Factsheet 19/03 Horticultural Bradbourne House Development East Malling Council Kent ME19 6DZ Hardy nursery stock T: 01732 848383 F: 01732 848498 E: [email protected]

Sudden Oak Death/Ramorum Dieback – implications for the HNS industry

By David Jones, Central Science Laboratory

Phytophthora ramorum is causing dieback of several nursery stock in the UK. In Western USA the pathogen is causing the death of some native oaks and other woodland species. Legislation has been introduced as a precautionary measure to eradicate the organism where found and to prevent spread within the EU. This factsheet provides valuable information about Phytophthora ramorum in the UK, including its symptoms, sources and spead and gives practical advice about how to reduce risks and what to do if you suspect the disease is present on a site.

Background then, it has been reported in Belgium, Outside Europe, P. ramorum has only Denmark, France, Poland, Spain, Sweden been found on the west coast of the Where has Phytophthora ramorum and the UK, and may occur elsewhere USA and Canada. It was only after been found? in Europe. In the UK, over 5000 nurseries, the death of native oaks in California Phytophthora ramorum was first found garden centres and amenity areas have was attributed to P. ramorum in 2001 in Germany and the Netherlands in been inspected since 2002 and that the potential threat that this 1993, but it was not described as a approximately 6% have been found to pathogen posed to the UK flora was distinct new species until 2001. Since have affected by P. ramorum. recognised.

1 Dark brown lesions on shoots of Rhododendron caused by P.ramorum Which plants are susceptible? cowberry. Concern has arisen because In addition, it causes dieback or In the UK, outbreaks of the disease these species show susceptibility in blight of many other woodland species have occurred mainly on species/hybrids experimental inoculation studies including the coast redwood, Douglas of Rhododendron and but undertaken in the laboratory. However, fir, and a number of understorey also occasionally on Camellia, , the potential of P. ramorum to infect and plants. Some ornamental Leucothoe, Pieris, Syringa and Taxus and seriously damage these plants in hosts have also recently been found (see Table 1 for more details). the natural environment is not known. affected in garden centres/nurseries Of major concern is that important On-going research will help determine in California, Oregon, Washington and components of UK woodlands, such risks more fully. British Columbia. as oaks, European beech and sweet In California and Oregon, P. ramorum chestnut could be threatened along with causes the death of native oaks, such heathland flora, such as bilberry and as tanoak coast live oak and black oak.

Table 1 Hosts of Phytophthora ramorum in the UK (July 2003)

Family Species

Caprifoliaceae Viburnum davidii Viburnum farreri (syn. Viburnum fragrans) Viburnum lantana Viburnum opulus Viburnum plicatum Viburnum tinus Viburnum x bodnantense (Viburnum farrerii x Viburnum grandiflorum) Viburnum x burkwoodii (Viburnum carlesii x Viburnum utile) Viburnum x carlcephalum x Viburnum. utile Viburnum x pragnense (Viburnum rhytidophyllum x Viburnum utile)

Ericaceae Kalmia latifolia

Leucothoe fontanesiana

Pieris formosa var. forrestii Pieris formosa var. forrestii x Pieris japonica Pieris japonica

Rhododendron augustinii Rhododendron balfourianum Rhododendron brachycarpum Rhododendron ferrugineum Rhododendron ponticum

Rhododendron hybrids – hybrids found infected have been derived from crosses involving: Rhododendron arboreum, Rhododendron campylocarpum, Rhododendron catawbiense, Rhododendron campylocapum, Rhododendron caucasicum, Rhododendron dichroanthum, Rhododendron facetum, Rhododendron forrestii, Rhododendron fortunei, Rhododendron galactinum Rhododendron geraldii, Rhododendron grande, Rhododendron griffithianum, Rhododendron griersonianum, Rhododendron maximum, Rhododendron ponticum, Rhododendron smirnowii, Rhododendron strigillosum, Rhododendron viscidifolium, Rhododendron wardii, Rhododendron yakushimanum and possibly other species

Oleaceae Syringa vulgaris

Taxaceae Taxus baccata

Theaceae Camellia japonica Camellia reticulata Camellia x williamsii (Camellia saluenensis x Camellia japonica) Symptoms

Symptoms that occur on known hosts in the UK are described below. Other pathogens can produce symptoms similar to those caused by P. ramorum so care must be taken with the diagnosis. Laboratory analysis is required before diseases can be positively attributed to P. ramorum.

