for cultivars like Frontenac and LaCrescent. Unfortunately, we don’t have such helpful ranges for determining trunk damage, which often comes with more profound consequences and is costlier to address.
Rapid temperature drops are often the most devastating in terms of the extent of damage. Fortunately, December temperatures this winter descended very gradually giving vines time to fully acclimate to cold weather extremes. In fact, recent data from the Cornell research group in the Finger Lakes region of New York shows that LT50 values for primary buds of several cultivars were close to, or at, maximum hardiness. Therefore, it is hoped that many Northeastern U.S. vineyards were well prepped and close to their hardiest when these cold events occurred. On the other hand, any given cultivar in central New York is likely to be a bit more cold hardy than that same cultivar growing in southern Pennsylvania, simply because vines farther north will have accumulated more cooling units than those farther south. So there is the possibility of bud and—worse yet—trunk damage in parts of PA, to the more sensitive cultivars of V. vinifera.
We also had a balmy warm period during the second week in January that pumped temperatures up into the 60s in some places before plunging back down into single digits. However, it’s unlikely the brief warm period was long enough to cause any deacclimation of vines before cold temperatures resumed, and little, if any harm, is expected from that event.
The capacity for cold hardiness is mostly determined by genetics. As I alluded to above, V. vinifera cultivars are generally the most sensitive to cold winter temperature extremes, French hybrids are generally hardier, and native V. labrusca cultivars are often the toughest. Nevertheless, other site specific factors can come into play to affect cold hardiness, and this is often the reason for the range in the LT50 values. For example, there’s vine health to consider; vines that finished the season with relatively disease-free canopies and balanced crop levels can be expected to be hardier (within their genetic range) than vines that were over-cropped and/or heavily diseased. At times like these, we can’t emphasize enough how important it is to maintain your vines and production strategy with a view to optimizing their chances of surviving every winter. Other stresses like drought or flooded soils (during the growing season) that we can’t do much to control, and infection by leafroll viruses, can also play a significant role in reducing vine cold hardiness.
If you suspect damage, you should delay winter pruning of your vines, according to Dr. Michela Centinari. Feel free to revisit her previous blog posts and others at psuwineandgrapes.wordpress.com. Type “cold hardiness” or “winter injury” into the search box, and you’ll quickly and easily gain access to several timely blogs.
Bud damage can be estimated from 100 nodes collected from each potentially compromised vineyard block. Typically, gather ten, 10-node canes from each area, but do not sample from blocks randomly, unless the block is relatively uniform. If a block is made up of pronounced low and high areas (or some other site feature that would affect vine health and bud survival) make sure you sample from those areas separately as they will likely have experienced different temperature lows (Zabadal et al. 2007). You may find that vines in high areas need no or less special pruning consideration than vines in low areas that suffered more primary bud damage and will require increased remediation.
Once you have your sample, bring the canes inside to warm up a bit and make cuts (with a razor blade) through the cross section of the bud to reveal the health (bright green) or death (brown) of primary, secondary, and tertiary buds. You’ll need a magnifying glass to make this determination as you examine each bud. You should figure that primaries will contribute two thirds of your crop and secondaries, one third when considering how many “extra” buds to leave during pruning. And remember that some bud damage, up to 15% or so, is normal. If you’ve lost a third of your primaries, leave a third more nodes as you do your dormant pruning. If you’ve lost half your primaries, double the nodes you leave, and so on. However, when bud mortality is very high (more than half the primary buds are dead), it may not be cost effective to do any dormant pruning as it is likely there are more sinister consequences afoot, like severe trunk damage that is much harder to quantify. A “wait and see” strategy, or at least very minimal pruning, may be best for severely injured vines (Figure 1) and trunk damage will manifest itself in spring by generating excessive sucker growth (Figure 2). And one more thing: Secondary buds are often more hardy than primaries, may have survived to a larger extent, and in some cultivars, can be incredibly fruitful. This is especially true of some hybrid varieties like DeChaunac. So, to make more informed decisions when winter damage is suspected, you have to know the fruitful potential of your cultivar; and in cases where primary bud mortality is high, it’s therefore important to also assess the mortality of secondary buds.
Another great fear is the appearance of crown gall, mainly at the base of trunks. This disease is caused by a bacterium that lives in the vine. However, the bacterium generally doesn’t cause gall formation on trunks until some injury occurs, usually from severe winter cold damage near the soil line or just above grafts on grafted vines (if you hilled over the grafts last fall). Another search at psuwineandgrapes.wordpress.com will bring up information on how to deal with this disease. You can also visit What we have learned about crown gall for an update on research into this disease from Dr. Tom Burr and his research group at Cornell University. Tom has devoted a lifetime to researching grape crown gall and many advances have been made over the years. But it’s still a huge problem for Northeastern U.S. grape growers; and crown gall problems will likely increase as our industry becomes more and more heavily invested in the most susceptible cultivars of V. vinifera.
With more sensitive detection methods, Tom’s group is getting us closer and closer to crown gall-free mother vines and planting stock, but they’re also discovering that the crown gall bacterium is everywhere grapevines are located. Not restricted to internal grapevine tissues; it’s also found on external surfaces of cultivated and wild grapevines. So, clean planting stock may still acquire the pathogen internally down the road and management of crown gall, once vines are infected, will continue to be an important part of life in any vineyard that experiences cold winter temperature extremes. However, there is potential for a commercial product that inhibits gall formation, which can be applied to infected vines. The product is actually a non-gall-forming, non-root-necrotizing version of the crown gall bacterium that is applied to grape wounds and inhibits the gall-forming characteristic of the pathogenic strains of the bacterium. This product is still under development in lab and greenhouse tests, awaiting field nursery trials soon.
