Threats to the plane tree in the UK

In the UK, the plane tree (Platanus x hispanica) has been, and perhaps still is, a very popular choice of tree for urban landscapes. In London, the recent i-Tree report suggested that up to 4% of Inner London’s tree population of 1,587,000 trees are plane (meaning 63,480 are plane trees), though in terms of leaf area is provides as much as 8.9%. In this sense, it’s evidently a tree that is usually found to be quite large, and this landscape dominance makes it a very prominent feature as well as quite a common one (for a larger tree, where feasible planting space is automatically more limited). However, the ability to clonally propagate plane, in addition to the report’s findings that 21% of London’s trees are clones, we can assume that at least 13,331 planes are genetically not unique (perhaps more, as many planes are assumed to the ‘Pyramidalis’ cultivar, though other cultivars may also be found and thus raise the total level of ‘clonal-ness’ amongst plane meta-populations). This presents problems, as such a lack of genetic diversity means pests and diseases can readily sweep through clonal stands, as there is no inherent level of variable resistance in the population. Beyond London, they also feature quite prominantly in some of the new towns, and particularly along main vehicle routes and within urban parks.

Introductory spiel aside, my remarks on pest and disease are quite pertinent to this post, which will look at a recent article published by Tubby & Perez-Sierra in the Arboricultural Journal, which looks into the current status of plane and those pests and diseases that may soon put the species at risk, within the UK. For those that get the print journal (like myself), then you may have already read this, though it’s got some great information and thus is a good one to share with an audience far beyond those who get (or have access to) the journal.

Arguably the most prominent aggressor of plane in recent years is what we know as Massaria (Splanchnonema platani). Despite this fungus being considered a “weak pathogen”, where it will ‘aid’ (in an adverse sense) with twig abscission and cause small cankers upon minor branches, it has been increasingly observed, across central and western Europe, to cause decay and dieback within larger-diameter branches at the upper branch crotch and along the upper side of the branch, and notably in the lower crown where branch diameter is between 100-200mm. The dieback has the potential to cause very wide decay strips, with potentially around 30% of the entirety of a branch’s circumference being damaged by the fungus (which induces a soft rot). Such damage will induce symptoms of decline within the affected branch (which may be highly discernible if the branch is large and bears a lot of foliage), as will cambial dieback create lesions that are evident from above the crotch (a pinkish-orange colour). Failure can occur in as little as three months following on from the onset of decay. Of course, as most plane trees are present within the urban environment in the south east of England (and are generally large in size), which incidentally is also the most densely-populated area of the UK, there is an evident risk to public safety. For this reason (amongst others), the LTOA (London Tree Officers Association) published guidance quite recently on the subject.

Cross-sections of the decay associated with Massaria. Source: LTOA.

Beyond Massaria, which is indeed present within the UK, we can also observe how plane canker (Ceratocystis platani) may be a very possible threat in the near future. Whilst it is a fungus not present within the UK right now, courtesy of transportation over from the US during WWII, it is indeed present on the continent, and ‘uses’ humans as its primary vector across the landscape. In both urban and forest-borne planes, in countries such as France and Switzerland, this fungus causes marked xylem staining (extending up to as much as 2-2.5m a year), thereby induces wilting, and may even cause the death of its host plane tree as a result. Because its spores may remain ‘active’ for long periods of time before finding a host to infect (usually following wounding), chainsaws (and other equipment), boots, and even the transporation of soil may enable it to spread over vast distances. For this reason, infected trees are felled and disposed of on site, and all equipment used is thoroughly cleaned (or replaced) afterwards. Such a rigorous sanitation measure is, unfortunately, necessary, and therefore its emergence in the UK could be very damaging and costly.

Remaining with fungal decayers, Fomitiporia punctata (syn. Phellinus punctatus) also has the potential to significantly infect planes within the UK (it can already be seen colonising some planes in the UK, however). On the continent it may indeed use the plane as a host species, and when it does it creates rather non-distinct sporophores (brown ‘splodges’) that can easily be missed. The fungus causes a very significant white rot of the sapwood and (depending upon host species, false-)heartwood region of its host, and therefore in the urban environment such decay may prove to be significant in terms of the level of risk posed by the host tree.

F. punctata on Quercus sp. Source: NHGS.

Similarly, whilst Inonotus hispidus may already be found colonising plane within the UK, a species of the same genus known as Inonotus rickii may be another potential fungal pathogen. Native to the tropics, it is currently in Italy, where it induces a white rot upon its host and may colonise, like Inonotus hispidus, through open wounds. The authors do note, however, that climatic differences between the UK and Italy may mean its emergence in the UK may not be for a fair few years, though it is likely to succeed at some point (and particularly in urban areas where it is warmer, courtesy of the urban heat island effect – and this is where most plane trees reside). Other wood-decay fungi that may use the plane as a host include the resident Ganoderma species (G. australe, G. lipsiense, and G. resinaceum), and also Perenniporia fraxinea.

A type of powdery mildew, with the scientific name of Erysiphe platani, is a further potential pathogen of plane. In fact, it was found in the UK in 1983, though does not appear to be at all significant at this moment in time (if it is even still here – it may have been eradicated?), and is instead far more significant as a pathogen in countries such as Spain. Much like other powdery mildews, it affects the foliage and causes leaf deformation, though may also impact upon the amenity value of infected plane trees. If, like oak mildew, there are also marked impacts to the health of the tree over the long-term, then this powdery mildew of plane may have adverse consequences in that regard as well.

Sticking with the leaf, a more evident problem for plane trees is anthracnose (Apiognomonia veneta). Peronal observation of this leaf pathogen (that over-winters on twigs, where it may blight them during milder years) is that it can be very readily observed in larger plane trees, where it can cause extensive leaf dieback (to the point that members of the public are concerned enough to report it). In younger trees, the entire foliage crown may prematurely be lost, and a second one formed. Such a loss of foliage (in plane trees of all sizes) has an adverse impact upon tree energy levels, and like Cameraria ohridella in horse chestnut, is far more than an amenity problem.

Again upon the leaf, though this time a pest and not a pathogen, the sycamore lace bug (Corythuca ciliata), since 2006, has been a UK problem. Adults will feed on the underside of plane leaves, which will cause the upper surface of the leaf to become minutely-dotted in white. In time, as feeding continues, the leaf may eventually be evidently chlorotic or bronzed, after which time the leaves will prematurely abscise. Again, this is far more than an amenity issue, and heavy infestations of this lace bug can play a role in the death of a plane tree (particularly when combined with other plane pests and pathogens).

The lace bug’s foliar symptoms. Source: SDIS.

From the source article, it is certainly evident that there are many looming (and some already present) threats to the plane tree in the UK. Whilst the plane doesn’t necessarily have as much direct ecological value as many other tree species found in the UK (notably native ones), it does have a substantial impact upon airborne pollutants, and its hardy nature means it can thrive where other trees species may suffer beyond belief. Therefore, if it does begin to succumb more readily to pathogens discussed here, then there may be problems with regards to its longevity and abundance. As many of our plane trees are urban-based, will risk management be a principal driver behind its decline? Only time will tell, though as always, protecting and safeguarding ecosystems from invasive pests and pathogens is absolutely critical and, as an island, the UK is (theoretically) greatly-poised to control the entry of these organisms. Of course, I say theoretically, as a control program is only as effective as its weakest link.

Source: Tubby, K & Perez-Sierra, A. (2015) Pests and pathogen threats to plane (Platanus) in Britain. Arboricultural Journal. 37 (2). p85-98.

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