Plane trees (Platanus spp.) are commonly planted in urban situations because of their tolerance of water shortages and high levels of pollution. Their habit of shedding bark allows them to cast off particulate pollutants, whilst their huge, stiff leaves make them excellent shade trees. However, there are several pathogens which could be very damaging to Plane trees in the UK.
The Plant Health (Forestry) (England and Scotland) Order 2005 was amended to implement recent changes to EU legislation. The amendment replaces national emergency legislation applying to movements of plane, sweet chestnut and oak.
The UK is now a protected zone for plane wilt disease (also known as canker stain of plane), chestnut gall wasp, sweet chestnut blight, and oak processionary moth. The new legislation for plane and sweet chestnut effectively bans the movement of plane, oak and sweet chestnut plants to the UK unless they have been grown in an area free of these pests and diseases, and accompanied by a plant passport certifying this.
Plane tree wilt / canker stain of plane (Ceratocystis platani, formerly Ceratocystis fimbriata f. platani)
The ascomycete fungus Ceratocystis platani originates from the eastern United States, and causes canker stain on a range of plane species including London plane (Platanus x acerifolia) and its parents, P. orientalis and P. occidentalis.
The pathogen was accidentally introduced from the eastern United States through a number of southern European ports during World War II on infected crating material, and spread rapidly through Italy and into Switzerland. Although its progress through France was initially slower, recent reports confirm the fungus is spreading northwards at a much faster rate than in the previous decade. It was also recently reported in Greece.
C. platani is a wilt pathogen causing pronounced xylem staining, severe wilting and tree death. Staining can extend longitudinally in the sapwood at a rate of 50–100cm per year, and can reach the heartwood along the medullary rays. Infected trees exhibit sparse chlorotic foliage and sometimes sunken, elongated or lens-shaped bark cankers which can become roughened and black with age. Infection commonly occurs through fresh wounds, although transfer between trees can occur across root contacts.
For unknown reasons the disease seems to have become less significant in the United States in recent years, but in Greece and south-east France there have been serious losses of shade trees, with infected trees dying within 3–7 years. This pathogen poses a significant risk because it affects a key urban amenity tree species, and can be spread easily through the movement of infected cuttings. It also produces resilient, long-lived spores which can persist in soil and on unsterilised pruning and cutting tools.
Because the disease mainly proliferates through human activity, the spread can be limited by sourcing plant material from regions free of the disease, and by disinfecting pruning tools with alcohol. Larger agricultural equipment such as terracing machinery should be jet-washed with water to remove any contaminated soil.
Affected trees decline so markedly that it is unlikely that the disease could be overlooked, so it is unlikely that if it had reached the United Kingdom it would have remained undetected.
On behalf of the Forestry Commission, the London Tree Officers' Association (LTOA) surveyed 2,979 London plane trees in 2014 for symptoms of C. platani. Inspections were undertaken at 53 sites across 28 London boroughs. More than half of the sites surveyed included potential hosts planted during the past 10 years, and the sites ranged in size from a minimum of 20 to, in some cases, more than 200 trees. No positive findings of C. platani were detected in any of the trees inspected.
However, we would be grateful if anyone seeing trees exhibiting the symptoms above were to report it to us. See 'Reporting' below.
The response to discovery of a case in the UK would be guided by our Contingency Plan for C. platani.
Another fungus, the basidiomycete Phellinus punctatus, is also found on London planes in France and Italy, where infection results in decay. P. punctatus can cause cankers on the bark, but because these are pale buff and flat, sometimes with perennating rays, they are extremely difficult to see on the mottled bark of plane trees. The fruit bodies merge into the bark, but the presence of a narrow to broad black zone at the top of the fruit-body is diagnostic. It initially causes a rapidly spreading canker before developing an intense white rot involving both sapwood and heartwood. Affected trees have frequently suffered from stem failure. The first report of P. punctatus on plane in Britain was identified in West Sussex in March 2008.
The basidiomycete fungus Inonotus hispidus has the capacity to colonise and cause decay on planes in the UK, and can appear as rusty brown fruit bodies on the main stem. For reasons as yet unclear, in central and southern France and Germany it also forms blackish, inconspicuous, long narrow cankers, and has great health and safety significance.
A canker of this type was found on a tree in Dulwich, London, in 2006, and this appears to be the first clear evidence of a major stem canker caused by this fungus in the UK.
With changes in climate and increasing imports of planting stock, it is possible that all these pathogens could become problems to tree managers in the UK.
Apart from branch and stem failures brought about through decay, information within the arboriculture community suggests there is a clone of plane (introduced about 40 years ago) with a propensity to develop weak forks. Although discontinued some years ago, some trees of this clone appear in the marketplace. Mature trees with this problem are easy to spot, and the advice is to source nursery stock carefully.
If you think you have spotted any of these diseases, please tell us using our Tree Alert form