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Dutch elm Disease: Ophiostoma novo-ulmi (Ophiostoma ulmi= Ceratocystis ulmi)

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Damage Type

Disruption of the tree’s metabolism and transportation of water/vascular system.

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Symptoms & Diagnosis

Syndrome A: At any time from early summer to early autumn, patches of leaves from within the crown, each patch arising from an individual branch, are wilted and yellowed or browned and falling. Some shoots among the affected foliage are dead and may be hooked over at their ends somewhat like little shepherd’s crooks.

Syndrome B:
A tree (unhealthy or healthy the previous summer) fails to flush or flushes and quickly dies; or, during summer, the whole of a previously healthy tree is wilting, yellowing or defoliating/leafless; adjacent trees (notably procera) die in succession over one or more years.

Confirmation

If a symptomatic branch is cut across, a ring of dark brown marks is evident in the wood; cut lengthwise, the wood shows longitudinal dark brown streaks. In fresh infections the stain may only appear in the annual ring immediately below the bark. Symptoms usually spread until the whole tree is dead- a process which may take only 3 or 4 weeks, or as long as 2 years.

Caution: The brown streaking is not always present in all parts of the affected branches. Verticillium wilt (relatively rare in elms) causes similar symptoms.

Status

By far the commonest fatal disease of elm. In the 1960’s, a new, highly pathogenic or ‘aggressive’ strain of the fungus (now named Ophiostoma novo-ulmi) was introduced into England from North America. By 1992, this had spread throughout England & Wales and almost as far north as Inverness in Scotland. The original or none aggressive strain of the fungus (now renamed Ophiostoma ulmi), that causes a far less damaging disease has been largely replaced by the aggressive strain novo-ulmi.

Significance

O. novo-ulmi is the cause of the most devastating tree disease this country has experienced in recorded history and is still a very serious threat to our remaining elms. Millions of trees were killed during the 1970’s and the new elm populations that are growing up in the aftermath of the epidemic from the surviving root systems are now being attacked. O. ulmi causes a far less severe disease from which trees often recover.

Host Species

The disease occurs only on Ulmus and Zelkova. Notably susceptible to O. novo-ulmi are U. procera (English Elm), U. hollandica (Dutch elm) and U. carpinifolia var. sarniensis (Wheatley or Jersey elm). U. glabra (Wych elm) and some varieties of carpinifolia (Smooth-leafed elm) become infected less readily and die more slowly. Zelkova, although sometimes infected, is rarely killed.

Infection & Development

Syndrome A: (as described in ‘Symptoms): the fungus grows in the bark of infected, dying trees where it sporulates in the breeding galleries of Elm bark beetles (Scolytus species). In spring and early summer, young beetles emerge, and carrying spores on their bodies, fly into the crowns of other elm trees to feed on the young, sappy bark in twig crotches. Spores become lodged in the feeding grooves where they germinate. The fungus grows into the water filled xylem vessels and there produces yeast-like spores.

Syndrome B: If roots of healthy elms are naturally grafted to those of infected trees, or if, (as is commonly the case with procera), trees in a woodland or hedgerow originated as suckers arising from a common root system, the fungus can move from tree to tree via the xylem vessels in the roots. As the fungus moves through the xylem vessels it produces toxic chemicals (phytotoxins) which disrupt the tree’s metabolism. Plugs of phenolic materials (the brown marks in the wood) are formed in the vessels. The tree, dying from the complex effects of the infections, which are not yet fully understood, is open to invasion from Elm bark beetles. In summer and autumn, the females lay their eggs in galleries (tunnels) which they bore between the bark and the wood often introducing the fungus to the bark in the form of spores carried on their bodies.

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Control

Therapy: Infections in specimen trees can often be arrested by use of fungicidal injections. For injections to succeed the tree must be:

  1. At least 30 m from other elms.
  2. Symptoms should not have appeared until after July 1st.
  3. No more than 5% of the crown should show symptoms.
  4. Symptoms should be at least 3m from the stem.
  5. One month after injection, the infected limb should be removed, or the tree should be pollarded in the following winter.
  6. The tree should be in an area where the disease is not prevalent.

Injections are likely to fail if

  1. Symptoms are the result of infection from the previous year (usually the case if symptoms appear before July 1st.)
  2. More than 10% of the crown shows infection.
  3. The tree has been pollarded recently.
  4. Symptoms are evident more than 3m from the main stem.
  5. Infection has been introduced via the roots.

The treatment must be given at the earliest opportunity after the symptoms appear in accordance with the directions specified.

Prevention

The prevention of the disease requires the implementation and determined continuation of a carefully planned strategy over a large area. This will involve the co-operation of all local authorities, landowners and private house-holders with elm trees. The plan requires the prompt reporting and felling of diseased trees and the rapid and safe disposal of potential beetle breeding material. Legislation exists to facilitate such measures in certain parts of the country.

Disease resistant trees

Several elm cultivars relatively resistant to the aggressive O. novo-ulmi have been bred in Holland and the USA. Some are on the market, others are still being assessed and the breeding programme still continues. Whether any of these will have enough of any of the attributes of any of our British elms, to make planting worthwhile, time alone will tell. Meanwhile, one approach to the problem of Dutch elm disease is to replace diseased trees with a different species altogether.

Remarks

The disease was first recorded in France in 1918, had reached this country by 1927, and North America by 1930. The cause, (Ophiostoma ulmi), was elucidated in Holland, hence ‘Dutch’ elm disease. In the early 1960’s, O. novo-ulmi reached England from Canada. In continental Europe the disease is further complicated by a second strain of O. novo-ulmi, making 2 races of fungi to attack in just as aggressive and fatal a manner.