Ash Dieback – Latest News

As I drive about here in Sussex and further afield, I often notice dying ash trees and from time to time whole groups – especially of smaller semi-mature trees: the wooded downland escarpment above Eastbourne; Filching, Glynde, Jevington; even west Wales.  Judging from the large numbers,  I can only assume ash dieback is the cause.  Added to this is the stress being caused by the current hot, dry weather exasperating many diseased and weakly trees of other species.

https://www.forestry.gov.uk/chalara.  (You can also report diseased ash trees via this link).   Confirmed findings as at midday on 1st June 2018:

10-kilometre grid squares with one or more Hymenoscyphus fraxineus (Chalara) infections confirmed in the wider environment:

  2012 2013 2014 2015 2016  2017  2018  Total % of all 10k squares in country
Scotland 7 5 33 125 10 27 12 219 19.9%
England 82 60 161 224 310 86 29 953 64.8%
Wales 0 2 5 31 96 64 13 211 79.6%
N Ireland 0 0 0 0 17 0 0 17 9.1
Isle of Man  0 0 0 1 1 0 1 7.1
UK (total) 89 67 199 380 433 178 54  1401 46.2%

 

The progression of numbers and appearance of new grid squares over time should not be interpreted as an indication of the rate of spread of the disease. It only indicates when infection sites were found, not when the fungus first arrived at the site, which in many cases cannot be known.

Symptoms

 

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Because ash trees have many genetic variants and occur right across the UK, they come into leaf at different times. Ash is also one of the last tree species to flush, sometimes taking as long as six weeks to do so, and can often occur as late as the end of May. Trees in the colder north flush later than trees in the warmer south. Some ash trees will break-bud, or flush, earlier than others, and some buds will produce flowers rather than new shoots. Some variation will be more apparent in older trees.

Some shoots on ash trees will fail to flush altogether, while others will flush normally before showing signs of ill-health or dieback later. These events might mean that the trees are damaged in some way, but shoot death and dieback in ash trees can have a number of causes.

So if an ash tree does not have any leaves on it in April and May, it does not necessarily mean that it is diseased or dying, but by mid-June all healthy ash should be in full leaf.

Mid- to late summer (August and September) is a good time of year to undertake surveys, because once autumn has begun, visual symptoms can be confused with leaves that are naturally changing colour.

ASH KEYS (Fraxinus excelsior)

In the autumn you might see clumps of sometimes dark-coloured ash keys (seeds), retained on the trees after the leaves have fallen. This is quite normal, but from a distance they can be mistaken for the blackened leaves which can be a symptom of the disease.

Reporting suspected cases

Tree Alert icon If you think you have spotted the disease, please check our symptoms section before reporting it using Tree Alert or one of the Further Information contact points below.

The science

Chalara is caused by a fungus called Hymenoscyphus fraxineus. This fungus has two phases to its life-cycle: sexual and asexual. The asexual stage, which grows in affected trees, attacking the bark and girdling twigs and branches, was the first to be described by science, and was originally called Chalara fraxinea. This gave rise to the common name of the disease which it causes.

The sexual, reproductive stage, which was only discovered some years later, occurs as tiny, mushroom-like fruiting bodies on infected rachises, or stalks, of the previous year’s fallen leaves. Infective spores from these fruiting bodies are spread by the wind on to the leaves of healthy trees in summer. This sexual stage was initially called Hymenoscyphus pseudoalbidus (H. pseudoalbidus) before a taxonomic revision suggested the name should be Hymenoscyphus fraxineus. The International Botanical Congress has also agreed that a single fungus should have only one name, even if different stages of the organism have previously been given separate names. Therefore Hymenoscyphus fraxineus is now widely accepted as the name to use.

Government scientists have set out the most up-to-date understanding of the disease. Their assessment concluded that:

  • the spores are unlikely to survive for more than a few days;
  • spore dispersal on the wind is possible from mainland Europe;
  • trees need a high dose of spores to become infected;
  • spores are produced from infected dead leaves during June to September;
  • there is a low probability of dispersal on clothing or animals and birds;
  • the disease will attack any species of ash;
  • the disease becomes obvious within months rather than years;
  • wood products would not spread the disease if treated properly;
  • once infected, trees cannot be cured; and
  • not all trees die of the infection – some are likely to have genetic factors which give them tolerance of, or resistance to, the disease.

Preliminary results from testing of selected fungicides for treating ash trees with Chalara dieback of ash for their efficacy in laboratory tests and field trials are now available on the Defra website.

Origins

The fungus is believed to have originated in Asia, where Asian species of ash are able to tolerate it after of thousands of years of co-evolution.

It entered Britain on ash plants imported from nurseries in continental Europe. However, now that infected, older trees have been found in South-East England with no apparent association with plants supplied by nurseries, it is thought possible that it also entered by natural means. These include being carried on the wind or on birds coming across the North Sea and English Channel, or on items such as footwear, clothing or vehicles of people who had been in infected sites in Continental Europe.

Video: history of the pathogen.

Last updated: 5th June 2018

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