Parasitic fungus introduced to attack Himalayan balsam

29th Aug 2014

Even if you love Himalayan balsam, it has surely become too much of a good thing as it takes over Britain's wetlands and riverbanks. But now it's facing a major setback - the deliberate introduction of a parasitic rust fungus from its native range in the mountains of Asia.

Could it be the beginning of the end for Himalayan balsam - the exotic purply-pink flower that has spread over, or invaded, riverbanks, damp woodlands, wastelands and soggy corners across much of the British Isles?

CABI, a non-profit research body, has just begun to release a rust fungus at locations in Berkshire, Cornwall and Middlesex as part of field trials to control the startlingly successful non-native weed (Impatiens glandulifera) using natural means.

The Environment Agency estimates that the weed occupies over 13% of river banks in England and Wales, its rapid spread enabled by the explosive pods which scatter its large black seeds over a wide area.

It can reach over three metres in height and competes with native plants, reducing biodiversity. Its presence also makes river banks more susceptible to erosion after it dies back in the late autumn.

Large scale chemical and manual control is often not feasible and not economically viable. Using existing measures, the Environment Agency estimates it would cost up to £300 million to eradicate Himalayan balsam from the UK.


Making the weed a 'good citizen'

But the weed is not all bad - though it's definitely far too much of a good thing. It adds colour to our countryside at a dull time of year in late summer and autumn when most native flora have finished flowering. And it is a rich source of nectar for bees and other pollinators when there's few others about.

So will the rust fungus eliminate Himalayan balsam altogether? No, says Dr Robert Tanner, Senior Scientist at CABI: "Biocontrol will address the imbalance that means it can create vast monocultures over acres of riverbanks - and bring it down below the level where it is no longer having a harmful effect.

"Unlike herbicide it will not wipe it out, but make it a benign plant in the ecosystem - as it is in the Himalayas where it has all its natural enemies to control it. We want the fungus to do its work where you get balsam in large swathes along our river systems, where it's highly dense and abundant, and act a self-thinning agent."

It will take some years for the fungus to spread and reduce the vigour of the species, he adds. "Over that time we hope to see the balsam reduce especially where it dominates ecosystems, and see recolonisation with native species. It won't disappear altogether, but will take its place among the nettles, willowherb and meadowsweet."


Eight years of research

The release of the rust fungus comes after an eight-year research programme funded primarily by Defra and the Environment Agency, with contributions from Network Rail, the Scottish Government and Westcountry Rivers Trust.

The aim of CABI's research was to find one of the many insects or fungi attacking the plant that had evolved to attack only Himalayan balsam, which could be released into the UK to control the weed while leaving indigenous species unharmed. CABI found that the rust fungus did just that.

During the course of the research, testing in quarantine laboratories has established that the rust fungus causes significant damage to Himalayan balsam and does not impact on native species.

According to Dr Tanner, it has been tested on 74 plants - including Britain's only native relative, Impatiens noli-tangere (Touch-me-not balsam), which grows (in the UK) mainly in the Lake District - and it only affects a single unimportant ornamental species, Impatiens balsamina.


A European 'first'

Defra Ministers took the decision to allow CABI to release the rust fungus in July 2014. This decision followed the Food and Environment Research Agency's (FERA) review of the scientific research and a public consultation on the proposed release of the fungus to control Himalayan balsam, which ran from May to June 2014.

Lord de Mauley, Minister for natural environment, said: "This is a great step forwards in tackling Himalayan balsam. This invasive weed prevents our native plants from flourishing, can increase flood risk, and costs the British economy £1m per year to clean up.

"The work CABI has done in identifying a natural control method will help us reduce the impact of Himalayan balsam without any negative effects on native species."

The rust fungus release is the first release of a fungal control agent in Europe - but surely not the last.


More information: see CABI's dedicated website on Himalayan balsam.

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