If you do happen to meet up with some crown gall development this spring, galled trunks can be nursed through the 2018 season to produce at least a partial crop while you train up suckers (from below the galls) as renewal trunks. When our Chancellor vineyard was struck with widespread crown gall in the 2015 season, we were able to harvest a couple of decent sized crops while trunk renewal was taking place (Figure 2), and we never went a single season without some crop. There’s also the issue of crop insurance to think of; adjusters may want you to leave damaged trunks in place so they can more accurately document the economic damage from winter cold.
Lastly, a great guide to grapevine winter cold damage was published about 10 years ago by several experts. In fact, information from that guide was used in composing large parts of this blog and I highly recommend you read it. It’s an excellent publication, the result of many years of outstanding research by a number of leading scientists and extension specialists from all over the Northeastern U.S. The details of that publication are found below and you can purchase a hard copy for 15 bucksby clicking here: Winter Injury to Grapevines and Methods of Protection (E2930).
For those of you who can spend hours reading off of a computer screen without going blind, you can also access a web version of the document at msue.anr.msu.edu/uploads/files/e2930.pdf.
Zabadal, TJ, Dami, IE, Goiffinet, MC, Martinson, TE, and Chien, ML. 2007. Winter injury to grapevines and methods of protection. Extension Bulletin E2930. Michigan University Extension
By Michela Centinari
The last month has provided a temperature roller-coaster going from a very, perhaps exceptionally, warm March to a cold beginning of April. Many grape growers are keeping their fingers crossed hoping to escape frost injury. As far as I am aware, no budbreak has been observed for grapevines grown in central Pennsylvania, but budbreak may be close in other PA locations. It looks like a good time of year to review some basic concepts related to post-budbreak freeze injury and frost protection options available for grape growers.
Freeze and Frost
We often use the terms “frost” and “freeze” interchangeably to describe a meteorological event, specifically related to air temperature dropping below 32°F (0 °C). However, “frost” and “freeze” definitions reported in the literature are variable and sometimes confusing. I personally like the definitions used in the book: Frost protection: Fundamentals, practice, and economics [Food and Agriculture Organization of the United Nations (FAO), 2005; 1]. In this book frost is defined as “the occurrence of an air temperature of 0 °C or lower, measured at a height of between 1.25 (49.2 in) and 2.0 m (78.7 in) above soil level, inside an appropriate weather shelter”, while freeze “occurs when water within the plant freezes”.
In other words a frost becomes a freeze event if ice forms within the plant tissues.
Keep in mind that:
- It is the ice formation inside the plant tissue rather than low temperatures per se that cause the damage. The formation of ice crystals can be either inter-cellular (space between cells) or intra-cellular (within the protoplasm of cells), the latest causing cell death  (Figure 1). The general hypothesis is that during spring frosts, freeze injury is mainly caused by inter-cellular rather than intra-cellular ice formation [1, 2]. The formation of inter-cellular ice crystals produces a water vapor deficit/gradient between the interior and the exterior of the cells. As a result, water migrates from the inside to the outside of the cells and deposits on the ice crystals formed in the inter-cellular spaces. If ice continues to grow, the cells become more desiccated and lose their turgor . Freezing-induced dehydration can also permanently damage the structure of cell membranes and other cellular components. This usually causes a flaccidity and/or discoloration of the damaged tissue . Thus, the current view is that dehydration injury is the main cause of frost damage. .
- Water within plants doesn’t always freeze during a frost event. Plants have developed avoidance strategies to avoid ice formation in the tissues, for example, by supercooling, and tolerance strategies (e.g., solute content of the cells) to survive inter-cellular ice formation without irreversible damage of the plant tissue .
The critical temperature is defined as “the temperature at which tissues (cells) will be killed and determines the cold hardiness levels of the plant” . Many factors affect the temperature at which damage occurs including: type of plant tissue, stage of phenological development of the bud/shoot, dew point and surface moisture, probability of an ice nucleation event and pre-frost environmental conditions .
Why budbreak is considered the onset of the most susceptible period for cold injury?
Growing organs have a high water content, which makes them susceptible to the formation of ice at freezing temperatures. Air temperature of –2, –3°C can permanently damage green tissues . Early spring growth is particularly susceptible to freeze injury. Freezing tolerance remains low during the most of the growing season and gradually increases late summer and fall (cold acclimation) and reaches its maximum peak in midwinter . In midwinter grapevines are able to tolerate freezing temperature through a complex process called deep supercooling. For example, the cells within the dormant bud become resistant to lower temperature through dehydration (i.e., movement of water to inter-cellular spaces) and accumulation of so-called cryoprotectant (e.g., soluble sugars and proteins). Those compounds lower the freezing point of the water within the plant tissue and stabilize cell membranes  making the dormant buds able to survive temperatures well below freezing. Also, during the dormant season buds are thought to be disconnected or weakly connected to the vine’s vascular tissues, which limit their potential to take up water .
