2014: The Year of Crown Gall
By: Bryan Hed
Throughout this grape growing season, we are reminded of the extreme cold temperatures earlier this year that damaged grapevines in many eastern US vineyards, especially those of Vitis vinifera. In addition to outright death of vine trunks and arms, there has been the widespread appearance of crown gall. What’s the connection? The pathogen causing crown gall is a bacterium, Agrobacterium vitis, that can be present in vines for years without causing any disease symptoms until a trunk damaging event occurs like the extreme cold brought on by the polar vortex in January. In spring, as cambium cells attempt to restore cold damaged vascular tissue, the bacterium causes the multiplying vine cells to create masses of unorganized callus tissue instead of new vascular tissue. Therefore, instead of repairing the conductive capacity of damaged vine trunks, the growth of callus tissue (the galls) progressively compromises injured conductive tissues and leaves many vines weak or dead. So, the presence of the bacterium does not in itself cause crown gall, but the susceptibility to crown gall is related to susceptibility to winter cold damage and we often find varieties of Vitis vinifera to be most affected by this disease in cold climate viticulture. Susceptibility among hybrid wine varieties can vary greatly and natives, like Concord, appear to be among the least affected. There may also be varietal differences in the tolerance to crown gall. For example, in our Vitis interspecific hybrid ‘Chancellor’ vineyard, most vine trunks appear to have exploded in crown gall this summer, and yet not a single vine has collapsed, canopies are lush and loaded with crop, and we hope to mature and harvest that crop. In contrast, in our Vitis interspecific hybrid ‘Chambourcin’, many galled vines have either collapsed or are turning yellow despite aggressive crop thinning, and the fate of that crop hangs in the balance. Site factors may also play a role in crown gall development as they affect low temperature extremes. Sites that minimize winter cold extremes will minimize problems with crown gall. There is no cure for crown gall and a vine that develops crown gall may die within the current season, or linger for a few seasons before collapsing. The speed at which galled vine trunks collapse within a season may be affected by stress factors, particularly availability of water.
Regardless of how quickly the disease is progressing, replacement of whole vines or galled trunks of infected vines will be necessary to maintain optimal vineyard productivity. If galls appear on newly planted vines, it would be best to replace them rather than spend time trying to renew them. When removing vines, it is important to extract as much of the root system as possible as the bacterium can be present in roots and remain viable in the soil for many years. For this reason, there is no guarantee that replacement vines won’t become infected from A. vitis bacteria already in the vineyard soil. On the other hand, older established vines can often be renewed if the rootstock is healthy and they are throwing healthy shoots from the scion below the galls. These scion shoots or suckers can be trained up to become new trunks. For example, ten years ago a rough winter at our site in Erie county caused our mature Chambourcin block to explode with crown gall; we had no idea crown gall infection was so widespread in the block. However, rather than starting over, crown gall expert, Dr. Thomas Burr, advised us to train up new vine trunks from suckers emanating from below the galls, and we enjoyed many more years of full production from that block. Unfortunately, 10 years down the road, we are now faced with repeating that renewal task. If you choose to renew, you have some options. If the old trunks are dead or obviously dying and will not ripen a crop, you can remove the old trunks now or at your convenience. On the other hand, if galled vines have developed some canopy on the old trunks, or if galled vines still appear reasonably healthy otherwise, you can leave the old trunks as (i) a support for training of new trunks (ii) as a competitive sink to reduce overly vigorous sucker growth (which will result in poor quality renewals), and (iii) as a means of harvesting some crop from what remains of the tops, if it appears cost effective to do so. In this case, old, still living trunks can be removed later during dormant pruning or in successive seasons, when new trunks are ready to make up a reasonably full complement of buds.
For the future, if you’re growing cold sensitive varieties (especially V. vinifera) in the northeastern U.S., maintain multiple trunks and expect to do regular trunk renewal as standard procedure. For grafted vines, hilling soil to bury grafts in fall will help to ensure the survival of scion buds for trunk replacement.
Down the road, through the Clean Plant Network and funding from USDA, a clean vine program is well underway to make ‘crown gall free’ vines more available from nurseries. I am told the new Farm Bill will provide funding over the next five years for this work to continue. The goal is to enable growers to start with a crown gall-free vineyard. And while we are not there yet, detection of crown gall in grapevines continues to improve, making the work of establishing clean mother blocks more trustworthy. Though the main source of crown gall remains infected nursery stock and nurseries may not yet be able to guarantee crown gall free planting material, the quality of planting stocks continues to improve.
For further information, these are some excellent references:
- Winter injury of grapevines. 2007. T. Zabadal et al. (eds.) 106 pps. Michigan State University.
- Tim Martinson and Thomas Burr. How Close are We to Crown Gall-Free Nursery Stock? Appellation Cornell; Research Focus 2012-1. http://nationalcleanplantnetwork.org/files/144948.pdf