By Aimee Fowkes (@AimeeFowkes)
A plant species is considered to be “invasive” in an area if it is not native and has a tendency to spread. Invasive plants can have a damaging effect on the new environment they find themselves in because they can outcompete the native flora and so interfere with the food web. Also, they may not have any natural enemies in their new location so they often show improved fitness and can grow uncontrollably.
Impatiens glandulifera is a large flowering plant that originates from the western foothills of the Himalayas. It was introduced into Europe in the early nineteenth century as an ornamental plant, and it has since spread across Europe and North America. It grows on riverbanks, waste grounds and in woodlands.
Conventional control of I. glandulifera using chemicals has so far failed because it tends to grow in places that are inaccessible, or where use of herbicides is prohibited. This has led to investigations into an alternative solution, biological control.
Biological control is the use of one organism to reduce the population of another pest species, for example, by importing a natural enemy of the pest species into the new environment. Insects have been used in biological control since the nineteenth century, but it is only recently that people have started looking into using fungi to control weeds and invasive plants. There are several cases of fungi being used as biological control agents and their narrow host ranges reduce the risk that they will have an impact non-target species.
A team of scientists went to areas where I. glandulifera is native and searched for natural enemies of the plant. Between 2006 and 2010, Tanner et al. gathered various insects they found feeding on the plants and samples of plants showing symptoms of fungal infection. Here is a photo diary of the team’s search in 2008:
Further testing showed that most of the sampled natural enemies were able to infect other species of plants that are closely related to I. glandulifera so they were all rejected, except for one fungus known as Puccinia komarovii. The researchers had found a variety of the fungus almost exclusively infects I. glandulifera, which is now known as P. komarovii var. glanduliferae. This fungus belongs to a group of disease-causing fungi known as rusts, named after their yellow/orange pustules of spores that can be seen on the leaves of plants. In general, the rusts have very narrow host ranges, and they will only infect a few closely related species.
Why could this fungus make a good biological control agent?
Tanner et al. tested P. komarovii var. glanduliferae against 75 species of plants and it could only infect I. glandulifera and the closely related I. balsamina. In fact, the other species that are closely related to I. glandulifera showed high levels of resistance to the pathogen, suggesting that the risk of cross-infection is low. Importantly, the fungus does not infect economically important species in the Impatiens genus, or a UK native species called I. noli-tangere (touch-me-not or yellow balsam). While infection of I. balsamina is unwanted, it is non-native to the UK and has very little economic value so is of little concern.
It is important that the biocontrol agent can survive in the new environment. P. komarovii var. glanduliferae originates from the Himalayas so it has evolved to cope with a range of temperatures. The spores can infect between 5 oC and 25 oC so they should be able to survive the British climate. As previously mentioned, I. glandulifera is problematic in river bank habitats in the UK. When the leaves drop from the plants in the Autumn, any spores on the leaves will also be incorporated into the soil. Although any spores below the flood plain could be washed away, it is thought that spores on plants in more stable areas (above the flood plain) should be able to survive the winter and infect seedlings the following spring.
Rust spores are carried on the wind and are able to cover huge distances. A species of rust (P. chondrillina) was previously used to control skeleton weed in Europe and within the first year, it was shown to have spread 300 km. Using a rust fungus can overcome the problems associated with chemical control, its specificity makes it more environmentally friendly, and it is less expensive because if the rust is able to establish itself it will not need repeat applications.
I. komarovii var. glandulifera was initially investigated under strict quarantine, but recently, the Department for Environment, Food and Rural Affairs (DEFRA) in the UK gave permission for outdoor trials to be conducted in England. If the trials are successful, this fungus could become the 29th fungus to be used to control a weed and it may encourage more interest in exploiting rusts as biological control agents.
About the author: Aimee Fowkes is a masters student at the University of Nottingham. She is interested in plant-pathogen interactions and is currently doing a project looking at the effect of an elicitor on defence responses in broad bean. She has recently started her own blog: When Plants Fight Back. Follow her on twitter (@AimeeFowkes)
Barton et al. (2011) Predictability of pathogen host range in classical biological control of weeds: an update. Journal of the International Organization of Biological Control.
CABI: Biological Control of Himalayan balsam. http://www.cabi.org/projects/project/32944
Shelton, Biological Control: A Guide to Natural Enemies in North America. http://www.biocontrol.entomology.cornell.edu/cite.php
Pal & McSpadden Gardener (2006) Biological Control of Plant Pathogens. The Plant Health Instructor.
Tanner et al. (2013) Impacts of an Invasive Non-Native Annual Weed, Impatiens glandulifera, on Above- and Below-Ground Invertebrate Communities in the United Kingdom. PLOS one.
Tanner et al. (Accepted article available online) First release of a fungal classical biocontrol agent against an invasive weed in Europe: biology of the rust, Puccinia komarovii var. glanduliferae. Plant Pathology.