Rhododendron dieback The pathogen primarily causes an aerial dieback on both nursery plants and mature bushes. Diseased shoots have brown to black lesions that usually begin at the tip and move towards the base. However, cankers may form on any part of the 3 Dark brown lesion on leaf of Rhododendron shoot (Fig 1) or stem. The internal tissue ponticum. P. ramorum has entered the leaf from of the diseased twig is killed causing a shoot via the petiole and is developing along a light brown discolouration which is 2 Dieback on a Rhododendron bush caused and out of the line of the midrib. Infection has distinctly different to the normal off- by P. ramorum also occurred along the margins of a wound (right) white colour visible when the bark is removed. Cankers near ground level can result in rapid wilting of shoots, causing occur at the leaf tip (where moisture Viburnum dieback the , which remain attached, can accumulate and remain for extended Infection usually occurs at the stem to hang down (Fig 2). periods encouraging infection), but can base (Fig 4) causing plants to wilt and The most characteristic leaf symptom form elsewhere. Eventually, entire collapse very quickly. The cambial is the blackening of the leaf petiole leaves can turn brown to black and tissue in the stem initially turns brown, extending into the leaf base (Fig 3). may fall prematurely. but eventually becomes dark brown. This may spread further along the Although P. ramorum has been Leaf spots or leaf blight may also midrib causing blackening of the leaf. isolated from roots and stem bases, occur, especially on evergreen species Leaves can become diseased with the pathogen is not considered to (Fig 5). Twigs and shoots may also be and without shoot infection. Diffuse cause significant rotting in these areas, affected. Minor root rotting has been brown spots or blotches frequently unlike other Phytophthora species. observed.

4 Dieback of Viburnum tinus, due to a basal 5 Lesion on leaf of Viburnum davidii. Infection has probably occurred at the leaf tip with the canker (arrow) caused by P. ramorum. The outer pathogen subsequently invading the lamina from the line of the midrib. Older dead tissue has layers of the affected stem have been removed changed from dark to light brown to show discoloured inner tissues. The exposed stems on either side are unaffected Pieris dieback Leaf symptoms are similar to those on Rhododendron. Black lesions can develop at leaf bases as a result of infections spreading down the shoot and along the petiole (Fig 6). Lesions also typically occur at the leaf tip or leaf edge where water accumulates. The whole leaf can also become blighted. Individual stem cankers lead to aerial dieback.

Kalmia and Leucothoe leaf spot Infection usually occurs at the leaf tip (Figs 7 and 8). The pathogen subsequently grows through leaf tissues towards the leaf base causing a brown to black discolouration. 6 Brown lesion at base of leaf of Pieris 7 Lesions on leaves of Kalmia latifolia. Camellia leaf blight japonica caused by P. ramorum. The pathogen Infection has occurred at the leaf tips where Brown to black leaf-tip spots (Fig 9) has entered the leaf from the petiole moisture accumulates. Older dead tissue has form that may result in entire leaf changed from dark to light brown death. Stems have been found to be susceptible in laboratory-based, inoculation experiments so it is possible that dieback could also occur.

Lilac leaf and bud blight Leaf symptoms observed so far on Syringa vulgaris have been similar to those seen on Kalmia, Leucothoe and Camellia. Necrosis begins on leaf tips and at leaf margins. Greyish-brown, diffuse, water-soaked lesions become evident later (Fig 10). Under laboratory conditions, the infection has not progressed into the shoot as leaves have abcissed. Bud blight leads to the brown to black discolouration of shoots as the infection spreads. 8 Brown lesions on leaves of Leucothoe fontanesiana. Infection has occurred at the leaf tips