There are two main types of frosts
Advective frost: an advective frost is usually a regional weather event. It occurs when strong, cold winds (colder than the critical temperature) blow into a region day and/or night. The rapid, cold air movement “steals away the heat in the plant causing freeze damage” . Unfortunately there is very little which can be done to protect against an advective frost. For example, wind machines are useless during an advective frost event.
Radiation or radiative frost: A radiation frost is the most common type of frost for many grape growing regions. Luckily, a radiation frost is also the easiest to protect against during a frost event. It occurs when a dry, cold air mass moves into an area when there is almost no cloud cover and no wind at night. Because plants and soil are warmer than the sky temperatures they will “radiate” heat back to their surrounding space and become progressively colder than the air .
Radiative and advective frosts may occur simultaneously, the classification depends on which is one is dominant (Table 1).What are the options available to protect your vines from freeze injury?
Passive or indirect methods (risk minimization)
Passive methods are avoidance strategies, efforts to reduce the probability and risk of freeze damage.
- Site selection
You have probably already heard this, but it cannot be said too many times: “The best time to protect your vineyard from frost injury is before it is planted” . Cold air flows downhill so mid-slope locations are warmer if there are no obstacles to cold air flow  (Figure2). Thus, when evaluating potential sites for establishing your vineyard, look for a site with good air drainage. Get historic records of low temperatures, number of frost-free days, and accurate information on percent slope, aspect or exposure and elevation. You can contact your local county Cooperative Extension office for information about site suitability for a vineyard, or utilize these resources here: http://bit.ly/VydSelectionTools.
- Cultivar selection
Grapevine cultivars may vary in the average day of budbreak by up to two weeks . To avoid or reduce the risk of freeze injury plant cultivars with early budbreak in the location within the vineyard with the lowest risk of frost.
- Training system choice
Many factors related to fruit quality and economics influence the choice of a training system. With regard to risk of freeze damage, a training system which places the buds high on the trellis may reduce frost hazard (Figure 3). Frost hazard is reduced by up to 0.36 °C each 10 cm (3.94 in) above the soil level .
- Pruning choices
- Delay pruning: Pruning too early may accelerate budbreak. Thus, prune as late as possible in frost prone areas of your vineyard.
- Double pruning: this is another option to delay budbreak for cordon-trained vines. The first step is to prune the canes to long spurs, 5 to 8 buds long . Buds at the end of the canes will open first and suppress the growth of basal buds (Figure 3). After frost risk has passed, do a second and final pruning to cut back the long spurs to two-bud spurs. Likewise, for cane-pruned vines one option is to leave long canes (first step) and cut them back (second step) to the desired bud number later, after the frost risk has passed. Some growers opt to retain extra canes as an insurance measure and then remove them later.
- Delaying budbreak by chemical means
Application of vegetable-based oils (e.g., Amigo oil) at nontoxic rates can slow bud de-acclimation and delay grapevine budbreak anywhere from 2 to 20 days depending on several factors including variety, number of applications and coverage [10,11]. Those oils are called “dormant oils” because they need to be applied when the buds are dormant. If you are interested in trying Amigo oil or a similar type of oil in your vineyard, begin with a small selection of vines. Be sure to record phenology, crop yields, fruit composition (Brix, pH, TA) and quality (fruit aromas and flavors, etc.) data for un-sprayed and sprayed vines. In this way, you can assess the impact of oil application on delaying budbreak as well as potential secondary effects on production and fruit quality parameters.
- Middle-row management
Mowing ground cover short will increase the warming of soil during the day and release slightly more heat during the night . Tall cover crops and weeds may also hinder cold air drainage.
Active or direct frost protection methods (frost management)
Active or direct frost protection strategies are efforts to modify microclimate conditions in the vineyard and increase temperatures above injury levels. Some of the most common active frost protection methods are:
- Wind machines (or fans)
Wind machines are well suited for radiational frosts because they use the inversion of air temperature that develops during this type of frost event. Wind machines pull down warmer air, from above the inversion layer, which may provide from 1 – 3°F of warming . The minimum size vineyard recommended for a wind machine is around 7-10 acres. Wind machines may become profitable on sites where there is a 20% (1 in 5 years) or higher probability of spring frost damaging events . It is worth mentioning that wind machines have been noted to produce a loud noise. Operating costs are higher than for over-vine sprinkling systems, but considerably lower than use of return-stack oil heaters and standard propane heaters .
- Over-vine irrigation
Over-vine sprinkler systems have been successfully used for frost protection since the 1940s . Sprinklers provide a constant amount of water covering the buds and shoots. As water freezes it releases a small amount of heat, which increases the temperature of the plant tissue. The level of protection is proportional to the amount of water applied . If properly used, this method is very effective in protecting grapevines from freeze injury. It is the only active method that doesn’t rely on inversion strength during a frost event . However, on the other hand, keep in mind that it requires substantial water resources, is labor intensive and if the system fails during the night/frost event it can cause more damage than otherwise applying no frost protecting strategy.
Heating the vineyard for frost protection is a very old practice. In ancient Rome (at least 2000 years ago) growers used to burn piles of pruned wood and other waste to heat their vineyard during spring frost events . Fossil-fueled heaters are rarely used these days because of the high cost of fuel and labor, low heating efficiency and contribution to air pollution.
Unfortunately there is not a perfect strategy which can provide complete frost protection in every situation. Quite often the combination of different methods is the best option.
