Syrah has become a popular wine grape variety in California in the past decade. Currently, there are approximately 19,000 acres planted to the variety in the state. Bearing acreage has increased dramatically from 1996 when there were only 931 acres in that category.
Unfortunately, the variety has been beset with various problems, and researchers haven't quite figured out what is causing them. Some of the symptoms appear similar to problems with “Syrah decline” experienced in other countries such as France. To address those issues that are similar to California, UC researchers organized an international Syrah Vine Health Symposium at Davis to explore current knowledge about the problems that appear to be specific to the Syrah (also know as “Shiraz”) variety.
Mark Battany, San Luis Opispo and Santa Barbara County advisor, started the conference with a summary and general categorization of various Syrah disorders that have been observed in California production.
“Category one is what I call leafroll virus symptoms,” Battany says. “The major complaint here is that the fruit quality is very bad. It has very poor color. The vines however, grow and survive just fine. We don't have any complaints of vines dying.”
Syrah exhibiting leafroll symptoms fall into one of two further defined designations. They are either identifiable in the laboratory as a recognized leafroll virus or fall into another unknown designation where symptoms are identical to leafroll, but are presently not distinguishable in a laboratory as a leafroll virus.
Another category of Syrah disorders in the U.S. is leaf burn. It occurs in the hotter, drier inland areas of California, according to Battany. “I don't see this in the coastal areas,” he says. “It occurs late in the season, and it is very inconsistent from season to season. Some think it is related to water stress. I don't know if it's at all related to leafroll. I don't necessarily see reddening in these vineyards, but if you look at some of the older literature there are descriptions of browning.”
When leaf burn occurs, the leaves fall off, the fruit is often damaged by sunburn and doesn't ripen. “This tends to be a very transient problem,” Battany says. “It may happen in a vineyard one year, but not the next, so it is not as reliable as some of the other symptoms.”
Finally, Syrah decline in California as a category is probably a little more similar to what is seen in France as opposed to the other categories, according to Battany.
“In my area, vine decline is less prevalent than leafroll symptoms,” Battany says. “The pattern here is that after several years of good growth we will start to see symptoms in vines that are 3-9 years old. The main complaint here is the vines are dying off for no apparent reason. However, the fruit quality is very good, or in some cases, stupendous. A lot of gold medals are being won from dying vines. Unfortunately, that just adds an extra level of frustration.”
Syrah decline is a well-documented problem in France where growers and researchers first observed the symptoms in Mediterranean vineyards in 1993 in Languedoc. The Syrah variety grew rapidly in popularity among wine growers in France in the 1970s due to its potential to produce high quality red wines with intense color and favorable balance.
After problems were noted in Languedoc, the symptoms of Syrah decline then began to be noticed in Provence followed by the northern Rhone Valley. Today it is a very pervasive problem in France that has yet to be pinpointed to a specific causal agent despite extensive efforts on behalf of the industry. In the past, Syrah decline in France has been mostly observed on vines that range in age from 6-10 years. However, in recent years the symptoms have been observed on vines as young as 4-5 years.
Anne-Sophie Renault-Spilmont, project manager with the Insitut Francais de la Vigne et du Vin (ENTAV, ITV France) gave an overview of Syrah decline in France, telling attendees that despite intensive research on the problem there is still much to be learned. Syrah decline is particularly concerning in France where Syrah is the fifth most planted variety, grown on over 160,000 acres (67,500 hectares) and is the second largest grafted variety.
Observations of Syrah decline in France have been characterized in two major categories: in the canopy and at the graft union. In the canopy, symptoms begin appearing early in the season with leaf yellowing, followed later in the summer by a reddening of all leaves.
At the graft union level, swelling and cracking occur associated with an abnormal enlargement and deep, parallel grooves, according to Spilmont. The wood hardens and necrotic zones develop.
