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Phytophthora ramorum

Overview

Phytophthora ramorum is a fungus-like pathogen that poses a significant threat to forests in the island of Ireland. Of most concern is the recent host range expansion from low value Rhododendron to Japanese larch in commercial forests in Ireland and Northern Ireland.

Phytophthora is a group of fungus-like organisms that contains over 140 species. Many Phytophthora species are important plant pathogens, for example Phytophthora infestans (cause of potato late blight).

Phytophthora ramorum is one of the most damaging Phytophthora species world-wide, infecting hundreds of plant species across Europe and North America. Disease caused by P. ramorum was first noticed in the early 1990’s almost simultaneously in plant nurseries in Europe and in forests in California. In mainland Europe, it causes a serious blight of ornamental plants, especially Rhododendron, Camellia and Viburnum. In North America it causes a forest disease called Sudden Oak Death, killing millions of Quercus trees in deciduous forests of California and Oregon. It also costs millions annually to the ornamental nursery industry in Europe and North America.

In Ireland, P. ramorum was first detected in 2002 on imported Rhododendron and Viburnum at several garden centres and in 2003 it was detected in the wild, infecting Rhododendron ponticum. In 2010 it was observed on Japanese larch in Ireland and Northern Ireland, and has since been recognised as a serious threat to forestry. Under current plant health policy, P. ramorum control has resulted in the removal of more than 1,300 hectares of larch forests on the island of Ireland and 17,000 hectares in Britain.

Phytophthora ramorum has been a regulated quarantine pathogen within the EU since 2002, with confirmed infected trees in forests removed and susceptible hosts within a 250 meter radius also removed as a precautionary measure. The harvested wood may only be moved and processed once a special phytosanitary permit has been granted by the regulatory body.

Symptoms

Symptoms of infection on Japanese larch often become evident during the spring and summer. The time between notice of first symptoms and tree death can be rather short, and this seemingly rapid mortality of larch trees has led to “Sudden Larch Death” being used to describe the disease epidemic. Infection on individual larch trees can be indicated by dieback of the crown, resinous cankers on the trunk, excess cone production and by blackening and retention of needles.

On Rhododendron, lesions along the midrib and at the tip of the leaf, stem decay and dieback can be a sign of P. ramorum infection.

On European beech, P. ramorum typically manifests itself as bleeding cankers.

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