Chestnut blight is a devastating forestry pathogen. It forms cankers that girdle and eventually kill chestnut trees. The disease was introduced to North America towards the end of the nineteenth century and resulted in more than four billion tree deaths, bringing the American chestnut (Castanea dentata) close to extinction.
Science fiction is full of strange biological manipulations: engineered plagues, living technologies, ecological rewrites, symbionts, and terraforming. The idea that you could cure a disease by infecting the disease itself, sounds like something that H G Wells might have used as a plot device. In forests across Europe, something very much like this is already happening.
Sometimes the best way to stop a disease is to give it another disease. Plant pathologists discovered that you can weaken the fungus responsible for chestnut blight by infecting it with a virus. This virus can then be spread between fungal individuals when their hyphae fuse, a process a bit like passing an STD. This is a real biological control strategy that has been used in Europe for decades.
Humans have developed fascinating ways to fight pathogens. Vaccinations train the immune system to recognise invaders. Selective toxicity uses substances more harmful to the pathogen than the host, such as mercury, historically used to treat syphilis. Antibiotics originate from chemicals produced over hundreds of millions of years in the ongoing war between bacteria and fungi, and we now utilise them to fight infections.
Plants face their own pathogens. The methods we use to fight plant diseases are equally fascinating. One of my favourites is that you can infect a fungus with a virus, which weakens it. You then spread the infected fungus, which passes the virus on to other fungi.
Fungi grow and spread via hyphae, and when the hyphae of two different individuals meet, they can fuse and exchange fluids. As with human intimacy, the fluids can contain viruses. This is not the same as true sexual reproduction, since there is no exchange of gametes (eggs and sperm, eggs and pollen, or similar). However, fusion allows cytoplasm and genetic material to mix, and the process is often described as parasexual. Cryphonectria parasitica, the fungus that causes chestnut blight, does this direct fusion and fluid exchange.
As with human mating, intimate contact offers an excellent opportunity for diseases to pass directly between fungal individuals (think mycelial networks, not mushrooms). Hence the title of this column: an STD could save your (chest)nuts. One such fungus-infecting-virus is Cryphonectria hypovirus 1 (CHV1). Infection with CHV1 (the “STD”) reduces the virulence of the chestnut blight fungus, leading to milder symptoms and less severe disease.
The process of giving a fungus an STD (on purpose) to save your chest(nuts) works like this: (1) you have a tree dying from chestnut blight; (2) you introduce a second strain of chestnut blight that carries the STD; (3) the two fungal strains engage in…ahem ... parasexual behaviour in the tree, passing on the STD; (4) the original chestnut blight fungus is weakened by the STD, giving the tree a chance to fight back.
In addition to its devastating impacts in North America, several introduction events have occurred in Europe, from both Asia and North America, infecting the European chestnut (Castanea sativa). The damage has been less catastrophic than in North America, but the disease still threatens chestnut populations, ecosystems, and livelihoods that depend on them.
In 1978, Grente and Berthelay-Sauret demonstrated that if an infected chestnut canker was re-inoculated with chestnut blight carrying CHV1, the canker would heal. This experiment showed chestnut blight strains infected with our “STD” mycovirus were weakened and could be used to fight the disease. The infected tree gains the chance to overcome the infection and recover. Even better, mycoviruses can also spread through fungal spores. This allows their beneficial effects to move beyond individual treated trees. Chestnut blight isolates infected with the STD have been used successfully in southern France as biological control agents for more than forty years.
Plant scientists, foresters, agronomists and gardeners often use solutions that sound like science fiction. There are many other fun examples out there in our fight against pests, for example the zombie-making fungus Beauveria bassiana. This fungus, reminiscent of The Last of Us, infects insects, taking over their bodies before producing spores to infect the next victim. Beauveria bassiana can be purchased as a commercial biological control product. Intriguingly, this fungus can also colonise plants such as maize and grapes in beneficial ways, promoting plant growth and increasing tolerance to environmental stress.
Plant science is full of strange and wonderful solutions to difficult problems. Plants themselves are remarkable organisms, and the diseases that infect them are many and varied. By understanding these complex biological relationships, scientists can combine them in unexpected ways to create new tools.
It’s easy to let the mind wander, speculating where this might lead. Can we use other pests to treat diseases, in crops or humans? Can we protect forests before outbreaks begin? What could we modify within ourselves to promote health or resilience to environmental stress?
Sometimes, giving a fungus an STD can be the best way to save your (chest)nuts.
References
- For an excellent description of CHV1, see the introduction of:
Feau N, Dutech C, Brusini J, Rigling D, Robin C. Multiple introductions and recombination in Cryphonectria hypovirus 1: perspective for a sustainable biological control of chestnut blight. Evol Appl. 2014 May;7(5):580-96. doi: 10.1111/eva.12157. Epub 2014 Apr 15. PMID: 24944571; PMCID: PMC4055179. - For a good summary of Cryphonectria parasitica, see: Rigling D, Prospero S. Cryphonectria parasitica, the causal agent of chestnut blight: invasion history, population biology and disease control. Mol Plant Pathol. 2018 Jan;19(1):7-20. doi: 10.1111/mpp.12542. Epub 2017 Apr 24. PMID: 28142223; PMCID: PMC6638123.
- For a debate on how to refer to biological controls, with interesting examples of controls throughout, see: Eilenberg, J., Ann Hajek, and C. Lomer. "Suggestions for unifying the terminology in biological control." BioControl 46.4 (2001): 387-400.
- For an interesting review of Beauveria bassiana: Zimmermann, G. (2007). Review on safety of the entomopathogenic fungi Beauveria bassiana and Beauveria brongniartii. Biocontrol Science and Technology, 17(6), 553–596. https://doi.org/10.1080/09583150701309006