If you are looking for detailed information about active frost protections strategies please check:
Understanding and Preventing Freeze Damage in Vineyards. 2007. Workshop Proceedings. University of Missouri Extension.
Evans, R.G. 2000. The art of protecting grapevines from low temperature injury. Proc. ASEV 50th Anniversary Annu. Mtg., Seattle WA, 19–23 June. p. 60–72.
Poling, E.B. 2008. Spring cold injury to winegrapes and protection strategies and methods. Hortscience 43: 1652–1662.
- Food and Agriculture Organization of the United Nations. 2005. Frost protection: Fundamentals, practice and economics. Vol. 1.
- Wilson, S. 2001. Frost management in cool climate vineyards. Final report to grape and wine research & development corporation. Available at: http://www.gwrdc.com.au/wp-content/uploads/2012/09/UT-99-1.pdf
- Poling, E.B. 2008. Spring cold injury to winegrapes and protection strategies and methods. Hortscience 43: 1652–
- Rodrigo, J. 2000. Spring frosts in deciduous fruit trees—Morphological damage and flower hardiness. Scientia Hort. 85:155–173.
- Evans, R.G. 2000. The art of protecting grapevines from low temperature injury. Proc. ASEV 50th Anniversary Annu. Mtg., Seattle WA, 19–23 June. p. 60–72.
- Keller, M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
- Martinson, T. 2001. How Grapevine Buds Gain and Lose Cold-hardiness. Appellation Cornell, Issue 5. Cornell University Cooperative Extension. Available at: https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2011-newsletters/issue-5/how-grapevine-buds-gain-and-lose-cold
- Hellman, E. 2015. Frost Injury, Frost Avoidance, and Frost Protection in the Vineyard. org Available at: http://articles.extension.org/pages/31768/frost-injury-frost-avoidance-and-frost-protection-in-the-vineyard
- Trought, M.C.T., Howell, G.S., and Cherry, N. 1999. Practical considerations for reducing frost damage in vineyards. Report to New Zealand winegrowers. Available at: http://www.nzwine.com/assets/sm/upload/eb/fl/ot/sp/frost_review.pdf
- Dami, I., and Beam B. 2004. Response of grapevines to soybean oil application. Amer. J. Enol. Vitic. 55: 269–
- Loseke, B.J., Read, P.E., and Blankenship E.E. 2015. Preventing spring freeze injury on grapevines using multiple applications of Amigo Oil and naphthaleneacetic acid. Scientia Hort. 193: 294–300.
- Wolf, T.K. 2015. Viticulture Notes. Virginia Tech University Cooperative Extension. April 2016.
By Michela Centinari
This past November and December were surprisingly warm months in Pennsylvania with temperatures rising into the 60s and even 70s °F (Figure 1a, b). In January temperatures dropped to single digits in many eastern U.S. regions  followed by a virtual temperature rollercoaster ride in the next months (Figure 1). On a positive note, viticulture specialists from Virginia Tech (T. Wolf and T. Hatch) did not observe any winter injury in Cabernet Sauvignon and Merlot buds and canes collected on January 8 (northern Virginia) .
In many regions of Pennsylvania temperatures in January and February did not reach the critical low threshold (Figure 1B) that tends to injure many of the cultivars grown in PA and we are not currently concerned about winter injury in the majority of the state. However, on February 14 temperatures of -10°F and below were recorded in some areas of the state (report from growers from northeastern PA and Figure 1A). The lowest temperature in Pennsylvania (-19°F) was recorded in Potter County on February 14 .
Although we are not aware of the extent yet, we anticipate winter injury in some of the cultivars grown in those areas.
A few reminders about the cold hardiness process:
In late summer/early fall, cold-tender grapevine tissues produced during the growing season gradually acquire cold hardiness and transition to a cold-hardy stage (known as cold acclimation) as a response to low temperatures and decreasing day length  (Figure 2). Bud (and other tissues) cold hardiness reaches its maximum level in mid-winter (known as maximum hardiness). Later in the winter as temperatures increase, the buds begin to lose hardiness (known as deacclimation) . The deacclimation stage ends in budbreak and active growth.
It is a well-known fact that bud cold hardiness depends heavily upon the grapevine species and cultivar. For the past two winters (2013-2014; 2014-2015) many growers in the eastern and midwestern U.S. have had the unfortunate opportunity to test this in their own vineyards. However, in addition to its genotype, the cold hardiness of a specific cultivar is determined by environmental conditions, such as seasonal temperatures and their variation, and by vineyard management practices . It is important to remember that exposure to decreasingly lower temperatures plays a major role in the ability of the vine to acquire its maximum cold hardiness. In other words:
- The colder the region, the closer a vine gets to its maximum cold hardiness potential . For example, bud cold hardiness of Chardonnay and Riesling in the Finger Lakes region of NY (cooler region) was found to be 2 to 3°F greater than that of the same cultivars grown in Virginia (warmer region)  (Figure 3).
- “The type of winter determines the extent of bud cold hardiness” , thus the absolute cold temperature that injures the same cultivar may vary between winters. J. Londo (Geneticist, USDA, Grape Genetics Research Unit, Geneva, NY) reported that the average mid-winter LT50 (lethal temperature for 50% of the buds) for labrusca varied from -24.71 °F in 2012-2013 (defined as a mild-cool winter in upstate NY) to -26.8°F in 2013-2014 (cold winter but with big swings in temperatures) to -26.5°F in 2014-2015 (sustained cold winter) . However, species and cultivars may vary in their response to different temperatures/winters. For example, Tim Martinson, senior viticulture Extension associate at Cornell University, did not observe a decrease in bud hardiness of Riesling vines this year as compared to last year .
