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.
In 2009 Royal Parks’ arboricultural teams in London began noticing large lesions on branches during their standard pruning operations. Similar symptoms had been seen in the past few years on plane trees within mainland Europe, most notably Germany and Austria, the Netherlands, and parts of France.
Spores of the fungus Splanchnonema platani, formerly called Massaria platani, were found associated with the lesions, 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.
It was found on twigs in Kew gardens in 2003, and again in Jersey in 2008, but until now had not caused any major problems in the UK.
The situation seems to be different 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.