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Journal American Rhododendron Society

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Volume 41, Number 4
Fall 1987

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Do Metal Wires On Plant Labels Cause Dieback?
August E. Kehr, Ph.D.
Hendersonville, North Carolina

Introduction
Strictly speaking, dieback is not so much a specific disease as it is a symptom of a condition that blocks the passage of water and metabolites in the stem of a plant. Dieback therefore can be caused by such things as fungi, stem borers or extremes of weather. In general, the symptoms of dieback from all these conditions have many similarities, and even plant pathologists are frustrated when they attempt to be specific as to the cause for dieback. Thus, despite efforts to the contrary, many instances of dieback have origins that cannot be pinpointed and must be attributed to factors unknown. This article deals with a type of dieback that does not seem to originate directly from any of the above causes but instead from metal wires on plant labels. In fact, it is difficult to assign metal wired plant labels as a definite causative agent of dieback, and exact experimental data is lacking. In an attempt to assess the possibility of metal plant wire labels as a cause of dieback, a survey of chapter experiences was conducted in early 1985.

The Problem
The problem (if there is one) of metal wires on plant labels causing dieback was raised in a letter written to me by Ted Van Veen as follows:
"Thought I would run an observation by you for your comments. Perhaps you can help.
        All of the plants here in Portland which are kept for observation and landscaping carry a metal identification tag similar to the sample enclosed. In excess of a thousand rhododendrons ranging in age from two years to fifty are tagged.
        It suddenly struck me a couple of years ago that a dead branch on a plant often seemed to be the one bearing the tag. While this certainly wasn't 100% true, this was a fact better than 50% of the time. And taking into consideration the many other branches of the plant not affected it is quite possible the metal tag might have been the cause for the dead branch.
        Yes, sometimes the branch appears to have had a canker such as a phytophthora, but definitely not always. Usually there is no ready sign for cause of death. Possibly at times it could be the result of winter injury. But why only one branch?
        At first I thought probably this observation was the result of laziness - the necessity to replace the tag after pruning off the dead branch. While I really haven't kept a tally, I am more convinced than ever that the tag did have something to do with the demise of that branch.
        Could it be a chemical reaction, or heat, or cold, or possibly some kind of radiation? Your thoughts would be appreciated."
        Instead of trying to answer Ted Van Veen's questions myself, I decided to poll local chapter research chairmen and ask those chairmen to discuss the problem with their respective chapters and report the results. It was hoped that from the overall experience of many members some answers to the problem might evolve.

The Responses
Following are responses from some of the chapter research chairmen - in alphabetical order:

Middle Atlantic Sandra McDonald
        This letter is in response to your request for a report on any suspected relationship between metal labels and branch dieback. I thought this was "common knowledge". I have been aware of this problem for a number of years. Plastic strip labels also seem to cause the problem. I think the wind blows the plastic or metal labels around and causes a mechanical or physical injury which then allows entry of disease. Thousands of my seedlings in the field have been labeled with plastic strip labels like the enclosed. The label bearing branch often dies. I have had the same thing happen with metal labels, like the sample you sent or the one piece aluminum labels. Now anytime I label a branch I attempt to put the label on a strong limb in a position I hope will not be too subject to wind (low on the plant), but of course this is not totally successful.
        I have seen many old healthy plants which have labels attached by copper wires. The wires girdled the trunk or limb, but the plant grew on around the wire, seemingly none the worse for the wire inside. Sometimes the heavy aluminum label is also embedded or the wooden label is rotted away. At that point I "let well enough alone."
        We had a discussion at our board meeting about this label problem. All the members had seen it and felt it was probably induced by physical injury. One member thought metal tags conducted heat or cold to the limb. Heat conduction would certainly take place if the tag was in a spot where the sun hit it, but often the labels are totally shaded.
        Here at the nursery I have made an observation which though not identical, may have some relevance. 'Nova Zembla' is especially susceptible to dieback (Botryosphaeria). The section of young plants is usually fairly healthy; but after the diggers walk through the section repeatedly, digging out plants here and there, the following summer Botryosphaeria increases. I think the diggers walking through the plants, accidentally brushing or bending limbs, produce many points of injury for entry of Botryosphaeria spores.
        Perhaps a study to determine "cause of death" of the branch would be in order. I suspect disease, but maybe it can be one of several diseases. The West Coast may have different diseases than the East. I thought the West Coast did not have a Botryosphaeria problem. Since in many cases the disease stops at the crotch, perhaps the disease is less virulent than Botryosphaeria.