A few of the things that Penn State Extension recommends to growers:
- Plan on pruning the hardiest cultivar first and finish with the least hardy . If a cold event occurs late in the winter, like in the middle of February, and cold-sensitive cultivars were not pruned yet, you can still assess bud cold damage and adjust pruning severity accordingly . Leaving extra buds on the vines, and adjusting shoot number after bud break certainly cost growers more money, but it also increases the chance to produce a crop, hopefully close to normal.
- If your site is an area with moderate to high risk of cold damage events, consider keeping some freeze-tolerant grape cultivars in the mix to reduce the economic downside risk. These are considered cultivars that you can rely upon to “pay the bills.” Cold-hardy cultivars, most of which were released by the breeding program of the University of Minnesota (i.e. Marquette, La Crescent, Frontenac, etc.), are increasing in popularity mostly, but not only, in regions where vinifera or other inter-specific hybrid varieties are not well suited or have struggled to survive and perform well in the long term.
Thanks to The Northern Grapes Project we are gaining a better understanding of how to optimize viticultural, winemaking, business management and marketing practices of these fairly new cold-hardy cultivars. For example, important information on Cost of Production in Cold Hardy Grapes was recently published in the Northern Grapes Project newsletter . The fact that most consumers may still be unfamiliar with those varieties and the wine styles they produce doesn’t necessarily mean that they not will have chance to stand alone as a varietal wine if they produce high quality wines.
- As Zabadal et al.  pointed out “Minimizing winter injury is usually not the primary goal of a grape grower; however it must be given attention because of its huge impact on profitability”. Each grower should carefully evaluate if the cost of vine management practices that reduce vine winter injury can increase the business profit.
- Finally, the most important step, making informed decisions before planting a vineyard and always applying good viticulture practices, which includes keeping the vines healthy and in balance.
Only time will tell what weather conditions the rest of winter and early spring will hold in store for us. However, if your vineyard is located in a frost prone area and you have dealt with spring (post-budbreak) freeze damage in previous years, this would be a good time to review the frost protection practices available and assess if and what options could be used for your specific situation (see for instance: Frost Protection in Orchards and Vineyards by R. Evans, USDA or Methods of Vineyard Frost Protection by P. Domoto, Iowa State University).
A two-year study was conducted by our research team at Penn State to evaluate “low-cost” frost protection practices for their efficacy to avoid/reduce crop losses due to spring freeze injury. We tested the effect of a vegetable-based oil (Amigo oil) to delay budbreak on two vinifera (Lemberger and Riesling) and two inter-specific hybrid cultivars (Noiret and Traminette). We also tested the impact of KDL (Agro-K’s Potassium Dextrose-Lac®), sprayed shortly (»24 hours) before a frost event, on reducing frost damage to young grapevine shoots. The impacts of Amigo oil and KDL applications on yield components, fruit composition and perceived wine quality were also assessed. Results from this trial will be presented at the 2016 Pennsylvania Wine Marketing and Research meeting: http://bit.ly/PAWMRB2016Symposium.
- Jones McKee L. 2016. Cold hardiness and dormancy, pp58-64. Wines and Vines, March 2016.
- Eherts F. 2016. February 2016- Pennsylvania Weather Recap. The Pennsylvania Observer. March 2016.
- Martinson, T. 2001. How Grapevine Buds Gain and Lose Cold-hardiness. Appellation Cornell, Issue 5. Available at: https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2011-newsletters/issue-5/how-grapevine-buds-gain-and-lose-cold
- Zabadal, TJ, Dami, IE, Goiffinet, MC, Martinson, TE, and Chien, ML. 2007. Winter injury to grapevines and methods of protection. Extension Bulletin E2930. Michigan University Extension.
- Londo J. 2015. The Big Chill: bud dormancy and cold hardiness in grape. Northern Grapes webinar, December 8, 2015. Available at: http://northerngrapesproject.org/?page_id=257
- Northern Grapes News. 2016. Vol. 5, Issue 1, February 18, 2016. Available at: http://northerngrapesproject.org/?page_id=213
By: Bryan Hed, PSU Research Technologist
The past two winters have ramped up concerns about crown gall in Pennsylvania and other parts of the Northeast. Wine grape growers are discovering, many for the first time, the horrors of this disease and the extent of the damage it can cause in their vineyards. While there is reason for great concern, I would like to start out by saying that research efforts are generating extensive information on management of this disease, and there are new solutions from research in the pipeline.
CROWN GALL AND INCREASE SUSCEPTIBILITY TO WINTER INJURY
After the past couple of harsh winters vines have been collapsing in your once “healthy” and productive vineyard. What’s going on?
In some cases, brutal winter cold has simply damaged or killed a vine that was not suitable for its site. It is well known that the many varieties of Vitis vinifera that vintners prefer are simply less cold hardy than many of the French hybrid varieties. The crown gall bacterium, Agrobacterium vitis, can also play a large role by rendering infected vines incapable of properly repairing the cold damage to their trunks. The most obvious symptom of crown gall infection is gall formation at the base of infected vines. These tumor-like growths eventually choke out the vascular connection between roots and canopy, and the vine collapses (Figures 1 and 2).
