Phytophthora ramorum

- cause of Sudden Oak Death in U.S.A and Ramorum dieback in Europe

BACKGROUND

The quarantine organism Phytophthora ramorum has been identified as the cause of Sudden Oak Death (SOD) in the USA. Since 1995 SOD has been killing large numbers of oak trees along the California coast of the USA. It has since spread to Oregon. While it is not yet known if European oaks are susceptible, no symptoms of SOD have been recorded on oak in Europe. In Europe P. ramorum has recently been found to cause the diseases rhododendron twig blight and dieback, viburnum dieback and Camellia leaf blight. Other possible hosts have been identified in Europe, including plants of the genera Castanea, Fagus, Quercus, and Vaccinium. Current scientific information suggests that there are many differences between the strain of P. ramorum found in Europe and that in the USA (such as pathogenicity, host range and mating type).

LEGISLATION

The European Community introduced emergency phytosanitary measures to prevent the introduction into and the spread within the Community of P. ramorum. These measures came into operation on 1st November 2002. The main provisions of the legislation are:

·  Member States are obliged to carry out a survey for the organism and report the findings to the Commission

·  Plants of Camellia spp., Rhododendron spp. (other than R. simsii) and Viburnum spp. may not be moved from their place of production unless they are covered by a plant passport

·  Host plants imported from the USA must be accompanied by a phytosanitary certificate and the plants must either originate in areas in which P. ramorum is known not to occur or alternatively the plants have been officially inspected and found free of the harmful organism

Action required when Phytophthora ramorum is found in EU

In the event of the organism being found in the EU certain action is now required including destruction of the infected material and all susceptible plants within 2 metres of the infected plant and retention of all susceptible plants within a 10 metre radius for at least three months for inspection and testing. All other susceptible plants on the nursery will be subject to intensive re-inspection following the finding.

SYMPTOMS

The symptoms on rhododendron and viburnum are similar to that caused by other Phytophthora spp. on these hosts. On Rhododendron spp. the pathogen causes a twig and leaf blight. Affected twigs develop a brown to black discolouration that spreads along the twig and can spread into the leaves via leaf petioles. Characteristic symptoms include blackening of the petiole (leaf stalk), leaf base and leaf tip and may extend along the mid-rib. Leaf infection can also occur without twig infection. In contrast, Viburnum spp. do not show twig blight and infection usually begins at the stem base and spreads upward, causing the plants to wilt and collapse very quickly. Brown to black leaf infections, especially on evergreen species, may also be observed and the pathogen can also affect flowers. On Camellia spp. the pathogen usually only causes leaf infections. Leaf lesions are usually brown to black, typically occurring at the tip or edges of the leaves.

HOST PLANTS

Adiantum aleuticum, Adiantum jordanii, Acer macrophyllum, Acer pseudoplatanus, Aesculus californica, Aesculus hippocastanum, Arbutus menziesii, Arbutus unedo, Arctostaphylos spp., Calluna vulgaris, Camellia spp., Castanea sativa, Fagus sylvatica, Frangula californica, Frangula purshiana, Fraxinus excelsior, Griselinia littoralis, Hamamelis virginiana, Heteromeles arbutifolia, Kalmia latifolia, Laurus nobilis, Leucothoe spp., Lithocarpus densiflorus, Lonicera hispidula, Magnolia spp., Michelia doltsopa, Nothofagus oblique, Osmanthus heterophyllus, Parrotia persica, Photinia x fraseri, Pieris spp., Pseudotsuga menziesii, Quercus spp., Rhododendron spp. (except R. simsii), Rosa gymnocarpa, Salix caprea, Sequoia sempervirens, Syringa vulgaris, Taxus spp., Trientalis latifolia, Umbellularia californica, Vaccinium ovatum, Viburnum spp.

ACTION IN THE EVENT OF SUSPICION

Please contact the Horticulture and Plant Health Division of the Department of Agriculture, Fisheries and Food, phone (01-5053585) immediately if suspicious symptoms are observed. Isolate the suspicious plant(s) and refrain from moving host plants from the nursery.

Figs. 1 and 2. Symptoms of Phytophthora ramorum on Rhododendron spp.

Fig. 3. Symptoms of P. ramorum on Viburnum spp. and Fig. 4 on Camellia spp.

Further information can be viewed on the website of the European and Mediterranean Plant Protection Organization (EPPO – www.eppo.org) by clicking on the following links:

Phytophthora ramorum

Phytophthora kernoviae