Paper published by RHS staff:
Henricot, B. and Prior, C., (2004). Phytophthora ramorum, the cause of sudden oak death or ramorum leaf blight and dieback. Mycologist 18(4): 151-156
Latest news
INTERCEPTIONS: There has been a 38% fall in outbreaks despite a 56% increase in inspections by DEFRA in 2004, which may mean that their eradication campaign is succeeding. There have now been 60 findings in parks, gardens and the wider environment. 75% of the findings in nurseries are of UK origin, 15% from the Netherlands, the rest from Belgium, France, Italy and Poland. There were only 4 findings on plant-passported imports (all destroyed), so the passporting scheme seems to be working.
UPDATE ON RESEARCH: Studies have shown that Vaccinium myrtilus (bilberry) is very susceptible, so are ash, elder and dogrose. Chlamydospores can survive winter freezing, but there is limited persistence (based on baiting) at sites where the litter debris is removed, indicating perhaps that chlamydospores remain in the litter rather than the soil. The fungus is not dispersed by wind in the absence of rain, but may be spread in wind-blown rain.
Occurrence
A
disease known as Sudden Oak Death in the western USA and as
ramorum dieback in northern Europe has been recognised since
the early 1990s. It was first found in the UK in imported
Viburnum in West Sussex, Dorset, Lincolnshire
and Lancashire in 2002.
Since then there have been over 300 cases in the UK, most in containerised plants but some more recently in trees and shrubs in established gardens. The disease is subject to eradication under plant health legislation and infected plant material in nurseries has been destroyed.
Currently the eradication measures have been suspended in a few of the garden outbreaks to allow scientists from the Department for the Environment, Food and Rural Affairs (DEFRA) to study the disease under natural conditions, prior to a decision on whether to proceed with eradication in these cases also.
The disease is notifiable in the UK and all suspect cases must be reported by law to the Plant Health and Seeds Inspectorate (a part of DEFRA). The name ramorum dieback is preferred in UK, because it appears that native oaks are relatively resistant.
Cause
The disease is caused by the fungus Phytophthora ramorum. It gives rise to different symptoms in various unrelated species, including viburnums and rhododendrons in Europe and native Californian oaks in the USA. Large numbers of native oaks have been lost in California and Oregon. Studies on the biology of the fungus suggest that it has arrived independently and recently in western USA and northern Europe from an unknown country of origin.
Symptoms
In
rhododendrons the disease appears as dark blotches on the
leaves and on the bark of twigs, whereas in viburnums it can
also cause wilting as a result of bark cankers at the base
of the plant. In other UK hosts it mainly causes leaf blotches
similar to those on Rhododendron.
In native oaks in the USA, Phytophthora ramorum causes bark cankers. The bark splits, oozing red-black sap. Once infected, trees can die in months. Shoot tips may show sudden wilt and foliage on branches or the whole tree may rapidly discolour. The fungus grows into the bark and destroys living tissue. Similar symptoms of bark canker have been observed recently on southern red oak (Quercus falcata), horse chestnut and beech in UK.
Hosts
The
major known hosts in the USA are: Lithocarpus
densiflorus (tanoak), Quercus agrifolia
(coast live oak), Q. kellogii (black
oak), Q. parvula var. shrevei
(interior live oak), Arbutus menziesii
(madrone), Umbellularia californica
(California Bay Laurel: not the same as Laurus
nobilis, the European Bay Laurel), Vaccinium
species, Rhododendron species, Viburnum
species, grand fir and Douglas fir.
In the UK and northern Europe, the great majority of cases have been on Viburnum and Rhododendron. In addition, occasional cases have been detected on Camellia, Pieris, Leucothoe, Syringa, Hamamelis, Taxus and Kalmia. There have also been cases in UK in southern red oak (Quercus falcata), beech, horse chestnut, Turkey oak, holm oak and sweet chestnut. A case of bark canker on red oak (Q. rubra) has been reported from The Netherlands.
Studies in quarantine suggest that native oaks (Q. robur and Q. petraea) are relatively resistant, although individual trees may vary in susceptibility.
Further information
Details are available from the Food and Environment Research Agency (FERA) and the Forestry Commission.
View the illustrated leaflet produced by DEFRA
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What gardeners should do
If the disease is suspected, gardeners
with web access should check the symptoms against the illustrations
on the FERA website (above). If still suspicious, they should
notify the relevant authorities:
for viburnums, rhododendrons and other shrubs,
ring the Plant Health and Seeds Inspectorate (headquarters
in York, 01904 455174);
for trees, ring Forestry Commission Plant
Health Service on 0131 314 6414.
If appropriate, the inspection services may visit the site
and samples may be taken for testing. The disease is subject
to eradication and affected plants will be destroyed if the
disease is confirmed. Because of the pressures on the inspection
services, DEFRA have indicated that where only one or two
plants are involved in private gardens, they may simply require
the owners to destroy the affected plants according to DEFRA
instructions, without inspection.