How did vines get contaminated with the crown gall bacterium in the first place and why is it now causing problems?
There are many sources of the crown gall bacterium and probably many ways in which vines can acquire it. It is now known that the bacterium exists in populations of wild grapevines and can be found on plant surfaces in the vineyard. The most likely or common source, however, is through contaminated nursery stock. Since the bacterium can live systemically as an endophyte inside vines used for propagation material, cuttings from that material will carry the bacterium as well. The bacterium that causes crown gall can probably live inside vines without ever causing any disease, without causing the growth of tumors at the base of the trunk, without bringing about the collapse of vine trunks. Cuttings, produced from symptomless, contaminated mother vines, may be contaminated with the bacterium from “day one,” but may never develop crown gall. This is probably the case in California and other Mediterranean climates where many of the world’s wine grapes are grown.
So why is crown gall such a problem here in the Northeast, and not in California?
The crown gall bacterium shifts from benign coexistence, as an endophyte inside vines, into a tumor-inducing organism when there is damage or injury to grapevine vascular tissue. When injury occurs to the cambium, the bacterium attaches to plant cells at the wound site and literally inserts a copy of a self-replicating DNA strand (called a plasmid) into the plant cells (infection). The plasmid contains genes that code for hormone production that leads to the growth of tumors. These genetically modified grapevine cambium cells begin to grow tumor tissues with poorly organized vascular structure (that is, not capable of adequately conducting resources needed by the vine) at the wound site instead of organized vascular tissues. The injured trunk areas are never properly repaired by functional vascular tissue and as the tumor tissues grow, the trunk becomes more and more non-functional and eventually the vine collapses. And what is the most common cause of widespread grapevine trunk damage in the Northeast? Severe winter cold—which does not occur in most parts of California and similar warm, wine grape production climates.
All is not lost—tips on vineyard renovation
A collapsed vine with healthy roots will throw new shoots from the base of the plant, and these can be used to make new trunks and restore the vine to productive status. Here in the “Great White North” of Erie County, we renovate vines almost every year (Figure 3). Vines “laid low” by crown gall are often capable of being completely restored to productive life. Rather than ripping out your 7 or 10-year-old vineyard and replacing it, it can be more cost-effective to train up new trunks with the potential for a partial crop this year and a full crop in Year 2. An exception to this remedy is when trunks of grafted vines were not hilled with soil in the fall and the base of scions experienced the full force of the severe cold. This can completely kill the scion and leave growers with nothing but the rootstock. In this case, growers may have to start over with new vines, unless there is potential for field grafting of new scion wood. Also, when very young or newly planted vines develop crown gall, it is best to remove the plants, and replace them. The bacterium can be found in roots as well as trunks and can survive for long periods of time (years) in the soil, and it is important to remove all parts of infected plants.
All of us would love to be able to train up one original trunk and rely on that single trunk for every vine, every year. Unfortunately for many in the Northeast, that’s a pipe dream. Now that you know about crown gall in your vineyard, you can assume that more vines are contaminated than you previously thought. For example, we have a Chambourcin vineyard at the North East lab in which just about every vine is host to the crown gall bacterium. I had no idea this was the case until the winter of 2003-2004, when brutal cold caused nearly every vine trunk to explode with crown gall the following spring (Figures 4 and 5). Apparently, nearly every vine was contaminated with the bacterium and the vineyard collapsed! After discussing my conundrum with Dr. Tom Burr at Cornell University, an expert in crown gall biology/pathology, we spent the 2004 season training up new trunks for every vine, using only shoots that originated from below the galls. From 2005 on, the vineyard was enormously productive for almost ten years. Then came the polar vortex of January 2014, followed by the severe winter cold of February 2015, and with it more devastating bouts with crown gall.
Improving your odds that a winter cold event will not lead to complete loss
Growers of V. vinifera in Erie County, PA have pretty much resigned themselves to losses from winter cold and crown gall every few years, and they deal with it in a number of ways. The first way is by growing vines on multiple (2 to 3) trunks. The logic follows that if one or two trunks collapse from crown gall there may still be one trunk that produces a crop and provides some income until new trunks can be groomed to replace the galled/damaged ones. Trunks do not need to be replaced as a matter of regular maintenance, but rather when they become injured and/or diseased. The maintenance of more than one trunk can greatly improve your odds that a winter cold event will not lead to complete loss.
Growers of the hardier French hybrids generally suffer fewer economic down times from winter cold-induced crown gall than growers of V. vinifera. We cannot escape bouts of brutally cold winter weather, but we can (and should) plan for the worst and try to wisely match variety with site in order to minimize or eliminate losses to winter trunk damage and crown gall. Simply put, cultivars of V. vinifera and cold-sensitive hybrids should be planted only on the best sites in Pennsylvania—sites that ensure good cold air drainage during the worst bouts of winter weather. Where a vineyard is already established, vine management that maximizes vine cold hardiness (balanced timely nutrition, effective disease control, proper balance between growth and yield, good weed and water management) is absolutely essential for minimizing trunk damage and the onset of crown gall after a severe winter cold event.
