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New and exotic pathogens

Annual Summary of Research
1 April 2002–31 March 2003

This programme is directed to the biological and taxonomic characterisation of new pathogens, emerging pathogens and the risk of genetically changing pathogens. It provides information vital to pest risk analysis required for pathogen threats which pose a risk to the UKs trees. Outputs during the year have included organisation of and contribution to a symposium at the International Mycological Congress on ‘The role of hybridisation in the evolution of fungal pathogens’ and a contribution to the RSF conference on ‘Invasive pathogens’.

Phytophthora ramorum cause of Sudden Oak Death in the USA

Host range studies
From June 2002 to February 2002, six consecutive log inoculation experiments on oak and other key woodland and forestry species have been carried out with Phytophthora ramorum, under a Minsitry of Agriculture Licence in high security quarantine containment chambers at CABI, Silwood Park. The object was to assess the risk posed by P. ramorum to UK/European tree species. In all, 23 host have been tested (see Figure 1 below). They are:

  • Quercus robur, Q. ilex, Q. cerris, Q. suber and Q. rubra
  • Fagus sylatica, Castanea sativa, Aesculus hippocastaneum, Carpinus betulinus
  • Betula pendula, Acer platanoides, Alnus glutinosa, Tilia cordata
  • Populus tremula, Ulmus procera, Fraxinus excelsior, Prunus
  • Pseudotsuga menziesii, Picea sitchensis, Taxus baccata, Chamaecyparis lawsoniana, Sequia sempervirens
  • Rhododendron ponticum

Figure 1 Susceptibility of a range of hosts to US and European isolates of P. ramorum

On the basis of these results, the various hosts have been categorised into more susceptible species (eg Q. rubra, F. sylvatica and Castanea sativa), less susceptible species (eg Q. robur, Aesculus) and resistant (eg Tilia). Some conifer species such as Douglas fir and Sitka spruce have also proved to be highly susceptible in these tests, although it should be rembemred that these tests involve wounding the bark so do not indicate the effectiveness of the pathogen when applied to unwounded bark. The experiments also indicated there can be significant differences between the resistance levels of individual trees of the same species.

Tests also revealed that the USA poulation of P. ramorum were much more variable than the European populations of the pathogen. The latter were morphologically uniform, fast growing, highly pathogenic and of a single mating type (A1). American isolates showed a range of pathogenic behaviour and growth rates, a variable morphology and all consisted of the A2 mating type. However, when the A1 and A2 isolates were paired together, normal mating rarely if ever occurred indicating that P. ramorum does not appear to have a normally functioning breeding system, probably as a result of genetic divergence in the two populations. Recently, German workers have reported that they have isolated a single isolate of the A2 mating type in Europe but it is not clear whether this fits the profile of the American population or is capable of mating with A1 isolates from Europe. The differences in the pathogenic behaviour of European and American isolates is shown below in three representative host species.

Figure 2 Susceptibility of Q. robur, Q. rubra and Fagus to American and European isolates of P. ramorum

 


These tests have recently been extended to leaves to determine if any of these tree hosts have foliage that is capable of supporting sporulation by the pathogen. The process of leaf colonisation and sporulation is believed to be vital in driving the epidemiology of the disease; sufficient inoculum may have to build up in the canopy before susceptible tree species then succumb to stem attacks by the pathogen.

Work funded by Defra is assessing the mating behaviour of the pathogen and gene x environment interactions.

Intensive tree surveys around nursery out break sites
Following the discovery that P. ramorum was present in some nurseries and also beyond the nursery confines in 13 separate locations intensive surveys of the trees, surrounding soils and litter was undertaken. The sites (in the Hamspshire.West Sussex area) are show below.

Characteristics of sites sampled intensively for P. ramorum:

1. Specialist commercial rhododendron nursery surrounded by mixed woodland;
2. Specialist rhododendron nursery surrounded by extensive arboretum;
3. Specialist container nursery surrounded by woodland and arboretum;
4. Specialist rhododendron nursery adjacent to a large arboretum.

The results of the sampling indicated that P. ramorum could still be found in the leaf litter layer and soil, even after infected rhododendron plants were removed. Where the plants had been pruned but left in place, rain run-off water falling though the canopy also contained propagules of the pathogen. Results for one of the sites is shown below.

Table 2 Results showing number of samples (PRC1 to 5) taken along a 5 m transect with the starting point of an infected rhododendron plant.

Sample point
Rainwater trap
+ve traps (7 days)
Soil/letter samples
+ve s/l samples
PRC1
3
2
3
2
PRC2
3
0
3
1*
PRC5
3
0
3
0
Totals
9
2
3
3


However, the intensive examination and sampling of the trees did not reveal any indication of infection by P. ramorum. Other Phytophthoras and other fungal pathogens were found instead and some caused symptoms on birch, sycamore and horse chestnut that were consistent with the type of infection that would be expected with P. ramorum and other Phytophthora diseases? These included stem cankers and bleeding, necrotic areas of bark. None of these symptoms were found on any of the oaks at these sites, although other classic symptoms of oak decline were in evidence.

Transfer of Knowledge
The occurrence of the Sudden Oak Pathogen P. ramorum in the UK has raised much speculation and interest. To meet the demands for information various presentations have been made including:

 

  • Presentation on the host susceptibility studies and the intensive survey work to the PHSI annual conference.
  • Presentation at the Phytophthora Workshop at the International Plant Pathology Congress on Phytopthora ramorum.
  • Poster on the hosts susceptibility studies with P. ramorum at the International Plant Pathology Congress
  • Presentation on the work carried out by FR on P. ramorum presented to a specially convened EU Plant Health Committee, to advice member states on the status of this pathogen in their review of the emergency legislation that has been applied to Sudden Oak Death.

Other work
As part of the work on New and Exotic Pathogens a review of the position of Red Band Needle Blight in the UK has been carried out, resulting in a draft IN. In addition, four EPPO Diagnostics (Chestnut blight, red band needle blight, brown spot needle blight, plane wilt) have been reviewed and one is currently being drafted.