Journal of Plant Pathology - Sipav.org

S1.36 Effects of prohexadione-calcium on grape cluster structure Journal of Plant Pathology (2011), 93 (1, Supplement), S1.33-S1.37
Fig. 5. B. cinerea disease severity in the four different variants
(bars=standard errors; n=100).
Fig. 6. Sugar concentration measured in KMW (Klosterneuburger
Mostwaage). Values are mean ± standard error
(n=100).
DISCUSSION
The field experiment showed that disease pressure
strongly influences results. The health of the grapes is a
primary goal of quality-oriented viticulture. B. cinerea is
difficult to control because it has a variety of modes of
attack, diverse hosts as inoculum sources, and it can
survive as mycelium or conidia. For these reasons, the
use of any single control measure is unlikely to succeed
(Williamson et al., 2007). B. cinerea infection is dependent
on some kind of damage to the berry skin to provide
a point of entry, for example parasite attack, hail damage
and strong water absorption which can lead to rapid
bursting of the berries or dislodged berries that are imprisoned
inside the grape cluster. The thick cuticle of
the grape berry usually provides a mechanical barrier
against B. cinerea infection (Ribéreau-Gayon et al.,
2006). Another reason for B. cinerea infection can be
grape compactness, which leads to burst berries. This
depends on the variety, on whether there is a high number
of remaining berries after blooming, but also on the
size of individual berries (Kast, 2007).
In 2008, prohexadione-calcium yielded less compact
grapes, hence preventing B. cinerea.attacks. Thus, it improved
quality and quantity of the yield without risk for
consumers or the environment. As mentioned, prohexadione-calcium
is a gibberellin biosynthesis inhibitor
whose mode of action differs from that of other gibberellin
biosynthesis inhibitors currently used in agriculture.
Many of these growth regulators, including the
quaternary ammonium compounds, substituted pyrimidines,
norbornenodiazetine derivates, and triazole derivates
(Graebe, 1987) function by interrupting the synthesis
of gibberellin early in the biosynthetic pathway,
specially at the synthesis of ent-kaurene. The use of another
plant growth regulator (gibberellic acid) leads to
the extension of the stems and blossom drop, which
can lead to yield reduction and should be avoided. Longitudinal
growth can be increased, although not without
limits, since a genetically caused border exists
(Spies and Hill, 2008). In this experiment, no changes
could be observed in length growth following the use of
prohexadione-calcium. A correlation between berry
weight and number of berries was found although the
results showed that the treatment prohexadione-calcium
resulted in many small berries with a diameter of 3
mm, which generally reduces compactness. It was confirmed
that the use of prohexadione-calcium leads tendentially
to more berries with a diameter around 8-10
mm as previously observed by Haas et al. (2009). However,
in the present study, no significant difference was
found in the induction of loose grape clusters and the
subsequent infestation of B. cinerea. Prohexadione-calcium
had minimal effects on pH or titratable acidity
and on sugar content of musts in 2009, in agreement
with Lo Giudice et al. (2004) who reported that an immediate
postbloom application had no impact on soluble
solids (Brix). In 2010, control bunches had a sugar
concentration of 13.5 “Klosterneuburger Mostwaage”
(KMW) whereas a higher sugar content was observed
in the treatment prohexadione-calcium with 14.3
KMW.
In conclusion, the data presented here show that the
result of the application of a plant growth regulator depends
on different environmental parameters, among
which the exact timing of application during flowering,
the rainfall and the pathogen’s infestation pressure.