For grafted vines, hilling soil around the graft union in late fall will protect the base of the scion and may ensure that scion bud wood will survive to throw shoots for replacement trunks the following spring. During the following spring, hilled soil should be removed from around the graft to prevent rooting of the scion, which would otherwise defeat the purpose of the rootstock. Although an added expense, this practice is commonplace in many wine growing regions of the Northeast. Farther south and especially in the mid-Atlantic region, many growers have been avoiding this management practice because it represents a substantial added expense, can contribute to erosion on steep sites, and can increase the odds that vines may become mechanically damaged. Unfortunately, severe cold during the past two winters caused heavy damage to the less favorable variety/site combinations even in parts of southern PA and the mid-Atlantic. Where grafts were not protected, the supply of scion buds that would have provided for new trunks was killed. In such cases, all but the rootstock dies and the vine must be replaced—a much more expensive operation than trunk renewal. So in these more southerly regions, the decision to hill or not, may be less clear. In southern PA, proper variety matched to the site along with multiple trunk maintenance may be sufficient for sustainability. However, on poorer sites that suffer more frequently from a severe winter cold event, annual hilling of grafts may be necessary or a grower may need to rethink his/her established variety/site combination. As in all matters of farming, growers must weigh the expense of a practice against the magnitude of the consequences for not doing so as well as the odds that he or she will get hit with another severe cold event. The prudent integration of these management practices will help to guarantee that farms can remain sustainable and profitable in the Northeast.
Research in the pipeline
Once contaminated, there is no practical way to rid a vine of the crown gall bacterium. The best long term solution rests with the production of crown gall-free planting stock so that growers can at least start with a clean vine/clean vineyard. To that end, through funding from the National Clean Plant Network, Dr. Tom Burr’s grape research program and others are devoted to the generation of mother vines free of crown gall that can be used to start clean sources of grapevine nursery material. The emphasis in this effort is the development, and ongoing refinement, of extremely sensitive tests used to detect the presence of the pathogen, in order to determine whether a vine that might be used for propagation is “clean” or contaminated. Clean vine material can then be confidently used to establish grapevine mother blocks that will serve as the foundation of nursery propagation stocks. In turn, the mother blocks and nursery stocks can be continuously monitored for the presence of the bacterium using these same tests. The latest research has indicated that plants free of the crown gall pathogen can be generated but they will need to be assayed periodically to ensure they remain clean. Remember, however, that the crown gall pathogen, once introduced into a vineyard through contaminated plants, can live in the soil for many years. Therefore, the availability of crown gall free planting stock is not going to end our encounters with this pathogen. Clean planting stock will reduce or help to eliminate the incidence of crown gall in new plantings, but the pathogen will likely always remain present and northeastern growers will still have to manage their vineyards with a view toward minimizing the incidence of crown gall.
Extension Support for the Upcoming Season:
- This blog post and others will continue to be made available at Wine and Grapes U. to assist growers with the latest information. We hope you find this useful for managing crown gall and we encourage feedback.
- You can sign up for the Penn State Extension V&E News listserv by email Denise Gardner at email@example.com. This will keep you current when we release crown gall related information.
- Bring your crown gall questions and concerns to the upcoming Mid-Atlantic Fruit and Vegetable Convention in Hershey PA (February 3, 2016); let’s discuss them.
- Bryan will discuss current disease updates at the 2016 PA Wine Marketing & Research Board Symposium at the Nittany Lion Inn (University Park, PA) on March 30, 2016.
- Participate in a series of webinars being organized by Tim Martinson, Cornell University, that will enable growers to tap into Tom Burr’s long standing research program on managing crown gall and what we have to look forward to in the future. Stay tuned for more details later this winter.
- Check out this recent presentation by Dr. Burr at this link.
Information used in composing this article was from personal communication with Dr. Tom Burr and:
Compendium of grape diseases, disorders, and pests. Second edition. 2015. Wayne F. Wilcox, Walter D. Gubler, and Jerry K. Uyemoto, eds. American Phytopathological Society Press. Pages 95-98.
Tim Martinson and Thomas Burr. How Close are We to Crown Gall-Free Nursery Stock? Appellation Cornell; Research Focus 2012-1. http://nationalcleanplantnetwork.org/files/144948.pdf
Wine Grape Production Guide for Eastern North America. 2008. Tony Wolf, ed. Natural Resource, Agriculture, and Engineering Service. Cornell Cooperative Extension.
By: Michela Centinari
Denise Gardner, Penn State Enology Extension Associate, and I visited the Lake Erie region, Northwest of Pennsylvania, on June 25 and 26, 2015. We first visited the Lake Erie Regional Grape Research and Extension center (LERGREC) and later met with wine grape growers and winemakers at the South Shore Wine Company. The second day I visited three vineyards with our hosts Andy Muza, the horticulture Extension agent in Erie County, Bryan Hed and Jody Timer, research technologists at Penn State that focus on plant pathology and entomology, respectively.
Regional visits are always a great opportunity to connect with growers and winemakers, discuss production issues, gather information on topics of interest for future educational workshops, and also to learn about grower/winery relationships.
The wine industry in the Northwest region of Pennsylvania is growing, both in size and reputation. Most of the growers I met are experienced juice grape growers transitioning to wine grapes. One of the growers pointed out that he chose, among his 200 acres of existing Concord vineyards, the very best location to plant and grow his Vitis vinifera wine grapes. He had a clear understanding of how site selection is critical to successful cultivation of V. vinifera varieties, and site selection is one of the key areas we discuss with growers upon entering the wine grape industry.
