WW-03765     1993  

History of Dutch Elm Disease in Minnesota

Slowly, Steadily, Tree
Infections Increase

The stage was set for maximum losses of elms in 1977 because of wholly inadequate sanitation in preceding years

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Dutch elm disease developed more slowly in Minnesota than in states to the east and south. Exactly why its spread was slower is not known. A logical speculation is that European bark beetle populations did not survive as well here as in states with milder winters.

Based on laboratory confirmed cases of Dutch elm disease, the numbers of diseased elms was, for instance, lower each year in Minnesota than were the cases in Michigan and Wisconsin for the comparable early years of their encounters with the disease. (Table 1)

Table 1. Laboratory confirmed cases of Dutch elm disease in Michigan, Minnesota and Wisconsin for comparable years following introductions of Dutch elm disease into each state.

Minnesota		Wisconsin		Michigan
1961	9	1956	63	1950	9
1962	2	1957	376	1951	74
1963	43	1958	1,832	1952	298
1964	54	1959	3,580	1953	1,119
1965	23	1960	5,342	1954	5,774
1966	49	1961	8,124	1956	7,614
1967	136	1962	7,196

Though the specific numbers comparing rate of increase of Dutch elm disease for different states can be questioned, the general trend does suggest that environment or some other factor held Dutch elm disease in check during its early existence in Minnesota. The diseased tree numbers do not, however, account at all for what happened outside of metropolitan areas.

European elm bark beetles burrow into dead elm wood. The primary carrier of the disease-causing fungus in the southern part of Minnesota, it is actually a very small, inconspicuous insect about one-eigth inch long. The better adapted native elm bark beetle spread the fungus in northern.

One can never be certain that 1961 was the year when the Dutch elm disease fungus was introduced into Minnesota. While the first Minnesota diseased tree was reported in 1961, that tree was dying in 1960. The fungus could have been introduced in other locations, especially southern Minnesota, and not have been recognized. The first infected tree in Wisconsin, in Milwaukee, had been dying for three years before it was known to be a case of Dutch elm disease.

If we consider only the seven county metropolitan area of the Twin Cities where probably the best data are available, the losses for 1971 were only 421 trees. In 1972 and 1973 the losses increased to 977 and 1,457 respectively.

It was not until 1974, when losses jumped to 9,792, that the rate of increase changed substantially. The losses increased greatly in the following years: to 27,044 in 1975; to 75,460 in 1976; to 192,211 in 1977.

Data shows that 1977 was the year of greatest recorded losses to Dutch elm disease. The factors accounting for maximum losses in 1977 can be explained in part on 1976 weather favoring the beetles, and because effective control programs were not underway until 1977. This is when major funds were made available by the state legislature.

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Rural Counties Also Hit

While it was not possible to determine accurate numbers of diseased elms in rural areas, an effort was made to detect the first case in each county. This provided Minnesota with an approximate record of how the disease spread throughout the state.

By the winter of 1982-83, Dutch elm disease had not been found in only three of Minnesota’s 87 counties. All three of these were in the northern tier (Cook in the extreme northeast and Kittson and Marshall in the northwest). Dutch elm disease was reported from the southern tier of counties between 1967 and 1969, supporting suggestions that the fungus first entered Minnesota, as reported, in St. Paul and Monticello.

A cross section and a branch stripped of its bark each show a characteristic discoloration that can indicate the presence of Dutch elm disease.

There can be little doubt that the fungus was brought into Minnesota by some person or persons. This was not an exceptional event as the disease has typically been spread along highways and has hitch-hiked on vehicles. The first known incidence in Manitoba, for instance, appeared in Selkirk following a mobile trailer convention in their local park area.

At first the losses were very high in southern Minnesota and less in the northern portions of the state. This is probably due to the fact that the European elm bark beetle, which has been known to be present since 1961, is commonly found throughout the southern third of the state, including the seven county metropolitan area of Minneapolis and St. Paul. In northern Minnesota the native elm bark beetle is the primary vector. As the elms were lost in southern Minnesota the incidence of the disease subsided. By contrast, northern Minnesota has faced increasing losses of elms.

The stage was set for maximum losses of elms in 1977 because of wholly inadequate sanitation in preceding years. If the legislative support program had been initiated earlier this may not have happened.

