As harvest begins to wind down, growers are starting to decide whether their canker infected vineyards are financially fruitful.
Vineyards affected by canker diseases can have reduced yields ranging between 15 percent and 80 percent according to some research. Making a decision on what to do with a declining vineyard will depend on the percentage of yield loss and the price received for the grapes.
Older vineyards planted to Colombard, Grenache, Rubired, Zinfandel or other varieties often display the greatest amount of disease due to the multiple large cuts made during winter pruning. Their value and health of the vineyard will dictate whether a vineyard is “pushed out” or if alternative management practices can help increase yields for a few more years. Valley winegrape vineyards typically have a life-span of 25 to 30 years before replanting is considered, but can last many more with proper canker management.
Grapevine cankers are caused by the fungi Eutypa lata and Botryosphaeria spp., and are both capable of infecting through shoot positions other large pruning cuts and wounds caused by machinery (mechanical harvesters), ultimately killing grapevines.
Although replanting is inevitable for vineyards displaying canker diseases, there are some effective short- and long-term management strategies that will help minimize or prevent future infections. Keep in mind that the short-term strategies will only extend the life of a vineyard approximately five to seven profitable years, at which time it will need to be replanted. Long-term strategies have the potential to double a vineyard’s life and increase yields, with some yield loss after implementation.
Recently, double pruning has been shown to minimize infection by Eutypa lata. A single pass using a mechanical pruner (hedger) eliminates the majority of one-year old wood. What remains are canes that extend approximately 20 inches from the cordon. Prepruning takes place prior to the first heavy rains. The extended canes are then pruned to a two-bud spur in early spring, thus eliminating any infections that occurred during the wet winter. This approach is feasible on a small scale but could be costly when acreage is large and scattered throughout the valley.
The most common method of preventing canker infections includes the use of chemicals (thiophanate-methyl) painted onto fresh pruning wounds. Additional applications may be necessary when rain is forecast over a period of weeks.
Chemical applications must be focused on the pruning wounds in order to maximize fungicide efficacy. To improve the application process, a dye is added to the fungicide mixture to help identify missed applications. The use of chemicals coupled with replacing spur positions can help increase yields. Blank spots along a cordon can benefit from developing multiple spurs at a single position, known as “rabbit ears”. When vines are healthy, rabbit ears are discouraged because they can contribute to over cropping. However, they can be useful in replacing spur positions and maintaining yields.
The removal of infected wood — dead arms and cordons — from vineyards will help reduce the inoculum load. New spur positions will often grow from dormant buds and replace the weak or missing spurs. If an entire cordon must be removed, one or two canes can be tied to the wire in its place. In the spring, shoots can be thinned to the appropriate number in order to space arms evenly along the wire. If the vineyard is north of Merced County, dead wood should be removed from the vineyard and taken to a local landfill so it does not become a source of inoculum.
One final short-term approach is to let the diseases run their course. Depending on the variety and inoculum load, the vineyard will succumb to the disease within 20-plus years. Growers will then have the opportunity to replace the vineyard with the same variety or with something more profitable.
Some vineyards may be candidates for “slick” pruning, a practice that involves removal of the spurs down to the cordon. This approach allows a vine to become rebalanced by encouraging new growth along the cordon. Often, buds lie dormant for long periods of time being suppressed by the dominant buds further up on spurs and arms. By slick pruning a vine, the dominance imparted by the apical buds is removed and the vine will display a renewed vigor.
This approach is often practical for older vineyards planted to varieties that are in demand. Vineyards that have reduce yields that are 40 percent to 50 percent of normal will often benefit from this practice. Prior to implementing slick pruning, an inventory of the entire vineyard should be taken to determine if all or part of the vineyard should be aggressively pruned. Cordons that are dead will need to be replaced with one to two canes tied to the cordon wire, as previously mentioned. It should be noted that slick priming will create numerous large wounds that should be chemically treated to prevent infections.
When vines and yields begin to decline significantly, ownrooted vines can be retrained. A grapevine with a complete loss of fruiting wood can be cut at the trunk base to encourage new growth. A sucker coming from below ground is then trained up the existing stake with cordons developed the same year.
Vines will often return to full production within two years. Individual vines displaying significant loss are good candidates for a complete retraining. A vineyard with a chronic infection from fungi causing cankers may have 10 percent of the vines undergoing retraining in any given year. The use of multiple trunks may be a suitable option for highly susceptible varieties. This allows yields to be maintained over the life of the vineyard but can be more costly to manage.
Before employing any of these management practices for canker diseases, growers should determine which practice best fits their circumstances. The amount of yield loss, vineyard age, frequency of infection throughout a vineyard and variety will determine what approach will be most feasible for ones operation. One or multiple practices may prolong the life of a vineyard making it more profitable than replanting.
Eutypa dieback: http://ucipm.ucdavis.edu/PMG/r302100611.html 
Bot Canker: http://ucipm.ucdavis.edu/PMG/r302101011.html 
Stephen Vasquez is the viticulture farm advisor in Fresno County. George Leavitt is the viticulture farm advisor, emeritus in Madera County.