Awareness has been growing in recent years and even more in recent months about a disorder that largely has been associated with Syrah. Originally seen mostly in Central Coast Syrah/Shiraz vineyards, the disorder has become problematic in North Coast vineyards as well. Yes, that's right. As long as the problem is thought to be restricted to a small growing region, then it's something that they need to deal with. But now that it's affecting our precious vines here in the North Coast, it's suddenly an industry-threatening scourge. Go figure. And I admit that I'm as guilty as anyone else.
The Syrah Vine Health Symposium was held on Nov. 6, 2007 at the University of California, Davis. The symposium was put together by Rhonda Smith and Mark Battany, farm advisors from Sonoma and San Luis Obispo/Santa Barbara counties, respectively. They assembled a remarkable panel that was diversified and international. And while there were plenty of differing opinions offered about what is causing the disorder, there were enough commonalities that at least a good faith guess can probably be offered as to what lies at the crux of the problem.
What Does the Disorder Look Like?
Well, it looks like many other things, including the ever-present leafroll virus (more on that in another column). Leaves turn red in the latter part of the season. It affects the whole vine, not just a few shoots or half a cordon--the leaves on the entire vine turn color and, eventually, fall off. Of course, the fruit cannot remain viable without leaves; so if the symptoms occur early enough (which they usually do), the crop is lost. A key symptom of this disorder is that the trunk swells right above the graft union, and the swelling is accompanied by grooving, pitting and splitting of the trunk above the union (rootstock wood is smooth). The grooving and pitting occur on the scion, not on the rootstock. Necrotic lesions form in the grooves and can merge, eventually girdling the trunk of the vine. The vine usually dies shortly thereafter.
The disorder's leaf symptoms are actually an expression of trunk girdling, not of the disease's (as it probably is a disease) influence on the foliage. Because the symptoms mimic leafroll to some extent, it has been confused as such. But leafroll does not exhibit the trunk symptoms that this disorder does. Besides leafroll, rodent damage (e.g., vole, gopher) may be confused with this problem. In fact, rodent symptoms would be the first thing to look for, in my opinion. Got a red vine? Are there gopher mounds? Is the trunk girdled at the base (vole)? If not, then check for swelling above the graft union. Then, strip off some bark and look for grooving. Take cross sections of the trunk if you dare. The swelling and grooving are the telltale signs of this disorder.
But What is This Disorder?
There seems to be quite a bit of confusion about what this disorder really is. In fact, there is a disagreement about whether it is a "disorder" or a "decline." With no disrespect intended to anyone, I have to ask if it really matters. The French researchers that were here to attend the symposium said that the California symptoms differed from the ones they have seen in the Rhône and Languedoc. But there seems to be too much in common to think that they might be different disorders. What seems clear to me is that it is probably caused by a virus disease, and the virus is a Rupestris Stem Pitting Associated Virus (RSPaV). Anne-Sophie Renault-Spilmont, along with her group at ENTAV-ITV France, conducted an exhaustive study of 38 vineyards, testing the vineyards over a five-year period. They ranked subject vines as to the severity of the symptoms, with trunk cracking and no leaves being the least likely to cause vine death, and trunk cracking accompanied by red leaves to be the most likely to cause vine death. Vines with red leaves and cracked trunks had up to a 26 percent chance of dying. Environmental stresses and/or water stress could increase the likelihood of vine death.
The group tested samples of 22 different Syrah clones for various virus strains and found that 97 percent of them were infected with RSPaV strains. Because they are plant pathologists, they cannot state that RSPaV is the cause of the Syrah disorder. But in this humble general viticulturist's opinion, it seems awfully coincidental if a causal connection is not eventually found.
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Whether a "disorder" or a "decline," it seems clear that it is probably caused by a virus disease, and the virus is a Rupestris Stem Pitting Associated Virus. |
Clones, Rootstock and the Grafted Vine
The ENTAV group found that, unlike Cabernet Sauvignon and Grenache grafted on the same 140Ru rootstock, the cambial junction between scion and rootstock was less efficient in Syrah. They tried two different bench grafting methods as well as a grafting hormone, but found that there was no effect on vine mortality. However, they did find that field grafted vines had a better rate of survival than bench grafted vines (2 percent mortality versus 9.5 percent mortality, respectively).
