Sudden Vine Collapse

Karl Lund, UCCE Madera, Merced & Mariposa Counties, from Vit Tips December 2019

Several areas in California are attempting to understand a new viticultural issue being called Sudden Vine Collapse. This problem was previously being called mysterious vine collapse, but as the problematic pathogens have possible been identified, it has been relabeled as Sudden Vine Collapse (SVC). The symptoms were first identified in the Lodi area, but have also been confirmed in San Luis Obispo and Santa Barbara Counties, with possible incidence in Monterey County as well. In SVC mature vines with growing canopies, along with developing clusters, will collapse and die in a short period of time, generally 2-6 weeks. Researchers from UC Davis have possibly identified a combination of viruses in combination with specific rootstocks as being the probable source of the collapse. The viruses identified in this complex include Grapevine Leafroll 3 in association with one or multiple other Grapevine Viruses (Vitiviruses). Rootstocks affected so far include Freedom, 3309C, and 101-14. There still appear to be additional factors that are left unaccounted for. Let us take a look at the symptoms that have been identified and what they may tell us about the progression of SVC.

A good starting point is to look at a much better understood and similar situation from walnuts trees called blackline. Blackline is a symptom in walnut trees caused by the Cherry leafroll virus. A grafted, virus free, walnut tree is planted in a new orchard. Once the tree is mature enough it begins to flower, and the infection cycle can start. Cherry leafroll virus is transmissible through pollen, so if any neighboring trees are infected with the virus, they can infect the new orchard during pollination. The virus doesn’t cause any immediate major issues, as the scion variety has little effect from the virus. It is when the virus makes it to the graft union that the real problem begins to arise. Many common rootstocks used in walnut production are hypersensitive to Cherry leafroll virus. This means that as the rootstock tissue in the graft unions becomes infected by the virus, the rootstock tissue kills itself. This prevents the virus from spreading into the rest of the root system.  However, as the dead tissue builds up in the graft union, it causes the graft union to fail. Once enough of the graft union has died, the scion variety is cutoff from the root system and will die itself. When the graft union is inspected postmortem, the dead tissue shows up as a black line.

Grapevines don’t have any viruses that cause as visually dramatic a reaction as blackline. However, virus induced graft union collapse are known in grapevines. This became especially clear in the replant after the failure of AxR#1 in California. AxR#1 (and St. George) is tolerant of viruses in grafted scions, and in some cases can even suppress viral symptoms. When AxR#1 failed to phylloxera different rootstocks were needed to replant the dying vineyards. Wood from the dying vineyards were used to graft onto these different rootstocks, which is when the next problem occurred. The graft union of the new plants failed. During the lifetimes of these vineyards they had acquired viral infections, and when grafted to some of the different rootstocks these infections caused the graft unions to fail to form. The two rootstocks most affected by this problem were Freedom and 3309C, which are both implicated in this current round of collapse. While Freedom and 3309C have both shown issues with virus induced graft union collapse they are far from unique in this characteristic. Many rootstocks can succumb to graft union collapse to the correct (or incorrect in this case) virus or combination of viruses.

A virus causing graft union collapse in plants, and grapevines specifically, isn’t that unusual of a problem (and another reason to use virus tested plant material when starting a new vineyard).  In the case of SVC, testing symptomatic vines has led researchers to suspect that this is another case of viral-induced rootstock collapse. The graft union is the first piece of evidence that points in this direction. While not seen in all cases, many failing and dead vines do show visual symptoms at the graft union. In extreme cases this can be more in line with blackline with large sections of discolored and dead wood within the graft union. In more subtle cases the phloem tissue of the scion variety can be seen trying to grow over the rootstock, creating a bulged graft union.

More direct evidence can be seen when testing for starch (sugar) storage in trunks below the graft union. The trunk below the graft union of collapsing vines have a limited supply of starch, while healthy vines see a large supply of starch.  This is piece of evidence really informs a lot about the possible cycle of SVC. A lack of starch below the graft union in collapsing vines could indicate that the sugar being produced in the canopy is not able to make it past the graft union.  In a sense the graft union is becoming a permanent girdle, starving the root system of carbohydrates. The virus is causing a disconnect between the phloem tissue of the scion variety and the rootstock. We can see this in the unusual graft union growth of some infected plants, or the discoloration and death of tissue within the graft union of other plants. More clearly it can be seen by the lack of starch in the trunk below the graft union indicating that sugars made in the canopy aren’t being transported to the root system.

