Building on CABI research into the biological control of Himalayan balsam (Impatiens glandulifera) using a rust fungus (Puccinia komarovii var. glanduliferae), a Natural Environment Research Council (NERC) funded collaboration between Royal Holloway, CABI and the University of Reading is investigating the role of the microbial community associated with the plant and how these microbes may be exploited to enhance biocontrol efficacy and aid in the recovery of invaded sites. It is hoped that the findings of the study may be applicable to biocontrol programmes more widely.
Himlayan balsam dominates the banks of many of the UK’s rivers and waterways, out-competing native plants and leaving soil exposed to erosion during the winter. Although it dies off in colder months, research has found that the plant can cause changes in the soil microbiota meaning that native plants are not able to re-colonise during this time. In addition, the plant can clog waterways and attract pollinators away from native species.
The use of herbicides to control Himalayan balsam carries environmental risks due to the plant’s typical proximity to waterways and although regular removal by volunteers has been valuable, it is an arduous task that must be repeated for a number of years at a catchment scale to be effective. Therefore the introduction of a biological control agent was desirable as an environmentally friendly method of reducing and controlling the invasive population, leading to the first release of the rust onto plants in the field in 2014.
Rust fungus pustules on Himalayan balsam leaves
Royal Holloway’s Professor Alan Gange outlines the NERC study: “We are conducting experiments to see if fungi inside leaves (endophytes) and inside roots (mycorrhizas) can affect the ability of the rust fungus to infect Himalayan balsam.” After a year in the lab, the results of team’s experiments show dramatic effects of each fungal group on the other. Going into the second year of the project means taking the experiments into the field where things get much more complicated. This is where the team will also get to assess the microbiome of the Himalayan balsam to see if the application of rust fungus starts to damage the plant so that the microbial community around the roots and inside the plant changes, and whether this change is beneficial or not. By comparing both treated and untreated plants, the team will be able to see differences such as whether an increase in microbes means that they are detrimental to plant health or act as an indicator of plant decline. As well as recording any effects on the plant and soil, the CABI team is also looking at the recovery of native plants and invertebrate communities after the introduction of the rust fungus.
The decision to utilise rust fungi, says CABI’s Dr Carol Ellison, was because they “are often extremely host specific, sometimes only infecting a limited number of genotypes of the target weed, which makes them very safe to use.” There are also a number of other examples where the fungi have proven to be an effective and sustainable long-term solution for controlling individual invasive plant species.
The research is expected to be valuable for the future of biological control on a broader scale as it opens the door to the use of this approach across a wide variety of alien species; contributing to a growing field of research that looks beyond pesticides. The rust fungus has been introduced into a selection of field sites and its impact is being monitored in a number of ways. The priority is assessing the effectiveness in reducing the dominance of the plant, but the team is also investigating the recovery of native species in place of the Himalayan balsam and the impact of changing plant populations on the soil microbiota and local insect populations.
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