A phytoplasma on our pine perhaps

This odd growth on the branch of the Canary Island Pine next to our new Arid Garden is quite possibly the result of a parasitic infection by a thing called a phytoplasma (more formally Candidatus). These micro-organisms were only discovered in 1967, and are related to another group of wall-less bacteria called a mycoplasma. 

Before that, witches' brooms were assumed to be caused by viruses, fungi or mites. Some still are, so I can't confirm that our broom is of phytoplasmic origin, but on gross morphology (i.e. what the broom looks like with the naked eye) I think it might be. I also think this dead tree, standing between Guilford and Newstead, near Castlemaine, contains a dead phytoplasmic broom as well.

If you do a search for images of phytoplasma in pines, you'll see plenty of things that look like both these oddities. To check, we'd need to extract some RNA (similar to DNA) from the growth and sequence (map) it for comparison with known sequences. 

The witches' broom in the Canary Island Pine isn't killing the tree so we don't really need to know what is causing it. While phytoplasma are known to cause many diseases with major economic impact in coconuts, peaches, grapes and apples, they can also hang out more or less inconsequentially in plants such as this one. 

The phytoplasma found in many pines is called 'Candidatus Phytoplasma pini', or 'Candidatus Phytoplasma australiense' if we take the geographical naming route - but let's not get distracted by the strange world of bacterial naming and its very own Code of Nomenclature. Whatever you call it, it produces growths very like the two illustrated here, although I need to repeat that without molecular sequencing the identity of the infection can't be determined.

Phytoplasma can only live as parasites inside the plant's phloem - the pipes used to transport sugars from leaves to those places that need energy to grow - and the insects that transport them from plant to plant.

This is what they look like, highly magnified using a transmission electron microscope. Most of the circles inside the larger (phloem) cell are apparently phytoplasma - bacteria without outer walls (albeit some with dark staining borders in this picture).

The typical symptoms observed in a plant with phytoplasma in its phloem are yellowing and twisting of the needles and stems, stunting and a proliferation of small shoots and twigs (the classic 'witches' broom').

There aren't many reports of phytoplasmas in Australia but they have been described as the 'sleeping tiger' of agriculture and natural ecosystems. It's certainly possible for both examples illustrated here to be caused by a phytoplasma, but I can't exclude a virus or some other infection. Just in case the botanic garden record represents a new incursion, or is of interest in mapping the spread of the infecting agent, we'll put this on our list for testing by the biosecurity people in Agriculture Victoria.

In the meantime the tree seems healthy enough with one fully formed, and one early-stage, witches' broom. I've marked the mature, purported-phytoplasmal outgrowth in this final picture.

Thanks to Mary Stevenson for the picture of the Castlemaine potential phytoplasma, and her question about its identity which got me interested in the topic. And to Neville Walsh, as always, who suggested a possible link to the odd growth on our Canary Island Pine and help me resolve it. The microscope picture of phytoplasma in a phloem cell was copied from an open access paper by Munhoz, E.M., Pereira, T.B.C. & Bedendo, I.P., published in Scientia Agricola in 2019.