ICCVE Viticulture Advisory: January 2010
Many vineyards around the region this past weekend experienced
negative temperatures in the double digits (°F) while many
more experienced temperatures that approached that level. Temperatures
of these extremes have almost certainly caused vine injury, primarily
to buds, which are the least hardy tissue of the vine, but also
perhaps to wood in less hardy varieties such as some of the viniferas.
Before pruning vineyard blocks, particularly of the vinifera
and less hardy hybrid varieties, you should do a bud injury analysis
to ascertain the level of primary bud damage and determine if
any adjustments to normal pruning are necessary to compensate
for the level of bud loss. To conduct the analysis, use the following
procedure:
1) Wait 7 to 10 days after the cold event to begin sampling. Sampling
too soon after the freeze will not give an accurate estimate of
bud injury. This is because the damaged tissues need time to oxidize
and turn brown. Early next week, especially with warmer temperatures
coming in for the latter part of this week, should be a suitable
time to sample buds. Alternatively, you can collect the samples
and bring them indoors. They will have to be kept damp to prevent
the tissues from drying out, so wrap them in moist paper towels
and keep them at room temperature for 48 hours before dissecting
them.
2) Collect approximately 100-120 buds from the type of wood that
would normally be retained for fruiting spurs or canes. If the
vines are trained to a low cordon training system, this means collecting
about 50-60 two-node spurs from the basal area of canes that you
remove. For high-cordon systems, collect 25-30 four-node or 17-20
six-node cane basal sections to represent fruiting canes. Collecting
the buds from the mid-cane section or near the tip can also give
a false estimate of bud injury as these areas of the cane often
do not fully mature in the fall and are more prone to winter injury
and dieback.
3) Slice horizontally through the buds to reveal the primary and
secondary buds. Remember that each grapevine bud is actually 3
buds in one: the primary, the secondary, and the tertiary buds.
The primary bud is the bud that normally pushes out in spring and
develops into the fruiting shoot. It has the greatest fruiting
potential of the three buds and is the one about which we are primarily
concerned. In most Vitis vinifera (i.e. – Cabernet, Chardonnay,
Merlot, Syrah, Riesling, etc.) and V. labrusca (i.e. – Concord
, Catawba) varieties it is the only bud that produces an economical
yield: the secondary and tertiary buds are only somewhat or poorly
fruitful if they bear flowers and fruit at all. Many of the hybrid
varieties such as Chambourcin, Vidal, Seyval, and Chardonel have
somewhat- to very fruitful secondary and lesser buds; other hybrids
such as Vignoles have poorly- to somewhat-fruitful secondary and
lesser buds; while for some of the newer hybrids we do not yet
know the fruiting potential of the secondary and lesser buds. The
primary bud will located in the center of the compound bud; the
secondary bud will be on one side of the primary and the tertiary
bud will be on the opposite side.
4) Count the number of live and dead primary buds and calculate
the percentage of dead primary buds (bud mortality) to determine
if and how much the normal pruning needs to be altered to compensate
for bud loss.
Figure 1. In this photo all buds are alive.
Once the extent of bud mortality has been determined, the following
course of action is recommended:
- Prune as usual if primary bud
mortality is 0% to 15%.
- Alter the pruning severity if primary
bud mortality is 15% to 50%.
- Halt all pruning if primary bud
mortality is more than 50%.
Modification of pruning severity in the 15% to 50% ranges can
be achieved by instructing pruners to leave extra buds. An additional
number of buds corresponding to the number of primary buds injured
should be retained. A cosmetic pruning operation should be done
on vines that have greater than 50% mortality. This should be a
hedging-type operation and should maintain the vine in its trellis
space. Eliminate developing shoots that are on mechanically injured
canes, too low, too far out in the row, or located in adjacent
spaces.
When primary bud mortality exceeds 50%, careful management is
required. As soon as immature clusters appear after budburst, an
estimate of the crop potential should be obtained. If a normal
crop appears likely, most cultural practices should be continued
in a normal manner. However, if a crop reduction seems likely,
cultural practices should be altered, i.e. - nitrogen fertilization
and irrigation should be reduced or eliminated to prevent excessive
canopy growth and shading.
Figure 2. In this photo the primary bud is dead, but the secondary
and tertiary buds are alive. |
