What is in this article?:
- For many years, Eutypa dieback, caused by the fungus Eutypa lata was thought to be the main dieback disease of grapevines, causing death of spurs and cordons.
- The disease resulted in a gradual but severe decline in yields. Although Eutypa is still present, its effects are complicated by other aggressive pathogens, mainly species in the Botryosphaeriaceae family, some of which are able to colonize wood tissue three times faster than E. lata. Nine Botryosphaeria species have been isolated from grapevine cankers from California. The disease they cause is referred to as “bot canker”.
Training and pruning
Once a vine is infected, the canker should be completely removed in order to reduce spore production, and the wood should be removed from the vineyard. In most cases, this means removing a portion of the cordon and retraining a cane to recreate the cordon. A cut of this size should be made no earlier than March, and preferably around bud break. On older vines, doubling of spurs to replace lost spur positions and extensive cordon retraining or use of “kicker canes” may be necessary to maintain production.
There are a few key strategies for preventing canker diseases. One is to use a vine training method that reduces or eliminates the amount of cuts made during winter, such as minimal pruning or mechanical pruning in the late dormant period.
Research conducted in Sacramento County in the late 1990s showed that minimal pruning and mechanical pruning resulted in far less Eutypa dieback than spur pruning, while maintaining yield and fruit quality. Cane pruning also significantly reduced incidence of dieback. However, these training methods have not been widely adopted. In the case of machine, box or minimal (hedge) pruning, winemakers have been reluctant to accept these as standard practice, while cane pruning requires skilled labor and an extra operation of cane wrapping.
Another method is double pruning – mechanically pre-pruning to about 12-14 inches in fall or early winter followed by hand pruning before bud break. By removing most of the vine brush, the double pruning can speed up the final selective pruning, thus allowing growers to prune large acreages more quickly.
Research in the North Coast showed that neither E. lata nor species of Botryosphaeria could be recovered from farther than 1.5 inches below the pruning cut. When pre-pruning occurred in winter months, E. lata was recovered from 40 percent to 65 percent of canes, compared to only 7 percent to 10 percent when prepruning took place in February. But the hand follow-up pruning removes these infections.
When possible, prune in dry weather, and preferably when rain is not predicted for a week or more. The susceptibility of pruning cuts to infection declines over time, so a week of dry weather after pruning should result in less infection than when rain occurs the following day. Of course, this may not be practical on large acreage, where pruning must be done through most of the winter. Pruning less susceptible varieties first may be one strategy. Also, results from an unpublished study in the 1980s suggested that late pruning and shoot thinning in the establishment of young vines can significantly reduce later onset of severe dieback.
Late pruning reduces exposure of wounds to rain events. It provides a good deal of control in a currently infected vineyard since spores are depleted over the course of the winter. It is a wise IPM strategy to prune as late as possible.