Symptom expression had something to do with what rootstock the Syrah was grafted upon. The worst offenders were 99R and 110R. Least symptomatic were 5BB, Riparia (Gloire), 1103P and 101-14. It would have been nice if all of the non-symptomatic rootstocks did not have V. rupestris in their parentage, but indeed 1103P and 101-14 do. Nevertheless, my colleague Daniel Roberts (Integrated Winegrowing LLC) claims that they have no problems with Syrah disorder when grafted to 420A rootstock (V.berlandieri x V.riparia).
The ENTAV group found clonal differences in symptom expression:
Most symptomatic: 73, 99, 301, 381, 382, 383
Intermediate or variably symptomatic: 100, 174*, 300, 525, 585, 877**
Least symptomatic: 470, 471, 524, 747
* Clone 174 was highly variable and was highly symptomatic in some tests.
**Clone 877 has been planted throughout Paso Robles and has been highly impacted by Syrah decline.
So, there are clear guidelines about which clones to avoid, at least within the ENTAV selections. They suggest that clone susceptibility is more important than rootstock effect. On the other hand, if my colleagues' experiences are representative, then even susceptible clones grafted on a non-symptom-producing rootstock might allow one to get away with it.
It's not the virus that causes the problems; it's the response to the virus that is the downfall of the vine. Note that in the study mentioned above, where 97 percent of materials tested positive for RSPaV, many of the materials were in the non-symptomatic category. Therefore, there seems to be clonal sensitivity to the purported virus at a genetic level, and clones may be selected based on their genetic makeup.
The ENTAV group found a microsatellite (DNA) marker that was tested to be used to screen susceptible from non-susceptible clones. They found a very good, but not perfect, segregation of symptomatic versus non-symptomatic. But in their testing, they were able to identify one "profile" for which all of the samples were "sensitive." That means, potentially, that if a clone has this profile, it might be rejected on that basis. Their research may have provided an excellent basis by which new selections may be screened for their susceptibility to this "disease."
Other Viral Studies
Dariusz Gosczyninski, a virologist from the Plant Protection Research Institute in South Africa, also said that there are clonal differences in the symptomatic and non-symptomatic vines and that the French clones were the ones who expressed the disease. Like the French team, his group found that a RSPaV was found everywhere, but that it was a different variant than the French had found. They had also found an apple stem pitting virus present in their clone 99B, which was the clone most commonly afflicted by Syrah decline.
Adib Rowhani, plant pathologist at UC Davis, corroborated findings of RSPaV in Syrah plant tissue. However, like most of the others, he remains reticent to confirm that the disorder is actually caused by that class of virus. That is because they have not observed a correlation between virus-positive samples and expression of symptoms in their test plants. He, like others, including Jerry Uyemoto (plant pathologist with the USDA-ARS) cautioned against getting knee-jerk reactive about any red Syrah vines seen in vineyards. They stressed several other reasons why grapevines can turn red all of a sudden, including those I've already mentioned along with bad grafting, cold damage, tractor damage and herbicides.
Wrapping it Up
Despite the hesitation of the scientists to declare the Syrah decline/disorder solved, it seems to me that there is too much commonality among the findings to prevent me from reaching a non-scientific, perhaps tentative, solution. I feel that the circumstantial, not strictly scientific, evidence associating the stem pitting virus with the Syrah variety and the apparent clonally-differential sensitivity to the virus supports at least a working hypothesis about the underlying viral cause of this problem.
While this disorder has been largely associated with Syrah (hence the name), it can affect other varieties as well. I have seen many vineyards with this symptom, and not all have been Syrah vineyards: Pinot Noir and Petite Sirah, for example. Besides those dying vines that have clearly been due to gophers, voles, tractor, blight, etc., there are times where a red vine in late summer has had me scratching my head. When the grower tells me that he sent it in for virus testing but it came up negative, it would be easy to check off that box and move to the next possibility.
But it is important to understand that, even with the most advanced DNA-based detection techniques, the methods detect very specific virus strains, based on specific DNA sequences and that new strains are being detected all the time, probably being created by natural mutation. So, it is highly probable that a pathogenic virus strain cannot be detected at any given time if it has not already been identified and its pathogenicity proven. And until it is confirmed to cause disease, it will not become part of a detection panel. No one is at fault here--it is just part of a learning process. And once this one is solved, another one is waiting around the corner. wbm