9 Brown lesion on leaf of Camellia japonica. Infection has occurred at the leaf tip where 10 Light brown lesion on leaf of Syringa moisture accumulates vulgaris. Infection has occurred at the leaf tip Yew leaf blight black oak, a typical symptom is the parts, including leaves, shoots, P. ramorum has been isolated from oozing or ‘bleeding’ of a reddish-brown branches and any place on the trunk. brown lesions on young leaves on to black viscous discharge from bark P. ramorum has not been found new shoots of container-grown Taxus lesions at the base of the trunk (Fig 11). causing a lethal disease of trees baccata. Yew leaf blight has only been Trees wilt and foliage turns brown. in Europe. found on plants at one nursery and Some oaks die quickly while the decline has not been seen on mature trees. of others may take several years. Some trees may survive infection. Sudden oak death in California Basal trunk cankers on tanoak and Oregon cause rapid mortality (Fig 12). However, On oaks native to the Pacific coast of unlike coast live oak and black oak, the USA, such as coast live oak and infections can occur on all aerial

11 ‘Bleeding’ canker caused by P. ramorum at the base of the main trunk of a coast live oak 12 Dead and dying tanoak (Lithocarpus (Quercus agrifolia) in California densiflorus) as a result of disease caused by P. ramorum in a forest in Marin County, California

Biology been found in stream water, ponds and Other organisms that invade weakened small reservoirs. Care must be taken to and wounded plant tissues may How does the disease spread? ensure sources of irrigation water are accelerate the process. Roots are The movement of infected plants in trade not contaminated especially where rarely invaded. is the main means of long-distance there is a risk of recontamination from spread. Plants may not show symptoms soil and plant debris. What conditions favour disease for some time after infection, especially Thick-walled, long-lived spores development? if growth of the pathogen is suppressed which allow the long-term survival of Phytophthora species, commonly by the use of fungicides with activity the pathogen, can be produced within known as water moulds, are dependant against Phytophthora species. Symptoms infected tissues or on the surface of on moisture for completion of their may develop after the plant has been fallen leaves under moist conditions. lifecycles. Wet environments favour sold or moved to a different location. In Growing media, debris and soil on spore production, germination and this way, the pathogen can be spread vehicles, machinery, footwear or live- infection. Therefore, conditions that on host plants from location to location stock can also harbour the pathogen. allow moisture to remain on plant and country to country. surfaces favour disease. Localised spread of the pathogen is How do plants become infected and Laboratory tests have shown that the by fungal spores (sporangia), produced develop symptoms? pathogen is a cool, temperate organism on the surface of infected leaves and Spores germinate on plant surfaces with an optimum temperature for shoots. These are spread by rain splash, and infect through wounds and natural growth of 20ºC. Its minimum growth wind-driven rain or irrigation water and openings, such as stomata and temperature is 2ºC and maximum 30ºC. possibly aerially or by insects. Sporangia lenticels. Once inside the plant, the germinate under moist conditions to pathogen grows through the tissue release water-motile zoospores. Both causing cells to die. After many cells spore types are short-lived and have have been killed, symptoms appear. Management and between countries in Europe is also • Soil that has supported the growth undesirable. Hence, the movement of of diseased plants could be cont- control most species of Rhododendron and aminated with spores of P. ramorum. all species of Viburnum is being This soil should be either removed What should you do if you controlled by plant passporting (see and deep buried or sterilised by an know/suspect a disease caused by Panel 1 for more details). appropriate method, such as steam P. ramorum? Currently, no compensation is offered or chemical treatment before reuse. The disease is notifiable. Inform your for losses due to disease eradication. local Plant Health Inspector if you Can the disease be controlled with know or suspect that species of How is the eradication being fungicides? Rhododendron, Viburnum or any other undertaken? Although fungicides active against plant genus is affected by Phytophthora • When an outbreak has been Phytophthora may provide some ramorum. confirmed by a Plant Health Inspector, protection against this disease, they Contact Defra Plant Health and all affected plants and all known will not eradicate it. Once infection has Seeds Inspector or the PHSI HQ, York, natural hosts within a 2m radius occurred, the use of fungicides will Tel: 01904 455174, Fax: 01904 455197 must be destroyed. only suppress disease development Email:[email protected]. and mask symptoms, as has been uk, Website: www.defra.gov.uk • All known natural hosts within a demonstrated at nurseries where 10m radius of affected plants cannot outbreaks have persisted despite What is being done to manage the be moved for three months. These regular treatment. Suppression of disease situation? plants have to be inspected by a symptoms by fungicides could Early detection followed by prompt Plant Health Inspector on at least seriously compromise eradication of quarantine action is the basis of two occasions during this period the disease. Therefore, fungicides effective disease management. and found free of disease symptoms. should not be used when eradication The Plant Health and Seeds is being attempted. Inspectorate (PHSI) are conducting • All known natural hosts must also surveys throughout England and Wales. be inspected and found free of What research is being undertaken to check for the presence of the disease before movement off site on disease management? pathogen in garden centres, nurseries can occur. Research is underway at the Central and public gardens/parks. Similar action Science Laboratory of Defra and the is being undertaken by equivalent • Follow-up inspections will take Forestry Commission to determine agencies in Scotland, Northern Ireland place every three months for nine risks to ornamental plants, trees and and the rest of the UK. When detected, months after eradication. ecosystems in the UK. The pathogen the pathogen is being eradicated. The has a wide experimental host range, origins of diseased stocks are traced • For infected container-grown stock, but this does not necessary mean that to determine possible sources of the surface on which the containers all species will become infected under infection. Sources can be from within were standing, such as benches natural conditions. The epidemiology the UK or elsewhere in Europe. and concrete platforms/slabs, must of the pathogen is being studied, as are The Forestry Commission is investi- be disinfected. Antec Farm Fluid the most effective measures for control. gating oak and other trees with S® (acetic acid, dodecyl benzene Disinfectants and fungicides are under suspicious symptoms and checking sulphonic acid and hydroxy evaluation in HDC-funded work. for the presence of P. ramorum. The hybridendes) is suitable for footwear Scientists in the UK liase regularly pathogen has not been implicated and equipment at a concentration with their counterparts in overseas in the decline of trees in any cases of 2%. Panacide-M® (sodium research groups. As a result, investigated. dichlorophen) should be used for information between programmes gravel and standing-out areas at a investigating P. ramorum is rapidly Why eradicate the pathogen? concentration of 1.66%. Surfaces exchanged and utilised. The eradication of the pathogen in the to be treated should be cleaned to UK would eliminate the potential threat remove soil and debris before that it poses to native oaks, other forest application of disinfectants. After and amenity trees, heathland plants treatment, surfaces should be left and ornamental hosts. The further for at least 10 minutes before spread of P. ramorum within and À washing off with water. À Panel 1 Plant passporting scheme

