by Sanne Kure-Jensen
Vine balance is an ideal blend of fruit and vine growth which determines grape yield and fruit quality at harvest. Researchers continue to search for the best vine pruning and canopy reduction practices to maximize crop load and quality. Paolo Sabbatini, associate professor of horticulture at Michigan State University, offered a presentation called “Vine Balance — What Is It & How Do We Achieve It” at the New England Vegetable & Fruit Growers Conference. Sabbatini focuses his research on grape physiology in Michigan’s cool climate.
Pruning and crop thinning can maximize crops and vine balance. Sabbatini said the best return on investment is winter pruning. He recommended vineyard managers strive for 15 viable shoots per meter, per row for ideal leaf density, shade and crop load. Unbalanced vines may have too many or too few shoots per vine or per linear cordon. Too many shoots can lead to internal shading effects and increased disease and pest pressure, as well as reducing grape quantity and quality. Over-zealous pruning can also reduce yields by reducing the number of fertile shoots and clusters per vine. This can potentially reduce the functional leaf area that can photosynthesize the fruit and vines.
Vineyard managers prune vines “to encourage a consistent annual yield for long tern sustainability,” said Sabbatini. Managers help the vine to mature the fruit by improving the sink and source ratio: fruit demand (sink) for carbohydrate coming from healthy leaf canopy (source).
According to Sabbatini, growers often prune too severely causing a double push of primary and secondary buds as well as dormant meristem buds. This often leads to a heavy canopy, which will shade too many buds. This will cause reduced bud break and shoot fertility for the following year, as well as lower average cluster and berry weight. Managers then have to bear the additional cost of shoot and cluster thinning during the growing season.
Vineyard Managers use a standard pruning ratio based on the number of buds and pounds of material removed. The first number in the series represents retained buds when trimming the first pound of 1-year-old branches. The second number represents the number of buds per additional pound of 1-year-old trimmings. After 4 pounds of trimmings, stop retaining buds.
• American grape varieties’ ideal pruning ratio is 30 + 10. They do not typically need cluster thinning.
• French hybrids’ ideal pruning ratios are 20 + 10 or 15 +15. They may or may not need cluster thinning.
• New hybrids’ ideal pruning ratios are 20 + 20. They may need cluster thinning.
• Vinifera grapes’ ideal pruning ratios are 20 + 20. They typically need cluster thinning.
Retain vine branches with reasonable vigor. Look for a 1/4-inch diameter at the fifth or sixth node on the cordon or from the trunk. Remove thin canes with short internodes or unusually thick canes with long internodes.
Sabbatini recommended a second canopy management pass to achieve proper vine balance before or at veraison, reducing the shoot density or thinning the crop if necessary. Late season or pre-harvest canopy management is not typically cost effective and will have only a small effect on fruit quantity and quality.
The Ravaz Index compares fruit to pruning weights and identifies how balanced a vine was during the previous season. Calculate this index by dividing the total yield per vine by its pruning weight the following winter. Balanced vines score 5 – 7. Aromatic whites may score 8 – 10. In cool climates, reds may score 4 – 6. Hot, sunny years will yield higher scores. More canes will be needed in cooler seasons or climates. Shoot tipping at bloom time improves fruit set as much as 25 percent and can lower the Ravaz index to 4 – 6. Managers use this index to determine how much to leave on vines during winter pruning.
Leaves and Canopy
Sabbatini recommended 12 – 14 leaves per cluster or 10 – 14 square centimeters of leaves per grape cluster. At this canopy level, most grape vines will yield 5 – 8 pound of fruit per pound of prunings. Too many leaves cause too much shade and lower Brix values. Cool temperatures late in the season can reduce color and reduce Brix.
Contact Paolo Sabbatini, Michigan State University via email at firstname.lastname@example.org or call 517-355-5191 ext 302.
by Sanne Kure-Jensen