| Phytophthora ramorum: The Pathogen |
There are over 60 species of Phytophthora known throughout the world and this list is increasing with
Phytophthora ramorum being one of the more recently described species. It was first identified
from isolates taken from rhododendrons with twig blight and leaf spot in Germany and the Netherlands.
P. ramorum was shown to be morphologically identical to a Phytophthora species found
causing sudden oak death in California. Molecular analyses strongly indicate the North American and
European isolates are of the same species.
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Symptoms |
In the USA, the North American population of P. ramorum causes a lethal disease of several
forest oak species native to California and southwest Oregon. It also causes severe to moderate
diseases of other native tree canopy genera and understorey plants.
Specific symptoms are varied and dependent upon the host. On trees, symptoms may include branch-tip dieback,
foliar lesions, stem and branch cankers and death of new shoots. In some hosts symptoms can lead to
death of samplings and large trees. On shrubs, symptoms may include foliar lesions,
stem and branch cankers, wilting, dieback and may lead to the death of the plant. |
SOD in California woodland
Wilting on Rhododendon
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Distribution |
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Separate European and American populations have been identified. The A1 mating type of
P. ramorum has been identified in the European subpopulation, whereas isolates of the
North American subpopulation are of the A2 mating type. Recently, on nursery stock, some A1 isolaes,
have been found in North America and one A1 isolate in Europe. Apart from the difference in mating type
distribution, there are also differences in adaptation between European and North American populations,
such as variations in average aggressiveness.
It is not known how P. ramorum originally entered Europe or the USA, but the mating type and
population distribution suggests that separate introductions into Europe and to North America may
have occured from a third unknown location.
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Geographical Distribution |
P. ramorum was first identified in the UK in 2002, and in the Republic of Ireland in 2003. In
addition, within the EU, the pathogen has been detected in Belgium, Denmark, France, Germany,
the Netherlands, Spain and Sweden. In addition to these coutnries, within the EPPO region the pathogen
has also been isolated in Poland from imported hosts and in Slovenia. More recently, it has also been identified
in Italy, Norway (Interceptions), Switzerland and the Czech Republic.
In the USA in 2003, the pathogen was distributed from one Californian nursery to a number of nurseries
in States where the pathogen had not previously been reported; At least 21 States now have P. ramorum. |
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Host plants |
In the USA, various tree hosts have been identified including Quercus spp. (Oak)
and also ornamental shrubs including Rhododendron spp., Camelia spp. and Pieris spp.
In the UK and Republic of Ireland the pathogen has been found mainly on species/hybrids of
Rhododendron and Viburnum. Findings on other hosts have been less common.
The list of hosts in North America and the UK is increasing. Since October 2003 the first
trees were found with disease in the UK and the Netherlands.
Further details of host species can be found in Work Package 1: Distribution.
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Biology |
Analysis of a number of variables suggests that canker (bark necrosis) of coast live oak is more
likely to occur in trees that are vigorous and/or fast growing (i.e. trees that are larger, more dominant,
less water stressed and not in decline due to other agents) than in trees that are suppressed and/or slow
growing. Spatial patterns of diseased oak trees indicate that proximity to the forest edge is an
important explanatory factor followed by potential topographic moisture, abundance of bay laurel
and potential solar radiation.
In initial inoculation experiments carried out in the UK on detached leaves of Rhododendron using
mycelial plugs, wounding was necessary for successful infection and disease development. Unwounded
leaves did not routinely develop symptoms. There is also now strong evidence for infection of leaves of
Rhododendron and many other species by zoospores in the absence of wounds. It is thought that
infection via zoospores in nature may occur both through natural openings and wounds.
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