12 June 2003
I've had five conversations from different growers this week about poor
fruit set in their vineyards. The calls came from SE Virginia, SW
Virginia, and northern Virginia. We saw similar problems last week with
some varieties at our variety planting at Blackstone, in the Eastern
Piedmont, particularly with Traminette. The problem appears either as poor
fruit set, or deterioration of entire portions of the rachis (the cluster
stem). Some of our Traminette vines at Blackstone only retained the major
branches of the rachis, with no berry set. Sometimes the more distal
clusters of a shoot exhibit normal set, and yields may not be greatly affected.
What's going on? The shedding of a portion of the rachis (including the
entire cluster) is likely related to the sustained cloudy weather that
we've all experienced for the last 40 days or so. Vines in the pre-bloom
period are at a low point in terms of root carbohydrate reserves. The
cloudy weather aggravates the situation as nitrate-reductase activity can
be reduced, reducing the conversion of mineral nitrogen into organic forms,
and carbohydrate production suffers as leaves are not operating at optimal
efficiency. You might notice a greater problem with poor fruit set on
vines that are looking a little chlorotic (yellowish) now. When the
carbohydrate supplies are insufficient, it appears that flower/fruit
clusters are one of the first organs to suffer, and we see poor set and/or
shedding of portions of the cluster (typically the basal cluster is most
affected, but all clusters of the shoot may experience the
deterioration). From our earlier work with bud necrosis, we can also
predict a high incidence of bud necrosis (BN) developing this year on
BN-susceptible varieties such as Riesling, Viognier, and Syrah.
Rain itself at bloom may also reduce fruit set for physical reasons, but I
believe that the primary problem is more related to the prolonged, cloudy
weather. An analogous situation is occurring in apples this season, with
above-average self-thinning ("June drop") occurring with some of the apple
varieties - and greater than expected thinning occurring where chemical
thinning aides have been used.
The problem is probably much more complex than just a carbohydrate
deprivation -- hormones (cytokinins) produced in the roots are also
important for fruit set. If vines suffer impaired root activity (as with
saturated soils), the cytokinin supply to clusters can be expected to be
reduced, leading to reduced set. Also, if shoots are rapidly
extending/growing, there may be further competition between sources of
carbohydrates and "sinks" for carbohydrates, including the clusters.
A further contributing factor can be stress imposed on the vines last
season. If vines went into the 2002-2003 winter in a "compromised" state
(overcropped, diseased foliage, early frost, etc.), that might well
aggravate the problem that you're seeing this spring.
You might be seeing some botrytis in the senescent clusters, and this
should be monitored and sprayed for if necessary, but still, I think the
primary problem is one of poor weather. Unfortunately, except for praying
for sun, there's not a lot you can do at this stage. The grower that I
spoke with in southwest VA thought about hedging shoots severely and
forcing second order shoots to develop which, for some varieties can be
fruitful. It sounds severe, and I can not give a wholesale recommendation
to try this, but it might work with early-maturing varieties such as
Malvasia or Muscat.
This may explain what you're seeing this spring - I realize it does not do
much to solve the problem. Again, think 'sunny' and 'dry' thoughts.
Dr. Tony K. Wolf
Professor of Viticulture
AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Rd.
Winchester VA 22602 USA
TEL: 540-869-2560, extn 20
FAX: 540-869-0862
http://arecs.vaes.vt.edu/arec.cfm?webname=winchester§ion=about_us&PID=vitis