This research has attempted to determine if water stress and the unusual drought response
of Syrah may be responsible for the expression of the Syrah Disorder symptoms.
Additional research looked at the effect of crop load, foliar phosphorous nutrition, and
possible viruses. A first objective of the research was to determine if the disorder
symptoms could be correlated to water stress. Irrigation treatments at the primary Paso
Robles site did not correlate to the red leaf symptoms that began appearing in July;
however, this planting was later diagnosed with leaf roll virus. Irrigation treatments at a
San Miguel site did see greater red leaf symptoms in the driest treatment. Irrigation
treatments at the second Paso Robles site were inconclusive, due to all irrigation being
terminated because of water supply problems; however, strong leaf burn symptoms were
noted in this planting in areas that had received high amounts of gypsum, and tests
confirmed that salinity levels was about three times as high as non-symptomatic areas. At
the Creston site, no symptoms were seen. A second objective of this research was to
determine if measurable vine stress parameters could be associated with the onset and
expression of the disorder symptoms. Measurements of leaf water potential and stomatal
conductance, and automated measurements of trunk sap flow were made at the primary
Paso Robles site. As no correlation with the red leaf symptoms was seen with the
irrigation treatments at this site, it was not possible to determine what level of stress
could cause the disorder to appear. A third objective of this research was to determine if
Syrah in California exhibited the same type of unusual drought response as documented
in Europe. Automated measurements of trunk sap flow at two sites supported this
fundamental characteristic of Syrah.
Three additional objectives not outlined in the 2004 proposal included a determination of
the effect of crop load, foliar phosphorous nutrition, and the presence of viruses on the
disorder expression. The crop load effects were tested at a Santa Ynez vineyard. No
differences in red leaf symptoms were noted for vines at three levels of crop load. No
visual differences in symptom expression were noted for vines that received two foliar P
applications. Of the 20 symptomatic plantings tested for viruses, three were positive for a
leaf roll virus; other viruses were present in minor amounts and were likely not
responsible for the symptoms.
In summary, the 2004 work confirmed that California Syrah does indeed display a
different drought response mechanism as compared to Cabernet Sauvignon. The
expression of the Syrah Disorder symptoms were correlated to irrigation amounts at
some, but not all, sites, while high salinity conditions did lead to significant canopy
scorch in late August. It was not possible to correlate Syrah Disorder symptoms to
specific vine stress measurements. The presence of known viruses seemed to play a role
at several sites, while many symptomatic vines did not test positive for any viruses.
Reduced crop load and foliar P did not alleviate the symptoms where tested.