by Eric Boa
Cool, calm and green. Residential area in Bogotá.
I first came across Fraxinus uhdei in Colombia in 1998. It’s a species of ash tree from Mexico, known in Colombia as urapán. Introduced around 70 years ago, urapán has been widely planted in Andean cities in Colombia, particularly Bogotá. It is also widely present in rural areas, a result of natural dispersal. Mature trees grow up to around 25 metres tall and have a commanding presence. Urban trees do many things: they soften concrete jungles, mute the cacophony of traffic and human activity, and prettify drab vistas of buildings and roads. The choice of urban trees is often restricted to tried and trusted hardy species, many introduced, such as plane trees across Europe and jacaranda in warmer climes.
In the late 1990s reports emerged of a highly damaging problem on urapán. Studies into the cause, led by Liliana Franco and colleagues at the Universidad Militar Nueva Grenada in Colombia for more than 20 years, have produced intriguing insights on introduced and native trees. The urapán disease appears to have come from the United States, where ash yellows disease was first recorded and mainly affects white ash, a widely cultivated native species. All the current evidence suggests that ash yellows in Colombia was introduced on infected urapánes from the US.
Change ‘was introduced’ to ‘migrated’ and the argument strengthens in favour of planting native trees, bolstered by a touch of botanical nativism. But it’s not so straightforward, as European readers will be aware. The common ash native to Europe, F. excelsior, has been devastated by a fungus first recorded in Poland in 1992, with origins linked to Japan and east Russia. Ash dieback arrived in the UK relatively late, probably in the second half of the 2000s. Planting native trees will minimize but not eliminate pest and disease outbreaks. Good phytosanitary measures and timely surveillance will reduce the risk of new pest and disease introductions but never avoid them entirely.
Still, it appears preferable to plant native trees. They support native wildlife and are well adapted to local conditions. They have been exposed to native pests and diseases, co-evolving and developing strategies to minimize damage. But there are flaws to this argument, too, as witnessed by ash yellows: native species, native disease. I worked latterly on a long-term clove disease project in Indonesia. A highly damaging disease affecting farmers in Sumatra and Java doesn’t occur on the Maluku Islands, also part of Indonesia, where clove originates. No chance to co-evolve. All the evidence suggested that the disease had come from plants related to clove in natural forests next to clove plantations. A tiny sucking insect provided the bridge. Again, native tree, native disease, even if the Maluku Islands are several thousand kilometres from Sumatra and Java and separated by lots of sea.
In the move towards promoting native trees the obvious question is why plant urapán in the first place when Colombia has a host of its own fine tree species? One answer is that native trees are often unproven when cultivated. They may also be slow-growing and urban planners need something to plant that grows fast. Yes, it’s short termism, but I doubt much thought has gone into selecting urban trees in many cities. The calamity of ash yellows, at least in Bogotá, has had a belated effect. Trials showed that a native oak species, Quercus humboldtii, also known as the Colombian or Andean oak, was a suitable alternative. Urapán is out of favour, even though many trees remain in place in varying degrees of disrepair – or simply dead. Few trees have been cut down, a costly undertaking riven with practical difficulties. Bogotá drivers are impatient and unsympathetic to further delays. I don’t blame them.
And then came the phytoplasma. Declining and dead Urapánes in Bogotá
Q. humboldtii is simply called roble in Colombia. It’s the only native oak and was named by a French botanist, Aimé Jacques Alexandre Bonpland in honour of his travelling companion, Alexander Von Humboldt, a man of insatiable curiosity and intellectual verve*; the original polymath’s polymath. Bonpland is the forgotten half of a formidable duo. Bonpland collected and described many new plants, including Q. humboldtii, during travels and explorations of the natural history, geology and peoples of a region from Mexico to Peru.