New Jersey Allan Anderson
        I brought up the question about a connection between dieback and metal tags at a chapter meeting. It was also discussed with a few members individually. It was an almost universal feeling that there is a positive correlation between such labels and dieback.
        In general, the metal label itself is not thought to be responsible but the wire or metal band on the label that goes around the stem. In other words, it's OK to use a metal label if it's tied on with something non-metallic or with an insulated wire. Several members commented on the thin gauge insulated wire used by telephone installers.
        I wondered if we all tend to note and remember when the label is on the dead branch more than when it is not. I must report, however, that mine was a minority opinion. Everyone that expressed an opinion at all felt that heat conductivity from metal around a stem contributes to dieback.

North Kitsap Alice P. Smith
        At the North Kitsap Chapter we had a long discussion about the dieback from the metal tags. The same chemical that kills moss on gutters, roofs etc. here in the Northwest could be the problem. If any of the following chemicals are in the wire tie or coated on the tag itself, they could impregnate the cambium layer and cause death to the branch of the rhododendron: zinc oxide, ferrous ammonia sulphate, zinc chloride.
        Some of the above are in sprays, others are already in the paint for the gutters of houses. Throughout the winters here with all the dampness and heavy rains, we have moss growing on every kind of roofing etc. Instructions on the bottle of zinc chloride says "not to be used near plants, bushes or trees."
        Most of our members are using aluminum tags with copper wires. Analysis of the metal in Ted Van Veen's tags would be the solution.

Piedmont Larry Mellinchamp
        Overwhelming agreement that "more often than not, a dead branch will have had a label (metal) on it". We do not know the cause, but most growers have observed the phenomenon and made the correlation between tag and dead branch. To my knowledge, no one has made a scientific study of this situation or performed experiments to get at the cause. We agree that it would be a good ARS sponsored research project.

Pine Barrens Ira Holzman
        An informal canvass of the Pine Barrens Chapter membership indicates no correlation between plant labels and plant demise.

Potomac Valley George Ring
        I have also noticed that limbs with tags sometimes die. Limbs without tags also die sometimes. I have noted rather casually that an uncontrolled observation does not tell you much as far as suggesting cause. Some notes:
(a)  Tags placed around plant bases (under mulch), regardless of type of wire, do not seem to cause death. These tags will not be as hot, or as cold, and will not sway in the wind. They often become part of the plant crown when bark overgrows the wire.
(b)  Copper, steel and other bare metal wires for tags placed high might change temperature of stem at tie point, possibly collapsing cells due to freeze damage.
(c)  Plastic-coated telephone wire seems to be less prone to cause death of a limb if tags are responsible.
(d)  Some have attributed stem death due to physical damage caused by wires cutting the bark and permitting a disease to enter.

Scotland Edmund A.T. Wright
        Over thirty years ago we experienced the same trouble when importing large quantities of nursery stock into this country from Holland. In our case the labels were wooden, but the cause was no doubt the same. The problem of dieback did not occur with stock that was sold, only with stock left in the nursery, and on those plants bearing the wooden labels. Removal of the labels soon became one of our first priorities when new stock arrived, with the result that our losses of stock virtually disappeared, or were attributable to other causes, one is never 100% successful.
        Now to the reason, it was obvious to us that it was only the plants bearing the wooden labels on which dieback occurred. Since it was hardly likely that the label was at fault, it had to be the method of attachment. The Dutch nursery firms all used the same form, thin steel wire, and whether this was twisted loosely or tight, the dieback always occurred on the branch or twig encircled by the wire. It seems to us that steel or iron wire is the main problem, copper wire seems less likely to cause the trouble, although even where this is used, we have noticed weaker growth, as well as dead material. It would seem that plastic covered steel wire is less likely to cause the trouble and we have tried this out with favourable results.
        To sum up, it seems to us that encircling a twig or branch with steel or iron wire is the main cause of the dieback described by Ted Van Veen, not the label itself. Why this is we have no idea, although Ted's suggestions that it could be chemical reaction, heat or cold, or maybe some kind of radiation all seem feasible and is much the same as our line of thinking. Perhaps the best answer to this problem is to attach the label to a stake at the base of the plant, because even plastic or nylon, unless continually looked at, becomes embedded in the plant.

Seattle June Sinclair
        I've had no feedback other than what we discussed at the meeting - something to do with a chemical reaction.

