Pierce's disease cure?

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

Bacterial diseases of animal and man are commonly treated with antibiotics. It has been known for more than two decades that Xylella fastidiosa (Xf), the bacteria which cause Pierce's disease (P.D.) in grapevines, are sensitive to some antibiotic materials [3,5]. One could assume that the bacteria (along with P.D. symptoms) might be eradicated from infected vines by injected antibiotic. The problem in the past has been that no method was available for successfully injecting antibiotics into gravepines.

Unlike the case of animals or humans, plants generally have not been injected directly by means of hypodermic needles because the plants rarely retained injected solutions for longer than a few seconds. Medication injected into plant tissue by needle promptly oozes out of the plant as soon a the needle is withdrawn and little, if any, is absorbed. In further contrast to animals and man, grapevine wounds do not quickly seal themselves. This is easily observed during the growing season when sap oozes, drips or runs out of vines for many days through fresh wounds. The continuous flow of sap through wounds acts as a flushing mechanism that cleanses wounds and ejects foreign materials from the vine. Unfortunately for vines that suffer from P.D., this cleansing action has defeated all previous efforts to inject the vines with bactericidal materials.

After losing several vines to P.D., the author began experimenting in 1997 on Pinot noir vines which showed symptoms of P.D. at the Peterson tree farm near Yountville in Napa Valley. These studies have now yielded a simple method for the successful injection of antibiotics into living grapevines. The method is presented here in the belief that it may be useful in other areas of plant research as well as for the control of diseases like P.D.

Materials and Methods

The method requires using a specially designed, inert screw (DP screw), which has a typical screw head or bolt head on one end and screw threads along the shaft. The threads are self-tapping and the threaded end of the screw is blunt rather than pointed. The screw shaft is hollow from the threaded end towards (but not all the way through) the closed "head end" of the screw. Finally, slots run longitudinally into the threaded end of the screw so that material contained inside the hollow screw will have a route for migration from inside the screw through the slots to the threads outside. The hollow portion of the screw is filled with a suitable medication, such as an antibiotic or other bactericidal material.

In operation, a hole is drilled perpendicularly into the central xylem tissue, but never completely through the trunk, cordon or cane of the infected vine. It is important that the diameter of the drilled hole be slightly smaller than the outside diameter of the screw's threads. The screw, fitted with a pliable washer or o-ring if necessary, is then screwed into the hole in the vine until it is snug and no sap can be seen seeping out of the vine around the screw head. All fruit and flower clusters are removed from each vine to avoid contaminating the fruit whenever that vine is so treated. The intent is that, as sap rises through the xylem, it will dissolve medication from inside the screw and carry it along to other regions of the vine, where it can perform its curative function.

Dosage?

Antibiotic dosages for field experiments were chosen based on common medical practice with humans. The normal dose of tetracycline (tcy) for an average adult human (150 pounds) is 250 mg four times per day. The average human body can lose tetracycline (through metabolism, sweat, urine, etc) at an average rate of about 250 mg per 6 hours. Thus, to maintain a steady-state concentration near 250 mg of tcy per 150 pounds of body weight, it is necessary for the patient to swallow 250 mg capsules at 6 hour intervals throughout the treatment period.

Based on the above, it was postulated that Xf might be controlled in grapevines by the equivalent of an average, steady-state dose of 250 mg of tcy per 150 pounds of vine weight. Since vines are not expected to lose antibiotic to metabolism, sweat or urine, the steady state concentration for a sick grapevine was assumed to be attainable by giving the vine one single dose. Assuming that the average grapevine in the experimental group weighed about 3 pounds, the proper single dose should approximate 5 mg in size. This amount can easily fit into a small screw. Accordingly, doses used in these experiments varied from zero (for controls) to 10 mg per vine. To test for tcy toxicity, doses of 25 mg were given to healthy vines without visible negative symptoms appearing in any vine throughout one season of growth.

No single dose of tcy antibiotic is readily available from pharmaceutical producers in such minuscule sizes that could be filled easily into small, hollow, nylon screws. Accordingly, 5 mg doses were prepared by dissolving exactly 500 mg of tcy in about 5 ml of distilled water. One hundred miniature cotton balls were added to the mix and stirred carefully to distribute the antibiotic solution more or less equally throughout the 100 cotton balls. Cotton balls were obtained from Richmond Dental, P.O. Box 34276, Charlotte, N.C. 28234 (cotton pellets, #4, 1/8 inch diameter).