Poplar scab and it's implications for The Manchester Poplar (Native Black Poplar) 

Nigel Blandford and Joe Walsh      August 2004

Manchester Poplar Disease

Summary

Since around the summer of 2000 a virulent disease has hit the Manchester Poplar which in most cases leads to death over a three year period. The disease has been initially diagnosed as the fungus poplar scab/blight (Venturia populina/ Pollaccia elegans ) by Dr David Rose of Forest Research although it should be noted that this has not been confirmed by laboratory analysis. Secondary diseases are also thought to be involved particularly poplar leaf spot (Marssonina brunnea) and rusts although the latter is not thought to play a significant part in the death of the tree.

Concern has been increased by the fact that recent genetic analysis by Dr Fiona Cooper of Nottingham University has recently shown that the Manchester Poplar is genetically no different to the wild Native Black Poplar. This rare tree is potentially under threat if the disease spreads to the rest of the population in England and Wales.
For those urban areas where black poplar was planted as a pollution resistant tree, the landscape impact could be considerable along with the health and safety and cost implications of removing/replacing dead trees. While mostly restricted to the native black poplar, there are a few instances of it affecting Lombardy poplars.

The disease

The infection is assumed to be caused by a fungal disease called Venturia populina or its asexual stage Pollaccia elegans . A number of Venturia species cause diseases on other species of trees, the most common being apple scab. For want of a useful common name, Poplar Scab is as good as any but the Italians have termed it 'Summer Leaf Drop'.

It should be remembered that at this point V. populina is the 'best fit' but this has yet to be confirmed in the lab by Dr David Rose of Forest Research.

British tree pathology text books do not give much information on the disease, most only giving a few lines and indicating that it is of minor and of passing importance on young trees, grown close together in nursery conditions. One textbook states that the disease is almost unknown in Britain. Prior to 2000 only 2 cases had been formally identified in Britian; one in Lancashire and on in south Scotland.

American books regard it much more seriously, as a potential fatal disease of mature poplars and likewise so do the Italians. However, abroad it is principally infecting varieties and hybrids of black poplar, all of which occur in Greater Manchester but remain unaffected. It is suspected that a new strain of Venturia has either developed or arrived from abroad. However this will require further detailed investigation by Forest Research. Why it has suddenly appeared in Greater Manchester with such virulence remains a mystery. There is a strong suspicion that climate change is playing a role. It is known that 2 weeks of wet weather in spring are needed for infection and if then followed by a hot summer, conditions are perfect. Once infected the tree remains infected.

On inspection of trees at Heaton Park in Manchester by Dr David Rose of Forest Research's Tree Health Division in September 2004, other pathogens were also found on infected trees. Rust was visible but not thought to be playing a significant role in mortality. The other disease, poplar leaf spot, Marssonina brunnea could be playing a significant role but is yet unconfirmed.

Note that rust can produce very visible symptoms but generally does not lead to fatalities. Be sure that you do not base felling trees merely on a severe rust infestation.

To date the disease is almost exclusively restricted to Manchester Poplar. Although there are a few reports on Lombardy poplar and one on a hybrid poplar "Robusta".

The symptoms

Initial symptoms are hard to detect in isolated tree unless you are aware of that trees history and morphology. It is more readily detectable in groups or rows of poplars where some will appear to have a slight browning of the leaves compared to surrounding individuals and canopy leafing is reduced.

In the following spring, trees infected the previous season leaf-out but they do not produce the very dense canopy normally associated with Manchester Poplars. By late June to mid July, they can lose up to 90% of their leaves. They make a brave effort to refoliate but continue to drop leaves until by early October the infected trees have no leaves left.

New leaves initially show black lesions followed by a light brown shriveled appearance similar to brown paper. Leaf fall in the autumn is an active process with the tree forming an abscission layer at he base of the petiole (leaf stalk). Because infected leaves are killed, they cannot form an abscission layer and so hang brown on to tree for some time.

As the leading new shoots are also killed back, new growth must arise from a point lower down the twig. Infected trees begin to develop a disheveled appearance, obvious even to casual observers. The blackened drooping leading tip is a good indicator as seen on the picture above.