Certain plants and plant products • The plants are, to the best of the harmful organisms, which now moving within and between coun- producer’s knowledge, free from include P. ramorum. tries in the EU must be accompanied all quarantine pests and pathogens Species/hybrids of Rhododendron, by a plant passport. A plant passport and, where appropriate, grown in (other than R. simsii, which is must include the words ‘EC Plant an environment which is also free resistant to P. ramorum in tests) and Passport’, a country identifier such from these pests and pathogens Viburnum are now subject to plant as UK, NL or FR and a 5 digit passporting requirements. The reference number. It may include • Plants imported from outside the conditions of the passport are that ‘ZP’ if the plants have a replacement EU have been landed by a the material originates in areas passport.Check that your supplier is registered importer, inspected on where P. ramorum is known not to providing a valid passport. Plants that arrival in the UK or in another occur or where there have been no do not have a valid passport will be Member State (or at an approved signs of the pathogen at the place destroyed. This passport provides destination) and found to be of production. In cases where signs documentary evidence that: free from quarantine pests and of the pathogen have been found, • The plants covered have been pathogens prior to being appropriate procedures for grown by a registered grower, passported for movement within eradication should have been whose premises are regularly the EU. implemented. inspected, and who is authorised The scheme aims to provide protec- to issue plant passports tion against the dissemination of

