This fungus belongs to the Ascomycota group of fungi.
Damage. The first symptom of this disease is a yellowing of foliage in one part of the tree in early summer. The foliage then dies off progressively from this area of the tree, often resulting in death within three months. Trees that survive 1 year's infection may fully recover in the following year. All common species and hybrids of elm growing in Great Britain are susceptible to the disease (see Figure 15.11).
Life cycle and spread. The causative fungus lives in the xylem tissues of the stem, and produces a poison that results in a blockage of xylem
vessels, causing the wilt that is observed. Associated with this disease are two black and red wood-boring species of beetle, Scolytus scolytus (5 mm long) and Scolytus multistriatus (3 mm long). These beetles bore into elm stems leaving characteristic 'shot holes'. Eggs are then laid, and a fan-shaped pattern of galleries is produced under the bark by the larvae. Later, as adults, they emerge from the wood, carrying sticky asexual and sexual spores of Dutch elm disease to continue its spread to other uninfected elms. Graft transmission of the disease from tree to tree by roots commonly occurs in hedge-grown elms.
Control. For the professional horticulturist, the cost of preventative control on a large number of uninfected trees is uneconomical. However, high pressure injection of a systemic fungicide such as thiabendazole which travels upwards through the xylem tissues has proved successful in some cases. Selections of hybrid elms have proved to have high levels of resistance. Examples are the Dutch/French cultivar 'Lutece' with a complex parentage; the Italian cultivar 'Planio' with resistant Siberian Elm male parentage; and the American 'Princeton Elm' which has shown resistance since the 1920s. The European White Elm (Ulmus laevis) exhibits a resistance to the vector beetles, not the fungus itself.
After an estimated 80 per cent removal of the elm population in the UK since the 1970s by Dutch Elm Disease there has been an understandable lack of confidence in planting elms of any species or cultivar around the countryside, especially the English elm. However, there is a growing realization that the native elm contributes considerably to plant and animal biodiversity in the UK. For example, the elm maintains one lichen species (Orange-fruited Elm lichen) and one butterfly (the White letter Hairstreak butterfly) which are both wholly dependant on the elm.
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