At some point before the actual death of the tree, it has been observed that the thick bark becomes detached from the underlying wood of the stem. Following wet weather, damp patches appear on the stem giving the appearance of a weeping canker. This is probably caused by rainwater gaining access higher up the stem and lodging in the space between the wood (the xylem) and the loose but intact bark. When pierced with a sharp implement, the rainwater flows out with a rusty red appearance. The bark is easily removed at this point and the inner bark (the phloem) presents a curious dark red stringy appearance. Once the trees die, the loose bark dries, cracks, and can be easily pulled off the tree in large sheets.

Trees that have been pollarded over the last few years succumb very quickly presumably due to their already being under stress and the new young leaves are more susceptible to infection.

It is also possible that three years of repeated stress caused by defoliation may leave the trees vulnerable to other pathogens. This may lead to infected trees falling in high winds. However, it is too early to be certain of this at this point, without further evidence.

Treatment

The related Apple Scab can be treated by a combination of pruning out infected twigs, spraying the entire tree with fungicide and removing all fallen leaves that harbour the fungus over winter and spread it to cause new infections in the spring. It is unlikely that any one of these treatments could be achieved, even on one large poplar tree.

Currently felling diseased trees is the only course of action available. As the disease runs it course and infection levels begin to tail off it is expected that a few trees may survive.

Implications of the disease

Landscape

As a landscape tree, either as a single specimen or as a clump or shelter-belt, their unique huge, round, dense crown, described as looking like a green thunder-cloud make an imposing statement and will be hard to replace. Within some parks in Manchester and other urban areas the poplar is the dominant mature tree. Their removal will have a localised landscape impact on a scale not seen since Dutch Elm Disease of the mid 1970s. The Manchester Poplar is Manchester's tree with a long cultural history (see appendix 1). What would be the outcry in London if it lost all it's London Planes?

Wildlife

We now know that the Manchester poplar is a male clone of the native black poplar. Dr Fiona Cooper of Nottingham University, using genetic techniques, has established this. The native black poplar is one of England's rarest trees. As such, Manchester is likely to have the largest population in England if not Europe or the world. Dr Cooper has established that the genetic variance in the English population is very limited. This is likely to make the remaining native black poplar in England susceptible to this disease.

Health and safety

The tree has a propensity to shed large limbs due to poor structural attachment and naturally weak wood. The disease compounds this. For this reason, not much has been planted over the last forty or so years and most Manchester Poplars have been removed from the streets of Manchester. However they still exist in many public places such as parks and cemeteries. Consideration also has to be given to where trees both in public and private ownership are adjacent to transport infrastructure. A tree or part of it coming down over road, rail or tramway presents a significant risk.

Financial

The removal of dead and dangerous poplars could place considerable financial burden on land owners who have a large native black poplar tree stock.

Recommendations

Though not at checkmate, we are probably into the end game in Greater Manchester, with little hope now of the situation improving. However, positive action can still be taken.

In Greater Manchester

• It may be prudent to contact the local media and explain what is happening before the public see the council felling hundreds of trees in public parks.

• Ensure tree contractors clean any trunks, logs of leaf and branch growth less than three years old if transported outside currently infected areas.

• Produce some good news tree planting/replacement stories to follow on from initial press announcement.

• Seek broaden the genetic base of Manchester Poplar by;

• Establishing a nursery containing a variety of origins and both sexes of native black poplar.

• Establish riverside mixed sex planting on suitable sites.

• Organise the public in tree planting events later on in the year.

• Ensure the production of a Greater Manchester Species Action Plan for Black Poplar.

• Contact park user groups and friends groups to explain the situation.

• Health and Social housing providers should be informed if it is thought that vulnerable poplars may be present on their land.

• Contact Metrolink, Network Rail and Highways Agency to inform them of the situation and distribute this information sheet to them.

• Local authorities should consider surveying their Manchester poplars in order to gauge and manage risk, and profile budgets for expected removals.

