History of Dutch Elm Disease in Minnesota

People proposing alternatives to sanitation were
failing to understand the basic biology underlying
the spread of the Dutch elm disease fungus

From the very beginning of the impact of Dutch elm disease in the United States, professionals (i.e. scientists) and amateurs have competed in the search for a magic cure. Miraculous cures have been publicized to the extent that much of the “tried and true” basics of dealing with the disease have been shoved aside and even ridiculed. The unfortunate truth has been repeatedly supported by the failures of these “miracle” cures; the recommended control measures which are effective can be accomplished only with hard work and money.

No Magic Bullets

People continue to search for that magic compound which will cure even diseased trees completely colonized by the fungus. Tremendous sums of money have been spent on fake cures, treatments which were known by those knowledgeable about tree diseases to be of no value. Often these cures, some of which receive national front page news coverage, are proposed by people who have little or no understanding of the fungus and the disease. Promotions by non-pathologists are not only publicized but are often also financially supported by the public.

The major attention that these magic cures often receive is to the detriment of continuing education of the public on the proven control program. Not only is money often spent foolishly but the treated trees are lost as well.

Despite all the efforts expended searching for an exciting new effective technique for controlling Dutch elm disease, the basic approach, known for years, remains the best. The simple answer to the control of Dutch elm disease is sanitation.

Sanitation includes detection and destruction of all dead and dying elm trees, be they American, red, Siberian, or any kind. The notion that Siberian elms are resistant, which they are, and thus do not serve as sources of infested beetles is nonsense. The idea that every tree had to be diagnosed in the laboratory before it could be condemned was expensive, delaying, and led to further spread of the fungus. The “pressures” brought to bear on behalf of some elms, because of their prominence or their ownership by a prominent citizen, were also disastrous to neighboring elms. Sanitation programs were constantly being blocked and slowed by individuals who proclaimed to be concerned about our environment.

Sanitation was further encumbered by the well-meaning Pollution Control Agency which prohibited burning, despite that being the most expeditious way to dispose of beetles and beetle environments. The efforts to slow Dutch elm disease would have fared much better and cost much less if burning had been allowed. Random uncontrolled burning was not what was sought. Supervised burning in prescribed locations would have provided great advantages to those fighting Dutch elm disease and any resulting pollution could have been considered a small price to pay. The permissible thousands of fireplace fires in a single evening in Minnesota’s cities have far more impact on the environment than would have been created with the few fires that were foolishly prevented from being used to burn elm refuse piles.

One of the major metropolitan elm disposal sites was fortunate to have accidently gotten around that burning ban. It was a site designed to promote the utilization of the elm wood from removed trees. Logs, branches, and brush poured into the site in conformance with magnificent plans to salvage the quality logs for use in a variety of products. Unfortunately, the amateur managers of the operation found themselves inundated in acres and acres of elm debris with more coming faster than they could manage. At one point their equipment couldn’t even move when it was in running order. Fortunately, on a day when the wind was blowing away from St. Paul and weather conditions were otherwise favorable, a fire started accidently.

What would have cost millions of dollars and taken too long for the operators of the site to process, was quickly gone. The mayor of St. Paul and the city’s fire chief apparently decided it was safe to let the fire burn and better to save the money that would have been needed to put it out and to haul the debris to another location. The fire, which injured no one and had little impact on the environment, saved a lot of money. It was hard on bark beetles as well.

Egg-laying galleries of the European elm bark beetle have a characteristic but macabre appearence. They run parallel to the grain of the tree, with the larval feeding tunnels at right angles to the main gallery. The beetle overwinters as larvae and emerges as an adult around June. A second generation can be produced by these adults in late summer or early fall, and in some years weather can permit even a third generation to be produced.

People proposing alternatives to sanitation were failing to understand the basic biology underlying the spread of the Dutch elm disease fungus. The fungus spores carried by the two beetle carriers of Ceratocystis ulmi are spores produced in the insect galleries and the fungus is introduced to the galleries by the female beetles as they lay eggs. Thus it made little difference whether a tree died of Dutch elm disease or was killed by an automobile, a girdling chain or winter injury. Any dead or dying elm could harbor beetles and the fungus.

In the early stages of the control programs the requirement that laboratory tests confirm that an elm tree had Dutch elm disease delayed removal of hazardous trees. In some cases the lab diagnosis was a month or more late. On other occasions the samples from a suspected tree were inadequate, and though the tree was infected the fungus was not isolated. Those trees remained to endanger all elms in their vicinities. Laboratory confirmation was not necessary because Dutch elm disease can be diagnosed accurately in the field.

Sanitation, in addition to detection and tree removal included separation of roots between infected and healthy trees, a technique often referred to as root barriers. With all the effort put into searching for a cure, practically nothing was done to develop a method of preventing movement of the fungus through common root systems. In some Minnesota communities, circumstantial evidence suggested that as many as 88 percent of all new cases of Dutch elm disease resulted from invasion through common root systems. The only known technique for disrupting these common root systems in cities was with the soil sterilant SMDC, but this method has not been well researched and doesn’t always work.

One major Minnesota city discontinued placing barrier chemicals around diseased trees when a study showed that their techniques were ineffective. They were ineffective because they were improperly placing the barrier, with the chemical applied only from curb to sidewalk. The roots of the boulevard elm trees extended much farther and were actually fused in the front yards where no barriers were placed. Even with barriers placed all around the diseased tree, they sometimes failed because the roots were not killed at all the points of application or because the fungus had already moved into the adjacent tree. Mechanical barriers are not a reasonable solution because of buried utilities.