From here the symptoms and evidence lead quickly towards the collapse of the vine.  The root systems of collapsing vines have been found to be infected with pathogens that are normally unable to infect healthy vines. More importantly these root systems show very little to no growth of new feeder roots. Feeder roots are the primary site of nutrient and water uptake, and need to be replenished throughout the growing season. As feeder root growth diminishes, so will the plants ability to uptake nutrients and water. The trunk also sees an increase in trunk disease infections, as well as the speed at which these infections advance. Lastly the canopy with reduced root growth and a compromised trunk is left unsupported.  In some cases, the canopy becomes stunted, much like in advanced cases of trunk disease. In most cases the canopy continues to grow normally right up to the point that the root system and trunk can no longer support the water and nutrient needs of the canopy. At this point SVC acts suddenly with a complete collapse of the canopy.

The two final pieces of the puzzle is what evidence has been isolated by researchers to suspect the combination of viruses and rootstocks being put forward. The symptoms were first identified in vines grafted onto Freedom, and all tested that has been described to this point has also been with vines grafted onto Freedom. This makes the case for Freedom being involved in SVC is strong. The remaining rootstocks have been seen to follow the same progression as Freedom and have thus been grouped together. This makes their involvement in SVC more associative, but still likely. The Lodi Wine Commission has been following the development of SVC and has other rootstocks they suspect may be involved.  These additional rootstocks will need confirmation moving forward.

The identification of the problematic viruses has progressed through testing of collapsing and healthy vines all within the same vineyard. All the collapsing vines showed a combination of Grapevine Leafroll 3 and a Vitiviruses (most commonly Vitivirus A and/or F). The healthy vines did not show this combination of viruses. This satisfies the first of Koch’s postulates for the identification of an agent responsible for a disease. The agent (in this case a combination of viruses) must always be found in association with the disease.  To confirm that this combination of viruses is responsible for SVC, the viruses will need to be inoculated into healthy plants to recreate the symptoms. Confirmation that SVC is caused by a combination of Grapevine Leafroll 3 and one of the Vitiviruses, in conjunction with a virus sensitive rootstock, will take some time. However, there is strong evidence that this combination is responsible for SVC seen in San Joaquin, Monterey, San Luis Obispo, and Santa Barbara counties.

The San Joaquin Valley also has all the pieces present for SVC to affect us.  Up to the writing of this article there have been no incidence matching SVC reported in the San Joaquin Valley south of Stanislaus County. If you have, or have had vines collapse suddenly in the past few growing seasons, please contact your local viticulture advisor. To fully understand this new issue, we need to see as many possible vineyards that have been affected by it. The name of the issue may have been changed from Mysterious to Sudden, but there are still many mysteries left to solve. The UC system as a whole is looking for more examples of this new issue in hopes of gaining more knowledge on the subject.

Vit Tips Staff

Contributing Authors

Gabriel Torres, UCCE Viticulture Advisor Tulare & Kings Counties 559-684-3316, gabtorres@ucanr.edu

George Zhuang, UCCE Viticulture Advisor Fresno County 559-241-7515, gzhuang@ucanr.edu

Karl Lund, UCCE Viticulture Advisor Madera, Merced & Mariposa Counties 559-675-7879 ext. 7205, ktlund@ucanr.edu

Editors

Matthew Fidelibus, UC Davis Viticultural Extension Specialist Kearney AG Center 559-646-6510, mwfidelibus@ucdavis.edu

Karl Lund, UCCE Viticulture Advisor Madera, Merced & Mariposa Counties 559-675-7879 ext. 7205, ktlund@ucanr.edu

Upcoming Meetings

 San Joaquin Valley Grape Symposium

Wednesday January 8, 8:00 – 1:00

C.P.D.E.S. Hall, Easton CA

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 Sunpreme Mechanical Pruning Field Demo

Wednesday, January 22, 8 AM

University of California, Kearney Research and Extension Center 9240 S Riverbend Ave, Parlier, CA 93648

Mechanical pruning field demo on Sunpreme raisin variety with high-wire and quadrilateral trellis