“Sometimes the scion declines and then dies, but the rootstock may stay alive,” she says. “The most characteristic symptom of Syrah decline is the development of huge cracked graft unions, followed sometimes by leaf reddening and often by death.”
Histological studies of the graft union have shown progressively smaller amounts of live tissue between cracking vines and reddening vines and a disruption in the cambial zone.
“At first, it was thought that the problem was the result of new grafting techniques,” Spilmont says. “In particular, the use of a new type of graft associated with the application of hormonal substances was in question. However, trials have shown that the type of grafting is not involved in Syrah decline.”
French researchers have also conducted trials to evaluate various rootstocks and clones. The possibility of transmissible agents was looked at, but has not been proven — at least not yet — to be a source of the problem. “If a pathogen is involved in Syrah decline, it may be unable to induce the same symptoms on other varieties,” Spilmont says.
No phytopathogenic bacteria which cause problems such as crown gall, bacteria blight or Pierce's disease were found in the samples analyzed. Researchers also looked at fungi, phytoplasmas and viruses. That work, in turn, led to efforts to identify certain characteristics of clones with genetic markers. That research is ongoing.
In California, the challenge in defining and explaining various Syrah disorders is further complicated in that various symptoms may occur singly or concurrently. Some maybe be sporadic from one year to the next. Some may be consistent within a vineyard or the same disorder may be sporadic throughout the vineyard.
On the Central Coast at least, leafroll appears to be the primary culprit under the general category of Syrah disorders. In one sampling project conducted in 2004-2005, Battany found that leafroll virus was present in about one-third of the samples tested. Most of it was identifiable as leafroll, but not all.
“Leafroll virus symptoms start showing up in late July to early August, and then we start seeing red leafs appearing,” Battany says. “Fruit quality is very diminished, but the vines survive. They actually grow very well. We tend to see increasing red leaf symptoms with increasing stress, so the drier the vines are or the heavier the crop load, the more we see symptoms. However, it is interesting that some of the vines we tested that look identical to textbook leafroll don't test positive for it. That seems to indicate there is another strain of leafroll that we're not detecting.”
UC researchers have been actively searching for reasons why the symptoms are sometimes worse than others and what may trigger the problem. Interaction with management practices has been studied as typical symptoms are often confused with irrigation or nutrition deficiencies. Color is less than ideal in affected vines. Often times the affected vines will have a higher pH and higher potassium levels.
“We've been looking at foliar phosphorus and potassium,” Battany says. “There was some discussion that maybe those nutrients were deficient, but I haven't been able to show that. Other suggestions have been that maybe we were just trying to put too much crop on the vines. I haven't been able to affect the symptoms even if I drop all of the crop off the vine. They still show the same symptoms.”
Often the puzzle just gets more puzzling, according to Battany. In one block affected with leafroll, extra drip emitters were installed in a vineyard interplanted with new vines and the symptoms disappeared temporarily in the vineyard. “But fruit quality was still horrible,” Battany says.
There is also some debate as to whether or not the problem is variety specific even though Syrah seems to be the most heavily impacted. In one block of affected Syrah vines that Battany was sampling, there are now two rows of Cabernet adjacent to it that are starting to show very similar symptoms.
“I don't know if it was just a coincidence for some other reason or not,” Battany says. “We'll be trying to follow up on that for a couple of more years.”
No one is quite sure at this point what is causing the various Syrah disorders that have been observed in California. No one is even sure how closely related they are to what is seen in France. There are many theories, but the scope of the symptoms as well as the sporadic nature of the problem in many cases is making it difficult to pinpoint exact causes. Some think it is possibly weather-related or at least exacerbated by drought.
“We had six years of relatively dry years that coincided with when a lot of this Syrah came into production,” Battany says. “I think that was probably a factor. Also there was a lot of interest in producing high quality wines. We saw practices such as deficit irrigation, even fairly severe deficit irrigation, to try to reach a higher quality plateau. It may have gone a little too far.”