Shelton William E. Hanson
        I brought the matter up at a recent meeting of our Chapter and although there were scattered comments, it was evident that no one considered it a serious problem. One member indicated that he had noted the effect, but that he had assumed it was due to cold damage; the thought here was that the metal, being a good heat conductor, would come quickly to the ambient temperature, thus producing a cold zone where the tag was attached or where the tag itself came into contact with the branch. The effect would be expected to be observed more often for plants showing greater sensitivity to low temperatures.
        A couple of other individuals noted that tags are ordinarily attached to the lower, more sturdy branches and that, as the plant grows, these branches are the ones which get less sun and, hence, tend to die back.
        I am personally not taken with either of these explanations. I have not noticed the effect on any of my own plants but, at the same time, I think we cannot dismiss this matter as of no consequence. Ted Van Veen is an experienced observer and I am inclined to think that the effect he reports is probably real. If I were to guess, I would say that the damage is more likely to be caused by heat which could affect any plant and not just those sensitive to cold. If the aluminum tag were exposed to strong sunlight during hot weather, especially when there were no cooling breeze, it is conceivable that the tag could become much hotter than the environment by radiative heating. We all know how hot a piece of metal can get when exposed to full sun. By contrast, such a piece of metal during cold weather would quickly assume the ambient temperature but get no colder. It would seem reasonable that exposure of a branch to such a piece of hot metal over a time period might cause the observed dieback. I can't see that any chemical or other effect could be involved.
        This is all speculation, of course, but I am now curious to find out how hot a metal tag can get under the right conditions. Maybe I can set up an experiment or two during the summer.

Southeastern Russell Gilkey
        The problem is that the branch of a rhododendron often dies, in time, when a metal label is attached to that branch. Observations are:
Branches that aren't labeled also die.
Metal-labeled branches do not always die.
Metal-labeled branches die more often than unlabeled branches.
The kind of metal (copper, brass, steel, etc.) of which the attached part of the label is made is not important.
Paper or plastic wrap around the labels do not cause the problem.
Hall aluminum labels (a kind of wrap around) do not cause the problem.
Plastic-coated wire is not as bad as bare wire.
Labels attached with soft materials (string, rubber-like, etc.) do not cause the problem.
Labels that are attached below mulch level do not cause the problem even though eventually they may be incorporated in the crown.
Labels that are attached by wire swing and sway with the wind.
Fungus diseases are known to enter damaged areas of plants.
Conclusions from these observations:
The problem seems to be related to materials that can cause damage to the branch.
The material composition of the label doesn't seem to matter as long as it can be abrasive.
The problem doesn't seem to be related to metal conduction of heat; otherwise the Hall aluminum label would be worse than wire.
If a disease organism is involved, it is evidently airborne.
A combination of factors is required before the problem occurs.

Susquehanna Valley Lewis H. Brown, Robert K. Peters and Tom Schuetz
        In response to your inquiry on the effects of metal tags on rhododendron injury, I have sent two replies from members of the Susquehanna Valley Chapter. In my own garden I have not observed any damage which I directly attribute to metal tags. I will tag some plants this year and keep controls to see if there is any significant damage.
        I have had problems with tags, but not only metal but the paper or plastic ones. Over the years I have found that if I left the paper or plastic tags on small plants when I set them out, I would invariably lose the plant over winter. It seems that bark splitting is induced under and below the tag band. I try to make a practice to remove all tags from plants when I set them out. Sometimes I don't have plant markers or stakes with me and leave the tag around the plant. If I forget to go back with the marker the plant is usually dead by spring. In the summer of '82 I planted a row of 19 rhododendrons on a bank with SW exposure. Unfortunately I left tags on about 8 plants. In the spring those 8 were dead - split bark. The others came through the winter OK.
        As for metal tags I have left them on larger plants and I frequently found that branches were dead in the spring so I have been removing them from rhododendrons. I haven't had a problem with metal tags on hollies or other broad leafs. I have had a girdling action from copper wire ties on older plants. I have noted that Tyler and some other arboretums are using only copper wire covered with cotton and waxed-like old bell wire.
        As far as rhododendrons and azaleas are concerned, I make an effort not to leave any tags on them.
        For the last 6-7 years, I've used aluminum tags for plants with aluminum magnet wire for attachment. The magnet wire is an enameled wire, the enamel being a flexible insulating organic coating. In that time I have observed no ill effects outside of an occasional girdling of a plant because it grew around the wire. Even that problem hasn't seemed to slow down the plant.

Summary and Comments
        Of the 13 responses received, 10 people believed there is a definite correlation between metal wires for plant tags, while 2 were non-committal, and 1 was negative. Perhaps members of the Society will want to make close observations of their own plants in regard to this problem.
        Fungi causing dieback on rhododendrons and azaleas can, and usually do, gain entrance into the plant through damaged tissues in the stem. This is especially true of Phomopsis species and Botryosphaera ribis. While dieback caused Phytophthora cactorum may not always be associated with a visible injury, phytophthora dieback is ordinarily found on young stems and kills the tissues so rapidly it usually can be readily distinguished from the somewhat slow developing dieback discussed in this article. Likewise, borer dieback can readily be recognized by the tell-tale borer holes.
        Certainly this is a problem that could be solved with some experimentation. If you are interested enough, why not submit a proposal to solve the problem to the Research Committee? Your local chapter research chairman can give you an application form.

Dr. Kehr, member of the ARS Research Committee, is the former chairman of that group. He is a frequent contributor to the Journal.


Volume 41, Number 4
Fall 1987

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals