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76 WINES&VINES October 2015 evaluated each cultivar for bud damage. At each node there is a compound bud; a complex of three shoot primordia. The primary bud is the largest meristematic tissue of the three and is in the middle of the compound bud. The secondary bud is located toward the base of the cane, between the primary bud and the petiole scar, while the ter- tiary bud (generally not fruitful) is apical to the primary bud. For most of the cultivars grown in the east- ern United States and Midwest, the primary bud possesses 70% to 75% of the cropping potential of the compound bud (primary + sec- ondary + tertiary). Knowing this simple bud morphology and mak- ing cross-sectional cuts through the bud with a sharp razor blade al- lows growers to identify the sur- vival status of each bud at the node. If the buds are alive and healthy, they should be green; a brown or black color, unfortu- nately, is an indication of mortality (see the images on page 74). Grower response to this circum- stance is dependent on knowing the number of buds healthy enough to offer potential for the season's growth and production. The num- ber of nodes retained per vine at dormant pruning depends on an assessment of the amount of bud damage sustained after the dor- mant season nears completion. This assessment is simple. Col- lect a sample of 40-50 node posi- tions on canes of desired quality from positions appropriate to be retained upon final pruning. If all buds are dead (dark in color), the chances of finding living buds is very unlikely, and there is no need to continue the assessment. Alter- natively, highly variable results (alive and damaged buds in sam- ples coming from the same cultivar and the same vineyard location) suggest that it's advisable to en- large your sample to 100 nodes to get a more accurate idea of the level of damage. If the vineyards are not uniform (different vine size, slope, soil), then it is better to keep samples from vines/areas separated to evaluate the potential impact of those variables. Secondary bud mortality is usu- ally similar to primary bud mortal- ity, yet some indication of the amount of secondary bud damage is important, especially when more than 60% of primary buds are dead. When the assessment of bud damage is complete, the next step is to adjust the dormant season pruning strategy accordingly. Gen- eral suggestions are provided in the table above. When primary bud mortality is greater than 70%, the pruning effort should be directed to reestablish the fruit-bearing zone of the vines and to balance the growth of the vines during the spring and summer to avoid exces- sive shoot vigor, which can lead to vine weakness and higher potential for winter injury. Damage to vines and recovery The past two winters resulted in vines that were severely damaged. Growers used every remaining liv- ing bud to cope with the damage, including retaining base buds close to pruning cuts that have the potential to produce shoots with needed leaf area. This physiologi- cal phenomenon dilutes the indi- vidual shoot vigor available from the large root systems of mature, bearing grapevines. Mature, es- tablished vines have large root systems containing large reserves of carbohydrates to fuel growth. Retaining multiple shoots creates a dilution effect that helps avoid excessive shoot vigor (e.g., bull canes), which are much less cold hardy. Excessively vigorous shoots tend to have long internodes with larger diameter, lighter periderm color, poor lignification and larger cell size that make them more sus- ceptible to future winter damage. Field assessment of the two con- secutive harsh winters indicated that poor choices of canes at prun- ing time after the first year's dam- age (winter and spring 2013-14) led to greater levels of damage in winter and spring 2014-15. The extreme cold during the past two winters killed cordons and trunks of several cultivars. Damaged cambium tissues were brown to black color (see D, E and F on page 74). Thus, when dam- age to permanent cordons and/or trunks was present, the pruning effort was directed at reestablish- ing vine structure. Renewal canes from the base of the trunks were the best options for replacing damaged bearing structure—es- pecially for Vitis vinifera cultivars. Severely injured but surviving trunks were generally replaced. In cold-climate locations, damaged trunks have proven to be highly sensitive to crown gall disease. The diseased portions need to be removed and replaced with new shoots of moderate vigor. Severely winter-damaged vines will often have shoots (suck- ers) arising from basal trunk posi- tions protected by snow cover. Sometimes, due to extensive dam- age, the suckers are the only re- source for leaf area for the vines in the spring. Several extension bulletins, supported by the anec- dotal experience of growers in cool- or cold-climate growing re- gions, suggest that the suckers should not be removed or even thinned to one or two per vine (the latter a common standard). The better option is to leave at least five or six suckers and to try to guide their growth to the trellis system. Other suckers, if present, are allowed to grow without train- ing on the ground to bolster pho- tosynthetic leaf area, resulting in additional carbon fixation during the summer. These additions help limit the production of excessively vigorous growth on the potential SUGGESTED PRUNING STRATEGIES FOR DIFFERENT LEVELS OF BUD MORTALITY Bud mortality Suggested strategy 10%-15% No need to adjust your winter pruning. 20%-50% Leave a higher number of buds (+20%-30%) at winter pruning (e.g. prune to four to five bud spurs rather than the standard two to three bud spurs and/or leave more spurs/canes per vine). 50%-60% Double the number of buds of your standard pruning strategy. More than 60% No dormant pruning, or, reestablish the bearing structure of the vine Typically, not all the trunks of a multi-trunked vine would be killed during a harsh winter. Damaged trunks are removed, and the trunks that survived—vinifera (left) and interspecific resistant cultivars (right)—are the bearing structure for the following spring. For cold-hardy cultivars or in viticultural regions or sites characterized by moderate winter minimum temperatures, two trunks are adequate. PAT MURAD, MSU GRAPEGROWING WINE EAST