What occurs when different plant species grow side-by-side in a managed or natural ecosystem and one of them becomes infected by a virus disease which weakens it? This question is important; both for economically significant, mixed-species managed pasture systems and for environmentally significant mixed wild species populations. Moreover, it is also important, not only for the success of ‘regenerative agriculture’ in addressing climate change threats, but also for maintaining biodiversity in threatened natural ecosystems. However, past research on the impacts of virus infection on pasture species composition has been largely forgotten.
A series of field experiments with pasture plant species mixtures found that the delicate balance between different species is disrupted by virus infection. This is because the diseased species is no longer able to compete with non-host species. So, the proportion of diseased species is diminished whereas non-host plant species increase. The same thing also occurs when the species present are susceptible to infection, but one is sensitive and another is tolerant, resulting in the virus-tolerant species increasing at the expense of the sensitive species. Both of these scenarios also occur when virus-sensitive, susceptible pasture species compete with non-host or virus-tolerant weed species which results in ‘pasture decline’ due to takeover by weeds of little value as livestock feed. Furthermore, a similar situation was found later, when field experiments examined effects of virus infection on the species-balance in natural wild plant populations. There, a virus-tolerant invasive species displaced a virus-sensitive native species.
‘Regenerative agriculture’ addresses damage caused by global climate change while at the same time rehabilitating and improving depleted farmland. It aims to achieve this in different ways, especially by improving soil health, enabling cultivated plants to become more resilient to pest and disease attacks, and removing CO2 from the atmosphere or soil. An important means of achieving this is through greater use of managed pastures containing an optimal mix of sown pasture species, usually legumes and grasses. However, the effectiveness of mixed-species managed pastures declines when the species balance changes. The factors responsible include the diseases caused by viruses. Virus disease can not only replace the optimal balance between different pasture species with a sub-optimal one, but also accelerate pasture decline due to weed invasion. This in turn depletes a pasture’s livestock-feed value and diminishes its ability to improve soil health or remove CO2 from the atmosphere or soil.
Roger A.C. Jones published this review in Plant Pathology:
TITLE IMAGE: Single row plots of annual Burr medic (Medicago polymorpha) pasture plants: severely stunted alfalfa mosaic virus-infected plants (front) and vigorously growing healthy plants (back). All images used with permission of the author.