The disease in the UK and elsewhere
Plane trees (Platanus x hispanica) are commonly used in amenity plantings, and are a particularly important feature of many London streets and parks. One reason for their popularity is their tolerance to conditions typically found in an inner city environment, namely soil compaction, restricted root growth, drought, intensive management regimes and pollution.
Recently (2009 to present), arboriculturalists working within the Royal Parks in London began noticing large lesions on the upper sides of branches associated with branch drop. This caused obvious concern from a health and safety point of view and samples were sent off for analysis. Dutch scientists managed to isolate the fungus Splanchnonema platani, formerly called Massaria platani, from the lesions (below) although it is not yet clear whether this is the primary cause of the branch drop.
S. platani is usually considered to be a weak parasite causing only minor damage such as twig dieback in warmer Mediterranean climates.
However, in recent years it has been found associated with branch death and rapid decay within other parts of Europe, most notably Germany and Austria, the Netherlands, and parts of France.
It has been found on fallen twigs and small branches within England – Kew in 2003, Jersey in 2008 and Darlington in 2009, but until now had not caused any major problems in the UK.
The situation is different however in the southern United States where damage has been reported to be greater and more widespread.
The fungus is usually diagnosed on the basis of its very characteristic brown, multi-septate pycnidiospores (40-50 μm long x 12.5 to 17.5 μm wide) or ascospores (67-75 x 17.5 to 20μm).
In Europe the fungus seems to sporulate abundantly on the surface of the bark or cambium surrounding the lesion, and spores are easily seen under the microscope.
Spores seem to be more difficult to find on UK lesions although we have managed to isolate what appear to be S. platani spores from lesions from London and Bristol in 2011, and have used molecular methods based on sequencing regions of the ribosomal DNA to confirm this diagnosis.
During the molecular analysis other fungi, for example common basidiomycete decay fungi, were also found, and are possibly the cause of the extensive decay.
At this stage although S. platani is definitely present on at least some of the plane trees exhibiting branch drop, we are keen to obtain more, fresh samples of lesions from plane trees to allow us to:
- Compare our cultures with those from across Europe. This would give us a better idea of whether the S. platani isolates found within the UK lesions are actually native, and maybe are becoming more pathogenic because of other factors, (e.g. climate change or changes in management), or whether the fungus has been introduced from Europe (e.g. on un-sterilised tools, infected planting stock, or simply on the wind).
- Carry out Koch’s postulates to establish the cause of the disease and discover if S. platani is the primary cause of the lesions, or if the lesions are the result of subsequent infection with opportunistic basidiomycetes such as Auricularia spp.
More information on this subject is available in:
Tubby, K. V. and Rose, D. R. (2009). Problems facing plane trees. Arboriculture Association Newsletter. 144, Spring. 18–19.