Pierce's disease cure?

Wines & Vines, Nov, 1998 by Richard G. Peterson

All of the sick vines treated with one 5 mg dose of tcy between March and June, 1998 showed a healthy, rapid growth rate which was visually indistinguishable from their uninfected neighbors. In a few vines which had received only 2.5 mg of tcy the initial growth seemed normal but, by late summer, some slight leaf scorch symptoms had reappeared. It is believed that these vines received too small a dose to eliminate all the bacterial infection from the vines.

The original screws were replaced with 5 mg screws in August and these vines continue to be monitored.

DP screws were removed from two vines after three months of treatment because little visible change in vine growth rate had been detected since treatment. It was observed that those screws had been implanted into the graft union bulges because their larger diameters had made it easier to drill the vine and implant the screw. On their removal it was realized that a graft union is perhaps the worst location for screw implantation. Some of the tissue inside the graft union is alive and some is dead in most young vines, because of the geometry of the union itself.

In the case of these two vines, both screws had been implanted inadvertently into dry, brown, xylem and the removed screws still contained visible tcy. This antibiotic is vivid yellow, which makes it easily visible. The tcy had neither dissolved nor moved away from the screws, clearly because there was little or no sap flow through this dead, or dying, brown xylem tissue. New DP screws were installed into lower, healthy tissue of the same vines and new vine growth (with apparent recovery) was seen after two more months. By contrast, in vines which seemed to show a positive growth response to the screw installation, removed DP screws showed only traces of the toy they had held three months earlier. This is what one would expect if the toy had moved along with sap away from the screw towards the upper vine and leaves.

It often appeared that sick vines in which a screw was placed in the rootstock stem just below the graft union showed quicker recovery than when the screw was placed above the graft union. One plausible explanation is that some rootstocks can be infected and yet not show symptoms of the disease. Perhaps the Xf bacteria do not stop the sap flow through xylem tissue as effectively inside these rootstocks as they do in susceptible fruiting varieties. If so, the antibiotic movement (and effectiveness) should be more effective when inoculated into rootstock tissue. Only further research can give us the answer to this, as well as to other questions.

It will require further research to discover the most effective time of year for vine treatment. March to June is an obvious starting point because of the vine's normally rapid growth rate during that period. This also coincides with the rapid multiplication of Xf bacteria in infected vines. Bacterial populations often become quite high in grapevines before symptoms of P.D. are seen [1,2,4,6,7]. The authors' vines that were treated with 5 mg of tcy in July, 1998 were already showing improved growth by September. Purcell has suggested that treatment in the fall may be very effective because it might exaggerate the susceptibility of Xf to winter cold [8].