Do invasive plants escape diseases?

Results & impact 7 May 2024
When invasive alien plant species arrive in a new territory they are freed of their pathogens, which means they are able to establish themselves more successfully in their new environment. A team of scientists has put this hypothesis to the test, focusing on the viruses harboured by Bothriochloa barbinodis, a grass native to the Americas that was introduced into France in the 1960s. Today, the plant has colonized half of France, starting with the Hérault department.
Legend: A field invaded by Bothriochloa barbinodis at Octon, near lake Salagou in the Hérault department, in 2015 © Guillaume Fried
Legend: A field invaded by Bothriochloa barbinodis at Octon, near lake Salagou in the Hérault department, in 2015 © Guillaume Fried

Legend: A field invaded by Bothriochloa barbinodis at Octon, near lake Salagou in the Hérault department, in 2015 © Guillaume Fried

A plant often arrives in a new territory virus-free. Indeed, it is mostly seeds that travel and very few viruses circulate via seeds. Plant viruses are usually transmitted by insects. Even so, insects will ignore an unknown plant, at least initially. In this hypothesis, we find ourselves with an invasive plant free from the viruses of its original habitat that will initially not be subject to the viruses of its area of introduction either”.

This hypothesis, detailed by Philippe Roumagnac, a virologist from CIRAD, served as the basis for studying the viral community of an invasive alien plant species, Bothriochloa barbinodis. This grass can be found throughout southern France from July to October, with its stems topped by a small, white feathery tuft. The species is thought to have been introduced into France accidentally through the wool trade with the Americas between 1964 and 1976.

Initially free of its viruses, then targeted by local viruses

To ascertain whether Bothriochloa barbinodis did indeed free itself from the viruses of its original habitat when it arrived in France, the scientists inventoried the viruses of Bothriochloa barbinodis populations in Arizona, an endemic territory for the species, then compared them to the virus communities of Bothriochloa barbinodis populations in France, and of thirteen other grasses considered to be indigenous to Arizona and France.

The results confirmed the hypothesis. “However”, explains Virginie Ravigné, an ecologist at CIRAD, “we can see that the species is now interacting with local viruses. This is down to naturalization: after fifty years or so, invasive plants start to pick up pathogens from their new environment”.

As it is not weakened by diseases, the plant easily develops and becomes established in the new territory. After a while, though, insects get used to its presence and attack the new resident, biting and contaminating it.

Viral ecology, vital for controlling disease emergence

This phenomenon of freedom from viruses may be one of the factors explaining why an invasive alien plant species adapts so well to a new territory. However, as Philippe Roumagnac reminds us, a seed or plant is likely to arrive with viruses that could subsequently emerge on local plants. Consequently, introducing new plants always entails a major health risk for an ecosystem. 

This is particularly why the health quality of imported plants has to be guaranteed. It is a necessary, albeit insufficient, condition for limiting emerging diseases, as our scientific knowledge of the diversity, distribution and circulation of both animal and plant viruses in ecosystems also calls for improvement”, say the two researchers.

Indeed, compared to molecular mechanisms, which are very well documented, viral ecology is still a little-known aspect of virology. Yet, integrating ecology with virology could lead to a better understanding of the dynamics of plant viruses, hence potentially to the prediction of conditions conducive to the transmission of a virus from one species to another. This study, carried out as part of an ANR project coordinated by Philippe Roumagnac, was backed by an international consortium of partners from France (CIRAD, INRAE, ANSES, Tour du Valat, Conservatoire du Mascarin, University of Lille), South Africa (University of Cape Town, University of the Western Cape) and the United States (Arizona State University).

A quarantine service for cultivated plants

This type of study improves knowledge of how viruses are circulated via plants. Some crops, such as sugarcane, depend on international exchanges of cuttings. To that end, CIRAD offers a sugarcane quarantine service called Visacane, which endeavours to guarantee the transfer of healthy plant material. The cuttings remain in CIRAD’s glasshouses for two years. They are tested for all types of pathogens then cleaned. “This process of cleaning and exchanging healthy cuttings internationally is vital, because a contaminated cutting could go on to contaminate an entire region. Once a virus is present in the local fauna and flora, it is very difficult to get rid of”.


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