• There may be a few circumstances where private homeowners on low incomes may have infected and dangerous trees on their property. Thought should be given on how to assist this vulnerable group.

• Organise a Greater Manchester Tree Officers Group meeting and site visits.

• Those authorities that are badly hit should monitor (including photographic records) representative samples of any trees/groups that are unaffected or stabilising.

In the Region/Nationally

• Raise the issue with NW Tree Officers Group.

• Contact local authorities with large proportion of native black poplar as amenity trees these are likely to be northern mill towns and the midlands.

• Contact Native Black Poplar groups in other parts of the country and warn them of the impending danger.

• Raise the profile via press releases in trade journals such as Horticulture Week, Forestry and British Timber, The Arboricultural Association Newsletter etc.

• Encourage Forest Research to update FCIN57 in light of this disease and produce a Alert note similar to the recent on for Bleeding Canker.

• Support Forest Research in carrying out further work on the disease.

• Contact English Nature the government's nature conservation body who are trying to preserve the tree in the wild.

Further Reading

Jobling, J (1990), Poplars for wood production and amenity. Forestry Commission Bulletin 92

White, J (1993), Black Poplar: the most endangered native timber tree in Britiain. Forestry Commission Research Information Note 239

Stroughts and Winter (1994) Diagnosis of ill-health in trees. Forestry Commission Research for Amenity trees No. 2

Cooper, Jones, Watkins and Wilson (2002), Geographic distribution and genetic diversity of black poplar. Environment Agency, R & D Technical report W1-022/TR

Cooper (2003) The Black Poplar. Tree News, Issue 5 - Spring/Summer 2003,
Tree Council.

Cottrell, J (2004), Conservation of Black Poplar (Populus nigra) Forestry Commission Information Note 57

Appendix 1

The Tree

Manchester Poplar (Populus nigra betulifolia) is one of our rarest native trees. As a wild tree, it can grow to a height of 125ft (38m) tall and up to 8ft (242cm) in diameter at breast height. In Manchester, the tree rarely grows taller than 75ft (23m) with a trunk diameter of 3ft 3in (1m). The largest measured being a tree in an old church cemetery near Gore Brook, Gorton at 112cm diameter and another even larger specimen in Beech Road Park, Chorlton. It is a single sex tree with both males and females. The Manchester Poplar is exclusively male and from a single cutting e.g. they are all clones.

Outside of the Industrial North and Midlands, the tree is known as the Native Black Poplar. In the old days of industrial pollution, hardly any trees would thrive for more than a few years in the worst areas. Phillips Park (near Clayton) was very badly affected and the Parks and Cemeteries Committee found that Poplars and in particular the Native Black Poplar would thrive where few others could.

The council acquired land at the unpolluted south of the City at Carrington and opened a nursery producing trees, shrubs and herbaceous plants for the City's parks and cemeteries. Records exist showing that thousands of Poplars were being produced by the early years of the twentieth century. The Park's Handbook for this period 1917) calls the tree the Blackley Poplar. There is no explanation for the origin of this name but the Handbook shows that more of the Poplars were being sent to this area than any other part of the City, despite comments that Blackley was not as badly effected by pollution as some other areas. It is also possible that the name arose due to the first cuttings being taken from a tree growing in Blackley, perhaps in Boggart Hole Clough. By the mid thirties, the tree was being planted in industrial towns and cities the length of England but mainly in the Greater Manchester area. Textbooks of this period now give the more familiar name of Manchester Poplar.

It has long been assumed that the Manchester Poplar is not the same as the Native Black Poplar or a selected clone of it. All the trees were produced by cuttings and so will be genetically identical i.e. a clone. All Manchester Poplars are male trees - the females produce abundant fluffy white seeds that blow for large distances, can stick to washing and generally make themselves unpopular. Many of the trees planted in Manchester Parks are known to have been planted during the depression, in the late twenties and early thirties by unemployed workers on an early 'Job Creation' scheme. These men had half a wage from the government and half from the council. They where trained to take cuttings of the poplars and sent out on bicycles with bags of cuttings and an iron 'pin' to plant them in the City's parks. The Park's Committee also gave away many of the poplars to Churches and other organisations.