During the two-day visit, most of the discussion focused on winter injury sustained by V. vinifera, inter-specific hybrids and also V. labrusca/native grapevines (mostly Niagara) over the past two winters. The last two years provided record low temperatures in Erie area during the 2013-2014 and 2014-2015 winters, which have proved challenging for farmers (Figure 2). Variation in cold hardiness among grape genotypes, the impact of a diversified crop, and vineyard management practices to reduce/minimize future winter injury losses were discussed to great length.
Growers have been busy assessing the extent of winter injury, training suckers to be used as new trunks and cordons (Figure 3) and making re-planting decisions. One industry member pointed out that winter injury losses have forced growers to make strategic decisions in terms of variety selection. It was recommended that growers use this opportunity to substitute varieties that did not produce well with others that are more suited for the region and individual vineyard sites. In terms of crop diversification, the use of cold-hardy hybrid varieties released by the University of Minnesota may be a viable option for growers, but wineries should be consulted with regards to their interest in those varieties. Local market opportunities still need to be explored in Pennsylvania with regards to cold-hardy hybrid variety sales in the tasting room.
From a research prospective, these two consecutive severe cold winters provided a good opportunity to evaluate the cold hardiness of V. vinifera and inter-specific hybrid wine grape varieties at the variety evaluation planting established at LERGREC in 2008, as part of the NE-1020 multistate project. For more information about the NE-1020 trial, please refer to the article “NE-1020… What? The Top 5 industry benefits affiliated with the NE-1020 variety trial” by Denise Gardner.
During our visit we noticed, as expected, further damage to the cold tender V. vinifera varieties. Among the V. vinifera, Grüner Veltliner and Cabernet Franc are recovering better than Pinot Grigio and Pinot Noir, which are either dead to the ground or showing a weak recovery (Figure 4a, b). Very good bud survival was observed in Chancellor (Figure 4c) which produced about 3-4 clusters per shoot despite the extremely cold events recorded over the last two winters. Overall, Marquette, a cold-hardy hybrid variety released by the University of Minnesota, has shown encouraging results. In addition to the cold sensitive V. vinifera varieties, a poor survival was observed in the NY81.0315.17 (Riesling x Cayuga White) selection (NE-1020 vineyard) and in Traminette at several vineyards within the Erie region (Figure 4d).
Protecting vines and fruiting potential from winter injury
Growers had several questions about best practices to protect trunks and the fruiting potential of the vines against winter injury.
Hilling up the soil around the vines in the fall and taking out hills in the spring is the most common practice used by growers to protect the graft union and some of the scion tissues above the graft union against low winter temperatures. A few growers showed concerns about the wet conditions of the soil in the fall/spring which makes this operation difficult. Heavy, wet soil may be thrown up under the trellis in large clods which can result in imperfect burial of the vine trunk, and makes the take-out operation difficult . Other growers mentioned using straw, or hay, or plowing snow (if there is sufficient snowfall) around the vines to insulate vine tissues. In the spring, it is critical to remove the soil around the graft union to avoid scion rooting, which defeats the purpose of rootstock and may result in vine decline .
Burying sucker canes under the soil or straw in the fall is a method that can be used to avoid winter injury and insure the retention of a ‘marketable’ crop the following year. A comprehensive description of the ‘burial cane’ technique can be found in: Winter Injury to Grapevines and Methods of Protection, Extension bulletin available for $15. Growers in Ontario and in New York State used this technique to protect some fruiting potential of the vines. As a brief explanation, shoots are grown on wires near the ground during the growing season and they are covered with soil or straw in the fall (Figure 5). Another option is to place canes near the ground after the vines go dormant. “A wire on the ground that is tensioned either permanently or temporarily is very helpful to hold canes near the ground” . The buried canes need to be extracted from the soil as soon as the risk of low winter temperatures is past.
The cane burial technique and its effects on vine bud survival and production have been examined at Cornell University with the help of cooperating growers of V. vinifera varieties. For detailed information on the study please check Understanding and Preventing Freeze Damage in Vineyards: Workshop Proceedings (21-38) .
The main finding in this study was:
- Sucker canes buried in the fall do avoid the freeze injury suffered by the aerial canes of the vines.
- After canes were unburied, buds appeared to suffer injury unrelated to freezing. Buried canes showed more “blind nodes,” they were less productive (e., significantly fewer clusters) than canes not buried. Buried canes also showed a delayed bud and shoot emergence compared to aerial, not buried canes.
- Cane burial is not a cheap operation: “typical extra costs per acre for cane burial in the Finger Lakes Region is about $400 to $500. This includes labor needs for laying out ground wire, wrapping them with suckers, hilling, and buried cane extraction, pruning, and tying in spring” .
- “In normal or warm years, burying cane can result in production and economic losses. However, in extremely cold winters, buried canes allow a “half crop” and no dead vines” .
- Zabadal, TJ, Dami, IE, Goiffinet, MC, Martinson, TE, and Chien, ML (2007) Winter Injury to Grapevines and Methods of Protection. Extension Bulletin E2930. Michigan University Extension.
- Goffinet, MC (2007). Grapevine Cold Injury and Recovery After Tissue Damage and
Using Cane Burial to Avoid Winter Injury. In: Understanding and Preventing Freeze Damage in Vineyards, Workshop Proceedings. University of Missouri Extension. 21-38.