Tuberculina/Helicobasidium: A fungal Jekyll and Hyde

By Nathan Smith

Taxonomy can often present itself as fixed fact; a sturdy rock in the uncertain storm of science. However this is not always the case, especially in understudied groups such as fungi. For example, Tuberculina a genus of fungus that parasitizes rust fungi. Rusts, such as Coffee leaf rust, Asian soybean rust, and wheat stem rust, are plant pathogens with major economic impact and Tuberculina was seen as a potential biocontrol agent for their management.

Jekyll and Hyde, or Helicobasidium and Tuberculina?

Jekyll and Hyde, or Helicobasidium and Tuberculina?

Helicobasidium, on the other hand, is responsible for violet root rot, causing root rot, yellowing, and in extreme cases death of the host. It has a wide host range including apple, sugar beet, soybean, potato, cotton, peanuts, tea, plum, grape, and carrot. More than 24% of planted acres of sugar beet in the USA have economic damage caused by violet root rot with the losses being as high as 50%.

It appears then that Tuberculina is a genus that can be used beneficially and should be encouraged in crop fields whereas Helicobasidium should be controlled against and excluded where possible. There’s just one problem: they are the same genus.

These ostensibly separate genera actually represent different stages in the fungal life cycle. Tuberculina and Helicobasidium samples were found to have morphological and genetic similarity. Most importantly, inoculation of a host with Helicobasidium spores was capable of causing a Tuberculina infection.

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Helicobasidium on a carrot (credit Rasbak)

Tuberculina is proposed to form an amplification stage, where the fungus produces large amounts of genetically identical conidia. The fungus then enters dormancy and the Helicobasidium stage where sexual reproduction takes place, allowing the fungus to remain genetically diverse.

That Tuberculina and Helicobasidium are one and the same is strong evidence for the argument against the use of the fungus as a biocontrol. However, for some Tuberculina species, an equivalent Helicobasidium-stage could not be found. It’s possible that some Tuberculina could have completely abandoned the sexual Helicobasidium stage. If this is the case, Tuberculina may still have potential as a biocontrol agent, although this would require extreme caution.

Fungi are critically under-studied as a kingdom and basic research into their various life-cycles is much needed if we are to effectively control fungal diseases and manipulate fungi for our own benefit. The Jekyll and Hyde characteristics of Tuberculina/Helicobasidium show this clearly and, without fundamental fungal research, we could all too easily still be supporting the traitor in our midst.

See the original paper on Tuberculina and Helicobasidium here.

All I want fir Christmas

By Toby McMaster

The living room’s most popular festive feature, the Fraser fir, has no immunity against the phytophthora root pathogen, infamous amongst plant pathologists, with its greek name meaning ‘plant destroyer’. Professor John Frampton of North Carolina State University is out to change that.

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Credit: Teresa Sikora

A species of phytophthora caused the potato blight of the great Irish famine and no variant of the Fraser fir has ever been found to be even slightly resistant to the disease. However help is at hand from the Fraser’s Japanese cousin, the Momfi fir, which makes an awful Christmas tree itself but is incredibly resistant to phytophthora.

Fraser firs can be grafted onto the roots of a Momfi, to produce a chimera with the best of both worlds: resistant roots and the beautiful Christmassy look of a Fraser. Frampton’s lab is attempting to find genetic sequences encoding root-rot resistance but this is a slow process, genetic modification of Christmas trees is likely to be a thing of the distant future. Even when good trees are found and researchers cross them it takes firs 10 to 12 years before they can reproduce and after this another 6 or so to judge the quality of the resulting progeny.

However eventually researchers hope to get to the root of the problem.

Ash Dieback: What’s causing the rise in plant diseases?

by Nathan Smith

Ash Dieback is the latest craze sweeping the nation, and by ‘craze’ I of course mean ‘plant disease’. Like Dutch Elm disease before it, it threatens to destroy thousands of iconic trees and restructure the shape of British woodland.

Caused by the fungus Chalara Fraxinea, Ash Dieback was first noticed in Poland in 1992, though it is thought to have originated somewhere in Asia. It affects the crown of the tree (the bushy top bit) and causes it to die back, although it may not kill a mature tree for a number of years. Even then it is often the case that when a tree is killed it is through an opportunistic infection. C. Fraxinea may not kill, but it does significantly weaken the tree.

AshAttributedSo what’s to blame? Despite fears that the fungus may have come to the UK via infected plants in nurseries, the current view is that it came in the wind from Europe (at least in the majority of cases of the disease). Whilst this may seem good news (particularly for the people running the nurseries), it causes us to reach a depressing conclusionwe cannot ‘stop’ the disease. The reasoning behind this is that most plant diseases can be controlled in the early stages of an outbreak via selective removal of plants. These techniques will probably not work now: we are in the middle of a full blown Europe-wide pandemic and even if we could remove the disease from the UK, it could still come back on the wind from across the seas. This may all sound rather despairing, and work is being done to try and reduce the ecological damage, but the truth is the models still predict that in 10-20 years time the majority of ash trees will be infected, if not already dead.

In a world of increasing globalisation, ensuring plant security from biological threats is almost impossible. Whether from the soil on a backpacker’s shoe or on a tree imported from afar, new microbes will always be brought into environments they have not come across before. Ash Dieback is not the first major tree disease to affect the UK and it’s probably safe to say it will not be the last.

For all those interested in some light reading, please find the link for the Government’s plan on tackling Ash Dieback: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69620/pb13843-chalara-control-plan-121206.pdf