Invasives Blog

Rhododendron ponticum, native to southern Europe and south west Asia was introduced into the UK in the 18th Century. Since then, this plant has grown uncontrollably and is now a common sight throughout western parts of the British Isles in areas such as Cornwall, Wales and parts of Scotland and Ireland. Despite producing an attractive flower in the spring, Rhododendron can have damaging effects on the local environment. By growing rapidly this plant outcompetes native flora, decreases biodiversity and furthermore constitutes a sporulating host for the two devastating pathogens Phytophthora ramorum and Phytophthora kernoviae, meaning these pathogens not only infect but also reproduce on R. ponticum.

A stand of invasive Rhododendron ponticum in Windsor Great Park (Picture copyright CABI).

Phytophthora, (Greek meaning plant destroyer) is one of the most damaging genera of plant pathogens in the world. Although infamous for causing the Irish potato famine due to the devastating impact of Phytophthora infestans in the 19^th Century, this group of pathogens most definitely are not old news. More recently the pathogen Phytophthora ramorum has been identified as the organism responsible for causing extensive die-back – or Sudden Oak Death – of American southern red oak in California and Oregon in the US. Since then this damaging pathogen has been detected outside its US native range within Europe affecting a large number of plant species. It is this status of an “alien invasive pathogen” and its documented wide host range which has led to the development of an extensive program of containment to prevent further spread of this pathogen within the UK.

Typical symptoms of Phytophthora ramorum infection; (from left to right) seeping bark, stem cankers and leaf lesions (Pictures copyright CABI).

Both Phytophthora ramorum and P. kernoviae are “fungal-like” organism which combined cause disease on over 150 different species of trees and shrubs present in the UK. Symptoms of infection include die-back of foliage, wilting and blackening of leaves, the formation of cankers on tree trunks and infection often leads to the death of the whole plant. One type of spores (zoospores) are easily spread by wind and rain and also by humans. In addition to this, another spore type (chlamydospores) can persist in the soil and leaf litter for longer periods of time until environmental conditions become more favourable for infection. Since its initial identification in the UK P. ramorum has spread dramatically. In 2009 P. ramorum was found both infecting and sporulating on Japanese larch trees in the south west of England and more recently in Wales, Northern Ireland, the republic of Ireland and western Scotland. Unfortunately, there is no cure for this disease and a result, infected trees have to be felled and are removed in order to prevent further spread. In order to be sure that neighbouring trees are not infected, these too are cut down. In many of these sites where outbreaks occur, the invasive plant Rhododendron ponticum has been present and it is thought that this is the main route of the pathogen into forests.

Eradication and control of Rhododendron is the most effective way of controlling these harmful pathogens. Removal of rhododendron typically occurs by mechanical and chemical measures however; the plants often regrow and require continual cutting back and spraying with herbicides and risk infection by spores present in the soil. In 2009, DEFRA (Department for Environment, Food and Rural Affairs) launched a five year Phytophthora disease management program looking at methods to contain and reduce the spread of these two pathogens in the UK. Within this programme a Forest Research led project is investigating a number of different strategies to remove rhododendron as well as looking at methods of safe disposal of infected plants. Through field experiments the efficacy of a number of control methods including mechanical removal of Rhododendron plants as well as the use of chemical herbicides and of the native wood-rotting basidiomycete fungus Chondrostereum purpureum to control resprouting of cut stumps are being evaluated.

Treatment of a birch stump with the mycoherbicide Chondrostereum purpureum in Finland. (Picture copyright CABI).

Scientists at CABI have provided scientific input into this project focussing on the evaluation of the fungus Chondrostereum purpureum to control regrowth of Rhododendron through its application as a mycoherbicide to cut stumps. This method has been tried and tested in different parts of the globe, i.e. the Netherlands, Canada and New Zealand, and has proven to be environmentally safe and successful in controlling a number of woody invasives.The evaluation of all field trials is currently under way and all project results will be summarized in the final project report to DEFRA during the first half of the year.

More information on this project can be found at:

CABI – Looking for a treatment to halt the spread of rhododendron

Forest Research – Clearance and disposal strategies for Phytophthora-infected rhododendron

DEFRA -Determining best methods for the clearance and disposal of key host plants, especially invasive Rhododendron, for the control of Phytophthora ramorum and Phytophthora kernoviae