Selective pruning, although expensive and labor intensive, can effectively extend the lives of some infected elms. Many of the successful chemical interventions may, in fact, be attributed as much to accompanying pruning as to the use of the chemicals.

An important fact is that an elm infected through its roots can not be saved, but a beetle inoculated tree can be, through selective pruning, if detected early enough.

This other form of sanitation, pruning, can be practiced only in a small percentage of cases. Aside from cost, the major objections to using this approach on public property is that it is extremely difficult for city foresters to monitor individual trees, especially when losing thousands of trees per year. It is a viable approach, however, and elms can be saved very easily by pruning.

In University of Minnesota studies, over 95 percent of the diseased elms pruned experimentally have been saved. The technique prevents the fungus from entering the main stem and in turn the roots of that and neighboring trees. Many of the elms supposedly saved by injecting chemicals were probably actually saved by pruning, which is part of the recommendation in chemically treating diseased trees.

Additional control measures do exist, but all are secondary to sanitation. When DDT could be used, it did provide some additional protection to a population of elms. Meth-oxychlor, one alternative to DDT, does not persist on foliage branches and must be applied in the spring when weather conditions often limit spraying operations. It has not been part of control programs in Minnesota as it is not effective enough to justify the expense. Many people also objected to having their houses, cars, yards and children sprayed. Cities that had such programs abandoned them.

Systemic Treatments

There is a long history of systemic chemical treatments for Dutch elm disease, some directed at the elm bark beetle, but most directed at the fungus pathogen. The earliest systematic attempts were at the Connecticut Agricultural Experiment Station at New Haven in the 1940s.

Effective systemic fungicides came along quite late in the epidemic. After some unsatisfactory results, research in Wisconsin and Minnesota finally determined that Arbotect is effective when used in sufficient amounts and when properly injected. Initially the insoluble form of benomyl was announced as an effective fungicide but its results were not impressive. Several publications developed by extension services described its use: the requirement for rapid distribution, the need for pressure equipment, and the technique for solubilizing the fungicide with lactic acid. Later benomyl was solubilized with phosphoric acid and the latter material, the most promising of this series, Lignasan BLP, was labeled for control of Dutch elm disease. Arbotect came a bit later.

Despite their efficacy for the control of Dutch elm disease, both Lignasan and Arbotect were labeled at rates far below what was needed for effective treatment, one-sixth the needed amount of Lignasan and one-third the amount of Arbotect. Initial injections were in the main stem above ground and later through severed roots. Best results were obtained by below grade or root flare injections which could protect elms for two years, and possibly even into a third season. Root flare injections have resulted in far better distribution of the chemical within the tree.

The products currently labeled and used in the United States are water soluble acid salts of two benzimidazole compounds developed in the 1970s. Measured in terms of long-term prophylactic effectiveness in the tree, Arbotect 20-S (thiabendazole-hypophosphite) is currently the product of choice by the professional tree care industry. The product also has a therapeutic effectiveness if the fungus infection is not systemic, the highest label rate is delivered uniformly to all parts of the tree, and the symptomatic parts of the tree are subsequently removed.

Current research at the University of Minnesota and Virginia Tech with a relatively new fungicide, called Alamo (propiconazole), although too preliminary to report, suggests that the product may be effective as a therapeutic treatment for diseased elms. Prophylactically, the product is completely effective against overland transmission in mature trees, at least for the short term (one season).

Despite this progress on the chemical front, the economics of fungicide injection are entirely against the process, except for trees of high value. At more than $10 per inch of tree diameter (measured at at a height 54 inches above ground) it is expensive and certainly not logical for entire populations of trees. Injection will also not work in elms infected through the roots or when the fungus has already become established in the main stem.

It is unfortunate that the majority of elms injected with system fungicides were improperly treated. Not only were the procedures a waste of money, but many homeowners, having invested several hundred dollars on the process to save their trees, argued that their elms should not be removed even though they constituted a hazard. The fungus and beetles profited as a result and the sanitation program lost ground.

While systemic fungicide injection was of some value when properly administered, there were many other proposed cures for Dutch elm disease which, in spite of newspaper stories and self supporting statements, were of no value. There have, in fact, been well over 500, and possibly as many as 1,000 suggested cures for Dutch elm disease. Often people became unbelievably enthusiastic before any substantive evidence on a treatment’s effectiveness became available.

Many elms have been treated with substances never even approved as non-hazardous by the Environmental Protection Agency. Many which have had EPA approval, have erroneously promoted the impression that EPA approval also means that a proposed fungicide actually works as advertised. The EPA has always passed approval only on whether a compound is hazardous. The losers in these affairs have been the tree owners, paying out good money while still losing their trees.

Another negative attached to the proliferation of unproven and ineffective treatments has been how those failures delayed the introduction of an effective chemical control. Research at the Sault St. Marie Laboratory in Canada found that Dursban (chlorpyrifos) was effective in reducing populations of the native elm bark beetle, which is the primary vector in northern Minnesota and much of Canada.² But even with that evidence, it was several years before the use of Dursban was recommended in Minnesota. It has since become an important part of a control program, especially for northern Minnesota.

Injection of dozens of biological and chemical compounds have been proposed as magic cures for Dutch elm disease. Several have been tried by researchers at the University of Minnesota, on the university's own trees. While a few of the compounds produce some degree of control, the overwhelming majority have been found worthless. Some can even be dangerous to the tree or the individual handling it.

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