A case of mistaken identity: Dutch elm disease versus winter injury

Photo of tree with Dutch elm disease.

Dutch elm disease (DED), a devastating killer of elm trees, has been on the rise, in central Minnesota, over the last several years. The cause is not completely understood, but involves an increase in the number and maturity of volunteer elm, weather patterns that have both stressed elm trees and favored beetle populations, and a slackening vigilance on the part of some community DED programs.

However, not all that goes wrong with elm is DED. With the damage caused this last winter, managers are having a difficult time distinguishing between causal agents, winter injury and Dutch elm disease.

It is critically important for managers to verify DED before marking trees for removal. This can be difficult, because it means slowing down for a careful inspection in the midst of the rush to keep up with the number of trees that are infected with the DED fungus. Because DED can rapidly spread from tree to tree, any delay can mean more trees infected. So managers have to balance the need for a rapid response with the need to verify the cause.

Symptoms Observed

The most obvious symptom of winter injury seen early this year was the very late bud break and leaf expansion. By late May, many elms still had few leaves. Making the trees look even worse was a very heavy seed crop. As elm seed mature, they dry and brown and then drop during the month of June. The heavy seed crop combined with the lack of leaves, gave elm trees on overall brown appearance, as if rapidly wilting. Now that most of the seed has dropped and most of the trees have put on a crop of leaves, the trees still do not look particularly healthy. The leaves on many of the affected trees are considerably smaller than they ought to be which gives the trees a thin appearance. The space on the stems, previously occupied by the seed, remain bare now the seed has dropped, adding to the thin appearance. In some cases, the seed has failed to drop, so twigs still have a brown cast to them.

Now, add that to the early stages of DED disease development. Individual branches wilt and begin to yellow. Often it is just the tips of the branches that show symptoms at first, and these are scattered among branches with small undersized leaves and remnant seed crops that show brown among the leaves. It is nearly impossible to tell the two sets of symptoms apart during the typical drive-by inspection many cities use to detect DED.

Survey Methods

To get a better feel for the percent of trees being affected by DED and the percent being affected by winter injury, a survey was conducted in four neighborhoods within the north-central metropolitan area. Relatively speaking, this is a very small survey, so cannot be used to make inferences across the state. But the numbers highlight the risk of misidentification (see Table 1.).

In the survey, trees that were symptomatic at the time of the inspection plus the few that had already been marked and removed this year were counted as DED trees. Trees with at least 80% of their normal leaf area were considered healthy. Winter injury (WI) was ranked in three categories, low (50-80% of ¬?normal¬? leaf area), moderate (20-50% of ¬?normal¬? leaf area) and high (less than 20% of their ¬? normal¬? leaf area). Note the categories are based on leaf area and not number or percent of leaves. What is normal is difficult to define, but in this case was based on a combination of leaf size (1¬? average versus the more typical 3¬?) and the number of live branches that had leaves on them. In general, most of the trees had put on a full complement of leaves. But in many cases, they were much smaller than normal. Trees that had neither WI nor DED, and showed signs of other kinds of damage were counted as Other. Trees showing signs of both DED and WI were counted as DED. Trees showing signs of either DED or WI and other kinds of damage were counted in either DED or WI.

Roughly 50% of location 1 consisted of Siberian elm. Elsewhere, Siberian elm made up less than 25% of the total number of trees. The rest were American elm. All of the Siberian elm and some of the American elm showed traces of damage from leaf miners. Otherwise there was no apparent difference in response to damage agents.

Survey Results and Discussions

The incidence of DED ranged from 5% to 19% across the four locations with 9% of all trees tallied showing signs of DED this year (see Table 2.). The incidence of moderate to severe winter injury ranged from 9% to 25% across the four locations with 16% of all trees tallied showing significant damage due to winter injury. So significant winter injury was much more common than DED, even though an annual loss of 9% due to DED is high. The sheer volume of winter injury increases the risk of confusion when diagnosing these two damage agents.The good news is that 75% of the trees appeared healthy even after a severe winter. As long as communities do a reasonable job of getting the infected trees down and the wood destroyed, the winter injury is not likely to contribute to a significant increase in disease. A curious note is that many of the trees with moderate to severe winter injury had put on new growth that appeared nearly normal. So the leaves that emerged first were tiny in comparison to the second flush. Branch tips had larger, denser foliage, so had a ¬?fox-tail¬? appearance. That suggests that at least some of the winter injury was confined to over-wintering tissue only. These trees will likely recover as the new growth makes up for the limited food production capabilities of the first flush. Another reason, to go slow when diagnosing DED among these trees.

Table 1. Tally of elm showing varying levels of damage from either Dutch elm disease (DED) or winter injury (WI).

# of Trees

Location 1

Location 2

Location 3

Location 4

Total

Healthy

63

43

22

68

196

DED

12

25

8

12

57

Low WI

57

30

66

134

287

Mod. WI

22

31

20

20

93

High WI

3

2

6

2

13

Other

 

1

 

 

1

Total

157

132

122

236

647



Table 2. Summary of tree tally where ¬?healthy¬? equals healthy trees plus those with minor WI, and ¬?WI¬? includes those with moderate to severe winter injury, combining the two most severe damage categories.

Location

% Healthy

% w/DED

% w/WI

% w/Other

Total

1

76

8

16

 

100

2

55

19

25

1

100

3

72

7

21

 

100

4

86

5

9

 

100

Percent of trees across all locations

75

9

16

*

100

*Less than 1%