The results of a nation wide survey has shown that there are only a very few genetic types, probably not more than four or five and that there is no difference between a tree found in Manchester and one in Norfolk or the coast of Holland. The reason for this is probably that the tree has,for hundreds of years, been produced by cuttings rather than seeds. There may well be almost as many trees in Manchester alone as there are growing in the wild.

The tree has a propensity to shed large limbs due to poor structural attachment and naturally weak wood. For this reason it has not been widely planted over the last forty or so years and most of the trees are at maturity. Safety issues have also resulted in most Manchester Poplars being removed from the streets of Manchester. The majority of trees in the ownership of the council now reside in the city's parks and cemeteries. Several hundred grow on open land owned by the council with unknown numbers in churchyards and other private ownership.

It has long been said that the trees only have a life span of sixty or so years and that as all the Manchester trees where planted in the thirties they are coming to the end of their life. This is not the case. Trees in Manchester over 1 meter in diameter must be over one hundred years old (and are in good condition). As we now know that our tree is genetically identical to the Native Black Poplar and that, these 'wild' trees are frequently found at over 2 meters in diameter at an estimated age of over 250 years, then if the present disease had not intervened, the existing Manchester Poplars would not be even half way to their natural age limit.

In the wider context, the most worrying aspect is that if the trees in Manchester are able to succumb so quickly, with so little genetic variation and so few trees in the wild, the possibility of the Native Black Poplar becoming extinct in Britain in a very short time becomes a distinct possibility. The population of black poplar in England, Wales and the Republic of Ireland are estimated at 7000 trees. This figure is however thought to have originated from a survey of "wild trees" and planted specimens in the North West are likely to account for over 50% of the UK population.

Appendix 2

Survey of local authorities in Greater Manchester

During July and August of 2004 The Red Rose Forest initiated a survey of local authority tree officers in Greater Manchester. They were asked;

1.    Has their area been subject to poplar scab

2.    What year did you first notice the disease.

3.    To date, what percentage of your poplar tree stock has been effected by this disease.

4.    Approximately how many infected trees have you had to fell.

5.    Have any trees recovered after infection? If so what percentage?

6.    What poplars have been infected;    

7.    Are infection rates;     Still rising/Levelling off/Declining

8.    Do you consider this disease to be;     Extremely serious/Serious/A problem/ Minor problem/Nothing to worry about

9.    Are there any other comments or observations you want to make with regards to this problem.

All of Greater Manchester's local authorities had recorded the diseases with the bulk seeing it emerge between 2000-2003 but there are some observations from as early as 1995 which may or may not be Venturia.

The percentages of council poplars infected were obviously going to be a difficult question to answer. Of Greater Manchester's ten authorities, 2 could not give a figure, 2 estimated under 25%, and 3 between 25% -50% and 3 over 50% with 2 respondents saying all trees were infected. Almost 700 trees have been felled to date with many more programmed in. Six authorities stated that no trees recovered once infected, 2 thought the figure was less than 5%, one thought that most recovered and one declined to answer.

The disease is almost exclusively restricted to Manchester poplar although 4 authorities thought that some Lombardy poplars may be infected and one example sited of the hybrid "robusta" showing symptoms. Three authorities thought the infection rates were still rising, 5 thought they were leveling off, one thought they were in decline and one failed to respond. Five authorities thought the problem was extremely serious, 3 thought it was serious, one a minor problem and one failed to respond.

There were comments from three authorities, all of which were amongst the worst effected, that some trees or groups of trees, while infected were not showing the three year terminal decline exhibited by the vast majority of trees.

This survey was also sent to some authorities surrounding Greater Manchester and Merseyside. It is present in parts of Merseyside, South Lancashire and West Yorkshire.

• See also Pests/Diseases page for Poplar Scab
  

© 2003 Chris Skellern. AIE.    Home  | News | A-Z Index  | Resources  |  Contact AIE  |  Terms of Use