I Have Galls in my Vineyard: Should I Call my Nursery?
Grapes 101 is a series of brief articles highlighting the fundamentals of cool climate grape and wine production.
By Tim Martinson and Tom Burr
After two back-to-back winters with extreme winter low temperatures, many grape growers will see trunk injury affecting the phloem, xylem, and cambium layers just under the bark tissue. As the vine starts to repair and regenerate these injured tissues, growers will inevitably see more galls forming in their vineyards, due to the presence of Agrobacterium vitis, the bacterium that causes the disease – and environmental conditions that trigger formation of crown galls. Are nurseries to blame for spreading this bacterium around? If so, why haven’t they eliminated it yet?
What to expect: In the spring, vines reconnect their vascular system from shoot tips downward through the trunks, reactivating the vascular cambium and generating new xylem and phloem tissue. Expect to see galls forming from bloom through mid-summer.
How Agrobacterium vitis forms galls. The A. vitis bacterium can survive in xylem tissue and grapevine roots in the soil for years without causing galls to form. But the gall-forming process begins when the bacterium transfers an element of its DNA into the plant. This occurs generally in actively dividing cells at wound sites, where callus tissue is forming to repair damaged tissue. The DNA transferred to these callus cells from the bacterium gets expressed and causes the tissue to form galls instead of forming organized conductive tissues (e.g. Phloem, cambium, and xylem tissues) to repair the injury.
Crown gall and planting material: A. vitis can remain present (but hidden) in grapevines for many years without galls being formed. Since gall-formation is triggered mainly in damaged tissue in response to cold injury or in graft unions (where tissue is intentionally damaged and callus tissue forms at the union to bond the rootstock and scion together), A. vitis is hard to detect and eliminate. Grown in moderate climates, vines with A. vitis may never express crown gall symptoms. It is most prudent for growers to assume that A. vitis is present in at least some nursery stock.
Crown gall elimination: Is it possible? A concerted effort has been made to eliminate the crown gall bacterium through tissue culture from shoot tips, because A. vitis was thought to be absent from meristem tissue at the shoot tip. But a new, more sensitive testing method called magnetic capture hybridization (1000 times more sensitive in detecting the bacterium) has questioned that assumption. In 2013, the Burr laboratory found that A. vitis was detectible in about 20% of the 48 shoot tips and meristems they tested. In follow-up tests in 2014, none of the 49 shoot tips collected from infected vines had detectable A. vitis. So while elimination of crown gall bacteria is possible with shoot tip culture, it is not as reliable a procedure for ensuring crown-gall free vines as was previously thought.
Crown gall in the environment. With this new, more sensitive test we are finding that the strain of A. vitis that forms tumors is more common in the environment than previously thought. In infected vineyards, A. vitis was found in 11 of 30 shoot tips in 2013, and 16 of 240 shoot tips tested in 2014. It was also detected for the first time on leaf surfaces in the vineyard. Further tests on wild vines revealed A. vitis in 45 of 154 sample of V. riparia in New York, and 7 of 34 samples collected from wild vines in Northern California. In short, gall-forming A. vitis is more common in the environment than was previously thought. It can be found on wild vines both in the West and in the East, and surviving on the surface of leaves and shoots.
Keeping vineyards and nurseries clean. Even if crown gall-free foundation plants can be generated, it will be a challenge for nurseries and growers to prevent them from being reinfected after planting. Experience with a tissue-cultured block of Niagara vines planted in New York State showed that some vines were re-infected within three years. A. vitis can survive on infected roots for several years after an infected vineyard is removed. Given the widespread presence of A. vitis in the environment, it will be challenging for nurseries to establish and maintain increase blocks that remain free of crown gall.
Management. At this time, crown gall is difficult to remove from nursery stock, and the potential for eventual reinfection in the field is high. But its expression in the vineyard is associated with environmental events that trigger gall formation – such as cold events associated with trunk injury that were widespread throughout the Great Lakes region and northeast this past winter. Growers can minimize the impact of crown gall by:
- Site selection. Avoid or leave a buffer from potential frost pockets. An example is adjacent to tree lines downslope of the vineyard block.
- Hill up grafted vines. Burying graft unions in the winter will protect them from fluctuating temperatures and preserve scion buds for trunk renewal.
- Multiple trunks. Maintaining multiple trunks allows for more rapid trunk renewal, and the opportunity to eliminate an injured trunk without replacing the entire vine.
- Drainage. In heavier soils, sub-surface drainage tiles may prevent soils from being saturated with water in the winter, and mitigate trunk injury caused by expansion of water during the freezing process.
- Manage vigor. Crown gall is often more prevalent in over-vigorous vines that keep growing actively past veraison. Balanced vines with appropriate cropping levels and moderate vigor should be less prone to developing crown gall.
Nurseries and crown gall: Given our current knowledge of crown gall, nurseries today are not able to promise to deliver ‘crown gall-free’ vines, and it is likely that within a matter of years after planting at least a portion of the vines, vines will eventually become reinfected with the pathogen. This stands in contrast to virus testing and elimination – which has and will continue to be successful in helping growers avoid virus infection. We are optimistic that, in the future, nurseries will have the capacity to reliably deliver crown gall-tested material. By doing so, growers will have the needed tool to avoid problems with young vines during vineyard establishment, and potentially forestall crown gall until vines are mature and better able to recover from crown gall injury.
Tim Martinson is senior extension associate in the Section of Horticulture, and Tom Burr is professor in the Section of Plant Pathology and Plant Microbe Biology at the School of Integrative Plant Sciences. Both are based at the NY State Agricultural Experiment Station in Geneva, NY.
For more information:
Burr, T. and T. Martinson. 2015. Grape Crown Gall, Factsheet of the National Clean Plant Network – Grapes.