Bogotá city authorities started to plant roble about 30 years ago following successful trials. There are now around 30,000 trees in the city. It’s an attractive tree with large acorns and a slightly fuller crown than urapán. What could possibly go wrong? My colleague Liliana became intrigued by the unusual appearance of urban trees other than urapán. Ash yellows is caused by an unusual group of bacteria known as phytoplasmas. They are spread by insects and in planting material, but not in seed. They disrupt the growth of plants in all sorts of strange ways: unusual development of branches, altered shapes of leaves, small leaves and condensed bunches of branches known as witches’ broom. Growth is reduced and ultimately, as with urapán, some hosts are so weakened that they die.
Manuela, Liliana and Daniel process oak samples in Málaga, Santander (the alcohol is for the insects - honest)
Powerful lab techniques help to confirm the presence and nature of phytoplasmas yet old-fashioned observational science still plays an important part in diagnosis. Liliana and her research team have done a great job in characterizing phytoplasmas associated with symptoms in non-urapán hosts. The list of other urban tree species continues to grow and includes not only introduced species, such as black elder (similar to the elder found in Europe), but also a native willow, Salix humboldtiana, known in Colombia as simply sauce. The species name was given by another botanist, adding to the huge number of eponymous labels that celebrate Humboldt’s achievements.
So that’s another native tree affected by a phytoplasma. Highly damaging, too, and readily spread in cuttings which are quick to establish. It seems almost inevitable to announce that roble also has phytoplasma-associated symptoms, confirmed in the laboratory, and seen in Bogotá. Roble is an endangered species and there are protected areas of natural populations – I hesitate to call them ‘forests’ since the areas I’ve seen are more like patches – in several regions of Colombia. I’ve been on two trips with Liliana, accompanied by her students Manuela and Daniel, looking to see whether phytoplasma symptoms occur in natural populations. And yes, not only have we seen suspicious symptoms, but major damage occurring in mature trees.
There are several intriguing parts of the disease jigsaw puzzle that we are trying to figure out. Are native oaks (and willow for that matter) a source of phytoplasmas or a destination? There are at least two known groups of phytoplasmas affecting trees (and crops – but let’s not go there). DNA analysis will help to establish relationships and possible origins. The role of insect vectors is being elucidated, providing further clues as to how disease is being moved around, including the discovery that a symptomless and widespread grass from East Africa (Kikuyu grass) harbours phytoplasmas.
We’re still puzzled – and amazed – at how ash yellows has got into remote regions of the Andes, far away from the original outbreak of ash yellows in Bogotá. It’s more than likely that the earliest symptoms were missed so it’s difficult to be certain of when phytoplasma diseases first occurred. I should add that the initial response to ash yellows was that it was an insect problem, delaying action to address the disease. But we’re beyond that now and while we relish the scientific challenges it’s vital to think of the practical consequences of the wealth of knowledge generated by the efforts of Liliana and her team.
There’s a need to explain phytoplasma symptoms and how to distinguish them from adverse growing conditions. I watched the at times puzzled response of Gustavo, a farmer hugely knowledgeable about trees, as Liliana and I attempted to enlighten him about phytoplasmas. We’re planning a visual guide that people can use at leisure, with web access too. There’s little that can be done to control the phytoplasma diseases, but we can help devise strategies to minimize future risks and figure out what to do with the urapánes that are beyond saving. There’s no immediate danger that the oak forests are going to disappear, though there is an undoubted threat to add to agricultural encroachment. The viability of small patches is uncertain and natural regeneration of roble is poor, for reasons not known.
Stop planting Kikuyu grass is another way forward, though we’d have to find suitable alternatives. Above all else, it’s important that we share what we know about these diseases that have affected introduced and native trees. They are of deep scientific fascination, but this counts for nothing unless the practical consequences of discoveries are fully explained to urban planners, city authorities and the folk who admire the trees they see every day.
* Further reading
Andrea Wulf (2015): The invention of nature: Alexander von Humboldt’s new world. A wonderful and wide ranging book.
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