Elm losses in 1977 were to a large degree due to substantial numbers of dead and dying elms in communities where beetles increased their populations. If the figures were available we would have known that the bark beetle populations were at very high levels in 1976. In addition to poor sanitation programs, the weather was favorable for the bark beetles. Adding to the problem, drought conditions in late summer predisposed the elms to beetle attack, especially in September and October of 1976.

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Pitchouts and Infection

A total of 12 city parks and adjacent areas were surveyed in the fall and winter months of 1976-77 to determine how many elms had been attacked by elm bark beetles and approximately what percentage of those trees attacked became infected with Dutch elm disease. The results of this evaluation are summarized in Table 2.

Table 2. Number and percentage of elms with pitchouts (indicating beetle attack) and number and percentage of these elms that became infected as a result

Location	Elms Per Acre	Elms With Pitchouts		Pitchout Trees With Dutch Elm Disease
		Number	%	Number	%
Loring Park	214	30	14	30	100
Stevens Square	112	42	37.5	40	95
Pearl Lake Park	77	0	0	0	0
Minnehaha Park	100	2	2	2	100
Kenny Park	16	1	6.3	1	100
Como Park	100	1	1	1	100
Phalen Park	104	8	7.7	7	88
Douglas Triangle	18	3	16.7	3	100
Cedar Ave. (Plot 1)	53	39	73.6	—	—
Cedar Ave. (Plot 2)	38	0	0	—	—
Kenwood Park	30	0	0	—	—
Lake of the Isles	10	0	0	—	—
TOTALS	872	126	14.4	81	64.3

Of 872 healthy appearing elms observed at random, 126 of them, or 14.4 percent, had been attacked by bark beetles. Of these beetle attacked trees, 81, or 64.3 percent, died of Dutch elm disease in 1977. It’s very likely the fungus actually killed more of the 126 trees, however, these trees were not checked in 1978 or subsequent years to see if additional elms had died.

The drought conditions that were present at this time not only predisposed elms to attack by bark beetles but also resulted in mortality in trees of many other species. Tens of thousands of paper birch trees succumbed, as did many Norway spruce and other conifers. Rainfall in August and September 1976 was only 2.81 inches, considerably below the normal rainfall of 5.78 inches for this period.

Downside of the Epidemic

In the years since 1977, elm losses have continued at a high level in wild or rural areas, but have subsided in cities and towns with substantial control programs. Minneapolis, the largest city in Minnesota, had one of the largest populations of elm, and came eventually to have one of the most vigorous control programs. The city was estimated to have between 200,000 and 600,000 elms before the advent of Dutch elm disease. The lower of these two figures is believed to be the more accurate.

As was true for the state as a whole, the disease in Minneapolis developed very slowly at first, with only 150 diseased or dead elms reported from 1964 through 1971. Losses increased rapidly each year through 1977. As the legislative support program had its impact on control efforts, the losses were sharply reduced thereafter. (Table 3)

Table 3. The increase of Dutch elm diseased trees in Minneapolis through 1977; and the decrease following control program enacted in 1977.

Increase:		Decrease:
1972	222	1978	20,813
1973	225	1979	6,751
1974	937	1980	4,184
1975	1,688	1981	5,068
1976	7,239	1982	3,389
1977	31,475	1983	2,144
		1984	4,965
		1985	4,087
		1986	2,896
		1987	2,280

The losses in 1978 remained fairly high, almost 21,000 elms, because of the carry over from 1977. Since that time the losses have stabilized at a relatively low level. What is important is that because of an aggressive sanitation program, the city still has close to 100,000 elms. The actual number may even be greater because of new elms which were nonexistent or too small to be counted in 1963. The American elm is a prolific seed producer and there will always be new elms establishing themselves.

The cost of reducing the elm tree losses was substantial. The total cost of the program in Minneapolis, including tree and stump removal, trimming, insect and disease control, inspection and replanting was $8 million in 1978 alone. (Table 4) In addition to the costs from 1977 through 1981, an additional $2 million from the State Legislature was spent on the program.

Table 4. Cost to the City of Minneapolis for Dutch elm disease control, 1978 through 1987, including tree and stump removal, trimming, insect and disease control, inspection and replanting.

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987

$8.0 million $6.7 million $6.7 million $7.0 million $5.3 million $5.0 million $5.2 million $5.1 million $5.0 million $5.6 million

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