Photo 6: London planetree is a favored street tree because it is relatively pest-free and highly tolerant of urban conditions. This has resulted in overplanting in some areas. Although the fungus has been reported throughout North America, Massaria has yet to become a significant, widespread issue. Photo courtesy of Elisank79/iStock.

Massaria is a very unusual and potentially important disease. The disease name, which reminds one of MRSA (the famed antibiotic-resistant flesh-destroying bacterial disease of humans) comes from the causal fungus Massaria platani. Based on reports from mycologists and pathologists, the fungus can be found in most areas in North America where Platanus species grow. So why do arborists need to be aware of the disease, and what makes it so unusual?

Features of Massaria

The pathogen is an Ascomycete fungus (a.k.a. the “sac” or “cup” fungi that most arborists know as causing many common canker, blight, wilt, foliar and other common diseases of woody plants). Indeed, Massaria does cause a branch canker (Photo 1). However, it also can aggressively decay wood. It could properly be called a canker rot, a disease type that is more commonly known from a select group of Basidiomycetes (See the TCI Magazine article on canker rots from February 2026).

Massaria canker and decay can develop and cause significant damage in relatively short periods of time – measured in one to a few growing seasons! The end result of Massaria infections can be branch failure or branch death. The decay pattern caused by Massaria is unusual. Initially, it forms a wedge-shaped pattern in sapwood that can be seen in cross-section (Photo 2). Eventually, the pattern typically flattens as the decay expands radially in the sapwood (Photo 3).

The cankers are also rather unique. They develop before the decay, almost always starting on the top of a branch and at the junction or attachment point with the main trunk or larger scaffold branches (Photo 4). The cankers are long (in most cases, many feet long) and narrow, and characteristically end in a sharp point (Photo 5). Cankers are seldom seen and rarely cause significant damage on the main trunk (Photo 4).

Another unique feature is that Massaria canker infections have little or

minimal wound wood on their margins (Photo 2). Most canker diseases usually have wound-wood formation that marks the canker margin. Massaria cankers are usually without a defining wound-wood margin, and it is common for branches to remain alive as the decay and canker progress.

Massaria eventually can decay the entire cross-section and kill the branch. Epicormic sprouting near the base of the cankers can occur in response to the girdling of branch tissues by the canker.

Maybe you can see why this could be a veritable nightmare of a disease for arborists or urban foresters managing populations of London planetrees or sycamores in urban areas, given that:

• The cankers are very difficult to see from the ground, because they develop primarily on the tops of branches (Photo 5).
• In the initial stages, the cankers produce minimal discoloration of the bark and cambium, and the bark usually remains on portions of the canker for some time before it flakes off (Photo 2).
• Even as the infection progresses and wood is being decayed, there is scant, if any, wound wood to help locate cankers.
• Even if you can see the cankers, ground inspection cannot reveal the extent of sapwood decay.
• Finally, because the decay progresses from the top of the branch down into the sapwood, significant loss of wood strength occurs even with relatively small amounts of decay, because the decay affects tension wood on the top of the branch. This progression of decay is known to result in branch failures even with relatively small amounts of decay in branches – well before they die from Massaria.

To add to the potential mayhem caused by Massaria, London planetree has been extensively planted, one might say overplanted in some cases, because of its exceptional tolerance to urban conditions. London planetree tolerates air pollution and compacted soils, and has relatively few serious insect or disease pests. As a result, there are large London-planetree populations present in many cities across North America (Photo 6), as it is a favored street, park and yard tree.

Given the rapidity with which the disease develops and its likelihood of causing branch failures, Massaria could be a significant management issue in mature populations of London planetree. Despite this, Massaria in North America seems to be mostly an oddity and not apparently the cause of widespread damage to Platanus.

Here is what is also interesting. The fungus, and to a lesser degree the disease, have been reported in North America since the 1890s! It is well documented in the mycological literature (mycology is the study of fungal taxonomy and biology) from virtually anywhere in the United States where the hosts are present and mycologists have discovered it (Farr et al., 1995). In almost all cases in the literature, it is reported from seeing dead branches or as a “weak” pathogen on drought-stressed trees (Sinclair and Lyons 2005). There is a single report from Washington, D.C. (Shear and Davidson 1936) of it causing extensive decay of sapwood following cankering. In Europe, increases in Massaria incidence and damage have been associated with periods of drought.

Massaria is clearly present in and affecting London planetree across the United States, but apparently not at the devastating levels that one might expect. In New York, and specifically New York City, the disease has been found on London planetree and is known to cause branch failures only on rare occasions. Cornell University’s Plant Disease Diagnostic Clinic has only received a few samples of the disease in the past 10 years.

Identification

Identification of Massaria in the field can initially be made based on visual symptoms, given that they are so unique. However, microscopic confirmation or isolation from infected tissues is a good idea given the apparent rarity of the disease.

For technical correctness, the fungus goes by its sexual state, Splanchnonema platani. This is the state that produces small black fruiting bodies with ascospores that are airborne when released (Photo 7).

The asexual state, (Macrodiplodiopsis desmazieresii – the updated name for asexual Massaria platani), has macroscopically similar-appearing fruiting structures in the bark that can be used for identification with a microscope. There are other microfungi that also have been commonly found in canker margins that preclude the assumption that the presence of fruiting bodies alone is definitive for identification of Massaria disease.

The bigger picture and lessons learned

In Europe, Massaria has taken a very different trajectory. The disease appears to have been spreading into and across Europe in the past couple decades or so, moving northward. In England and Germany, for example, the introduction and increased incidence of Massaria have resulted in frequent aerial inspection and pruning to reduce the potential for branch failures. This level of preemptive inspection has not occurred in the United States, given the relative rarity of branch failures.

Once again, the overplanting of any individual tree species or related species can result in management issues if new pests are introduced, or, in the case of Massaria in North America, if environmental conditions change that might result in increased incidence or severity of the disease. If Massaria disease becomes more common or damaging in North America, it could be a significant management issue for municipalities or property owners with large populations of London planetrees. In any case, arborists managing and working in Platanus species should be aware of the disease and the potential for it to result in branch failure when it is found.

Christopher J. Luley, Ph.D., is a consultant and tree pathologist in Naples, New York. He published an updated “Wood Decay Fungi Common to Urban Living Trees” in 2022. The field guide can be purchased at the International Society of Arboriculture’s bookstore at isa-arbor.com, or as an online read-only version at treerot.com.

References

Farr, D. F., G. F. Bills, G. P. Chamuris and A. Y. Rossman. 1995. Fungi on plant and plant products in the United States. APS Press, St. Paul, MN. 1252 pp.

Shear, C. L. and R. W. Davidson. 1936. The life histories of Botryosphaeria melanops and Massaria platani. Mycologia 28:476-482.

Sinclair, W. A. and H. H. Lyon. 2005. Diseases of Trees and Shrubs. Cornell University Press. Second Edition. 660 pp.