PRUNING FOR CONTROL OF PIERCES DISEASE

• Author(s): Purcell, Alexander; Weber, Ed;
• Abstract: Because there are no practical therapeutic measures for grapevines with Pierces disease (PD), we tested the effects of drastic pruning as a method of rapidly regenerating healthy vines from vines with PD symptoms. If such a practice were to be successful, it could speed the replacement of affected vines by preserving an established root system to support vigorous regrowth. Our studies are designed to provide useful information for growers even if our pruning experiments do not produce methods that eliminate the disease from infected vines. Some growers have experimented with pruning practices against PD and claim worthwhile successes but have not compared their results to negative controls. These efforts may be a waste of money for labor and lost time in vine replacement. Our goals were to determine either that pruning would reduce the time required to restore a diseased vine to productivity or that such practices are not worthwhile. Finally, the results of our proposed experiments would provide new data on the distribution and overwinter survival of Xylella fastidiosa relative to PD symptoms. Preliminary pruning trials in 1977 discouraged further testing of pruning to eradicate PD from individual vines. The incidence of PD symptoms was mapped in a plot within a commercial vineyard of Ruby Cabernet in Fresno County. Vines that had only a single cordon with PD symptoms were noted and the diseased cordon was sawed off of every other such vine during October. In the fall of 1978, about 35% of the marked vines had no PD symptoms, but pruning had no influence on the likelihood of recovery. The same percentages of vines recovered regardless of severe pruning. In another pilot experiment begun in late October 1996, we mapped the occurrence of PD in about 1000 Cabernet Sauvignon vines in Napa Valley. We marked, photographed, and took samples of symptomatic leaves from vines that had very early symptoms of PD only on one or several canes in these quadrilateral-trained vines (4 cordons). The samples were stored frozen at -70 C to process with PCR at a later date to confirm our diagnosis of PD. We then immediately removed all canes with symptoms from half of the vines. The following fall, 12 of 19 of the early-pruned vines showed no symptoms of PD. However there were no disease symptoms in 16 of the 18 unpruned control vines. We concluded that removing canes with light symptoms was not a promising method to eliminate PD from infected vines. Some growers in Napa and Sonoma county have claimed to have eliminated PD from about 60% of the vines by cutting the trunk of the vine near the ground and retraining a new shoot to form a new trunk. None of these growers left similarly diseased vines as unpruned controls, so the effectiveness of their drastic pruning to control PD could not be assessed. If this pruning practice were to be adopted on a widespread basis without further critical evaluation, it is possible that a lot of expense may be incurred with no real economic return.
• Publication Date: Dec 2001
• Journal: 2001 Pierce's Disease Research Symposium