What can be done • Where possible, growers should try • Ideally, blocks of susceptible plants to obtain an agreement with their should be spaced at least 10m to minimise risks on supplier that payment will be subject apart. In this way, if the disease is the nursery/garden to the plants remaining healthy for a found on any plants, only the block specific period. around it will be subject to control centre? measures. • Inspect all susceptible plant species Although there are currently no fugi- for signs of the disease BEFORE • Consider if it is possible to reduce cides available to control P. ramorum placing them on the main nursery the number of highly susceptible on nurseries, garden centres and beds or sales areas. species on the site and if possible, amenity areas, there are a number of change crops on previously infected steps which will help reduce the risk • Consider establishing a ‘quarantine’ areas to non-susceptible genera. of infection and spread of the disease. area to hold bought-in material away from the main production nursery. • Implement hygiene measures to Guidance on disease prevention This would enable signs of disease reduce the likelihood of spread by • Nurseries and garden centres suppressed by fungicides to develop contaminated soil, equipment or staff. should not accept susceptible plant without risking the main stock. Disinfect pruning tools and Danish material without a plant passport trolley shelves regularly. It may be (see Panel 1 above). • Remove all leaf and plant debris worthwhile disinfecting footwear from the beds underneath and plant beds. Antec Farm Fluid • Notify your Plant Health Inspector if susceptible material. S® is effective on footwear and you are offered susceptible material equipment, whilst Panicide-M® is without a plant passport or an Steps to follow on the nursery suitable for gravel and standing invalid passport. Follow up action • Inspect all known hosts, especially out areas. by PHSI will help to cut the risk of Rhododendron and Viburnum, for importing the disease. signs of disease at regular intervals. • Where possible, avoid the use of Other pathogens may give rise to overhead irrigation as water splash • Growers buying in stock of similar symptoms and laboratory can spread the disease. Similarly, susceptible plants are advised to tests, arranged by PHSI, will be placing susceptible material under ask their suppliers for a written needed to verify the cause. protection will cut rain splash. assurance that the plants they intend to purchase have not been • Train nursery staff to look out for • If plants have been standing on sprayed with a disease suppres- symptoms so that any disease capillary matting, sterilise or replace sant within six weeks of the date present can be identified at an the matting before any new crop is of shipment. early stage. stood out. • Do not use hosepipes from infected • If in doubt, seek advice from the • Landscapers buying in stock of areas on uninfected areas. local PHSI Inspector. susceptible plants are advised to talk to their suppliers and ask for a • Fungicides active against Guidance to landscapers written assurance that plants they Phytophthora may afford some • Landscapers are required to comply intend to purchase have not been protection against infection, but are with all the relevant legislation relating sprayed with a disease suppressant unlikely to eradicate any infection. to P. ramorum, including the destruc- within six weeks of the date of tion and quarantine of infected and shipment. • Growers are advised to communicate suspected plants, as required by PHSI. to their customers that they have complied with all the necessary legis- • Landscapers must comply with all lation and advise them of the steps the requirements of the plant they have taken to minimise the risk. passport legislation and should not It might be an option to offer an accept or sell any plants without the alternative lower risk variety or plant. necessary passport documentation.

Further information for advice on the disease eradication The Horticultural Trades Association process). A handbook of plant could also be contacted for In England and Wales, your local passporting can be downloaded at information (Tel: 01189 303132). office of the Plant Health and Seeds http://www.defra.gov.uk/planth/publica If seeping bark cankers that could be Inspectorate (PHSI) should be notified if t/passport/pass.pdf caused by P. ramorum are seen on an outbreak of the disease is suspected. PHSI Headquarters is located in York trees, the Forestry Research Advisory PHSI can also be contacted for details (Tel: 01904 455174; Fax: 01904 455197; Service should be contacted (Tel: of the plant passporting scheme and Email: [email protected]). 01420 23000 or 01420 22255).

Acknowledgements Giltrap, Charles Lane, Paul Beales, David Rizzo of the Department Alan Inman and Claire Sansford of of Plant Pathology, University of This factsheet was produced in CSL are gratefully acknowledged for California at Davis. consultation with the PHSI and HTA. their help in reviewing the text. David Slawson of the PHSI, and Neil Photographs courtesy of CSL and

August 2003 Whilst publications issued under the auspices © 2003 Horticultural Development Council. of the HDC are prepared from the best available No part of this publication may be reproduced in information, neither the authors or the HDC can any form or by any means without prior permis- accept any responsibility for inaccuracy or sion of the Horticultural Development Council. liability for loss, damage or injury from the appli- cation of any concept or procedure discussed. Design and production: HDR Visual Communication