SYMPTOMS OF DUTCH ELM DISEASE
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Photo 3: External leaf symptoms of Dutch elm disease. (Photo 3A, B and, D: courtesy
of H. Kaljee, OMEGAM Groenadvies, Amsterdam, The Netherlands)
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The first external symptoms of DED are discoloration or drooping of the
leaves at the tip of a branch. Subsequently, the leaves yellow, curl, turn brown, and finally fall off the tree soon after they die. Alternatively,
leaves can dry out rapidly, turn dull green before dying and remain attached to the twigs for several weeks. The shoot tip may wither and droop suggesting a “shepherd's crook”. A branch carrying only a few
wilted leaves at the tip is called a “flag” {[230],[311],[332]}. When Scolytus bark beetles feed on elms, small flags develop. Hylurgopinus rufipes causes the formation of large elm flags {[230]}. Recently, Möller {[640]}
reported a correlation between elm foliar asymmetry and DED infection. As the disease progresss, the normal, inversely egg-shaped leaves of U. glabra become more asymmetric; this applies to both the width of each half of
the leaf and the inter-rib distance. Large leaves showed the highest level of asymmetry. The frequency of abnormal leaf formation increased fourfold
from the first to the second year of DED infection.
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Photo 4: Different stages of DED infection: the onset of leaf discoloration (a),
leaves turn brown (b), defoliation (C). (Photo 4A and B: courtesy of H. Kaljee, OMEGAM Groenadvies, Amsterdam, The Netherlands)
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Internal diagnostic symptoms of DED are found within twigs of affected
parts of the tree. Immediately beneath the bark of the diseased branch, a brown discoloration appears in and around the water-conducting vessels
of the early wood of the outer growth ring. After removal of the bark, brown streaking and mottling can be observed, in contrast with the largely white wood in healthy specimens. Twigs stripped late in the season may
not show these symptoms because the discolored layer will have become overlaid by late wood. However, a cross-section of the infected branch will expose dark brown spots or a continuous brown ring in the current season's growth ring (Photo 5 and 6,{[230],[331]}).
| Photo 5: A cross-section of the stem of an elm that died as a result of Dutch
elm disease shows brown discoloration of the vascular tissue in the youngest annual growth ring.
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External DED symptom expression and internal fungal colonization are not
necessarily linked {[263]}. Sutherland et al. {[1]} reported that in seasons with high temperatures and a high number
of hours of sunshine, DED-infected elms had low defoliation levels (< 20%) yet extensive vascular browning throughout the crown (>80%).
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Photo 6: Internal symptoms of Dutch elm disease. After removal of the bark, brown
streaking can be observed (a). Cross-sections of infected branches or stems show a continuous brown ring or dark brown spots in the current season's
growth ring (b-d). (Photo 6a and b: courtesy of H. Kaljee, OMEGAM Groenadvies, Amsterdam, The Netherlands)
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Although the first external DED symptoms may appear as early as June, they
usually appear between July and autumn leaf-fall. These first signs of the disease may be masked in autumn by normal changes in leaf color. Low levels
of nutrients, summer drought, replantation of elm trees, or insect and mite injury can result in leaf discoloration, wilting, and leaf-fall (D. Elgersma, personal communication, {[230]}). Therefore, DED may remain unnoticed
if the appearance of the first disease symptoms coincides with the presence of one or more of these stress conditions. On the other hand, since Ophiostoma ulmi s.l. is not the only pathogen able to induce wilt in elm, the tree may also
suffer from an additional wilt disease (see Additional wilt diseases).
A beetle-mediated infection with Dutch elm disease usually starts on one
or several branches and then spreads to other parts. An entire tree may die gradually branch by branch over a period of several years. Alternatively,
dispersal of the foliar symptoms throughout the whole tree may occur within a few weeks, causing death of the elm by the end of the season. In the latter situation, infection of the elm often results from root graft transmission
of DED (D. Elgersma, personal communication). Occasionally─as was found on a wide scale during the first DED pandemic─trees infected in late summer may recover and show no disease symptoms
the following year {[332]}. However, often the wilted branches die during the winter, and in spring the disease continues to spread to new parts
of the tree. Eventually, the elm dies as a consequence of Ophiostoma ulmi s.l. moving across into a new year's growth. Twig die-back detected in early
spring is indicative of autumn infection {[230]}.
