Pest & Disease Management - Ash dieback

Ash dieback is caused primarily by the fungal disease Chalara fraxinea. The number of infected trees in Europe has risen over the past few years, partly due to wet summer weather.

Symptoms of ash dieback - image: Dove Associates
Symptoms of ash dieback - image: Dove Associates

Affected trees have been found in parks, gardens, garden centres, forest stands and commercial nurseries. The disease has contributed to the obliteration of around 90 per cent of Denmark's native ash tree population.

Ash trees were first recorded dying in large numbers from this form of ash dieback in Poland in 1992. It has since spread rapidly to other European countries. It took until 2006 before the fungus’s asexual stage, Chalara fraxinea, was first discussed as the potential culprit and in 2010 its sexual stage, Hymenoscyphus pseudo-albidus, was identified.

As a result of a pest-risk analysis and the number of confirmed cases on young imported trees as well as established trees throughout the UK, legislation was enforced on 29 October 2012 restricting all ash imports (plants and seeds) into the UK and movement within the country until sites have been officially inspected and given disease-free status. Northern Ireland and the Republic of Ireland also passed similar legislation on 29 October 2012.

How to recognise it

Early stages of infection can lead to leaf drop. Late-stage infection produces crown dieback, which can ultimately lead to the death of the tree. Chalara fraxinea has also been isolated from the roots of symptomatic trees.

There are other disorders of ash trees that may be mistaken for Chalara fraxinea infection, including:

• "Ash dieback" in which mature trees show severe crown dieback with new shoots being produced lower down on the plant. This "dieback" is usually associated with root disturbance on trees growing in arable situations.
• Stem cankers from infections by Nectria galligena and feeding damage from the bark beetle Leperisinus varius, which both cause bark damage and dieback. Ash trees weakened by infections may be more prone to bark-boring beetles.
• The activities of the ash bud moth (Prays fraxinella) in spring — moth larvae mine into the base of shoots causing them to wilt and die.
• Physical issues including frost and drought can cause similar damage.

Symptoms

Damage can occur on all plant parts, particularly leaves, stems and large branches. At high infection levels, the crown of an affected tree can die out and shoot regrowth can be seen further down the plant as it tries to stay alive. Brown/black spotting can be seen on leaves along with wilt symptoms — damage usually starts from the leaf tip downwards.

Small brown spots can be found on stems and branches (typically around the stem base) that extend to form large cankers leading to dieback. Wood is stained brown or grey under the canker sections and beyond into what can look like good wood.

Treatment: biological control

No specific biological controls have been identified yet.

Treatment: cultural control

Fraxinus excelsior has been found to be the most commonly affected species, with F. ‘Pendula’ and F. ‘Jaspidea’ being particularly susceptible. Also included in that level of susceptibility are
F. mandshurica, F. nigra and F. angustifolia. Ongoing work in Denmark and Germany suggests
that F. ‘Altena’ is more tolerant of the disease and further promising results have been obtained in trials with F. americana and F. pennsylvanica. It is currently thought that F. bungeana, F. ornus, F. sieboldiana and F. texensis are also unaffected.

Researchers are still unsure of the status of F. caroliniana, F. latifolia and F. velutina.
Young plants are more vulnerable to this disease than mature specimens. However, larger plants can be infected but symptoms tend to show through more slowly. It is unclear how the disease spreads but local spread may be from rain splash or from insect feeding.

Long-distance spread is most likely due to the movement of diseased ash plants and the movement of logs or wood from infected trees.

Defra’s options for destroying leaf litter — the preferred choice is listed first — include:

1    Burning material on site either on the ground (small volumes) or in mobile incinerators (large volumes).
2    Burial in the ground (householders only).
3    Composting. Work is ongoing into whether the spores from Chalara fraxinea will be effectively destroyed during the composting process. In-vessel composting may provide a more secure method because of the heat generated.
4    Incineration or landfill off site.
5    Composting off site.

Treatment: chemical control

Work in Germany has shown that products applied preventatively may have some control effect in the spring months.  Products applied after plants have been infected will provide little control – applications need to start in spring for optimum effect.  Application before leaf fall will also help in breaking the cycle of infection in the following spring.  FERA are carrying out laboratory-based fungicide testing using 16 different active ingredients shortlisted from suggestions made by companies and individuals.

The statutory instrument laid before Parliament can be downloaded at www.dovebugs.co.uk/ashnotice.doc. Any symptoms must be reported to one of the following:

• The Forest Research Disease Diagnostic Advisory Service
Tel: 01420 23000. Email: ddas.ah@forestry.gsi.gov.uk.
• The Forestry Commission
Plant Health Service
Tel: 0131 314 6414. Email: plant.health@forestry.gsi.gov.uk.
• FERA Plant Health &
Seeds Inspectorate
Tel: 01904 465625. Email: planthealth.info@fera.gsi.gov.uk.

FERA has produced videos on disease symptoms and a brief history of the disease:



Fully updated by Dove Associates.

Use plant protection products safely. Always read the label and product information before use.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.

For pest and disease alerts, fact sheets and management, see www.HorticultureWeek.co.uk/pests.


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