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Phytophthora diseases

Annual Summary of Research
1 April 2002–31 March 2003

Phytophthora Disease of Alder (Project PB8831)

The text of a nine chapter FC Bulletin which reports on co-ordinated research work on alder disease across Europe (the output of an EU Concerted Action project) is now in press. A condensed version of the text has already been published as an EA R&D paper which reports on the biology and spread of the fungus and management strategies to control the disease.

Details of the work on the alder Phytophthora were also presented at the RSF Annual Meeting on Invasive Species.

Alder Disease Survey
Last year the survey was delayed because of reduced access to survey plots as a result of foot and mouth disease, and this year has required some catching up on some of the plots that had been excluded. Some plots have also had to re-established because of tree losses, in some cases as a result of trees being uprooted during winter flooding and in others due to riverside felling. This year the all the plots were assessed and the survey was conducted between July and early October 2002. Transfer of the statistician who undertakes analysis of the data has delayed the process, but the annual incidence of the disease appears to be in excess of the 1.9% for 2001 – corresponding to incidence of disease seen in previous years. Observations again indicated that the disease is very severe in the southern part of the country, particularly the west country, and diseased alders can be seen on most river systems in this part of the country.

Coppicing for regeneration
The experiment set up in 1998 at Hadley Brook in Worcestershire to examine the impact of coppicing on regeneration of alder was assessed for the third. This area was selected for coppice because it was one of the first river systems where the disease was recorded. It is also interesting because not only the standard type of the alder Phytophthora has been found there but at least three segregant genotypes which are less aggressive than the standard type. This may indicate more than one introduction of alder Phytopthoras and it may also explain why the progression of the disease has been relatively slow despite the length of time the pathogen has been infecting trees along that particular river system.

When coppicing was undertaken all the trees, healthy and diseased, were coppiced along both sides of the river along a 250m stretch. In all, 50 trees were coppiced and just prior to this 16% of the trees were dead (with possibly some roots still alive); 27% were diseased; 14% of the stools had both diseased and healthy stems; 42% had healthy stems. Since then, shoot growth from healthy and many diseased stools has been very vigorous (up to 11m in height), with some stools bearing more than 50 shoots. In some cases, shoot growth from diseased stools has been very similar to growth from healthy stools (see Table 1 below).

Table 1 Impact of coppicing on growth of Alnus glutinosa with and without the alder Phytophthora.

Condition of the coppiced stools Number with live regrowth in 2000 Number with live regrowth in 2002 Height of regrowth (m) Mean no. of shoots per stool >2 cm diam. Mean no. of shoots per stool > 4 cm diam.
All stems dead 3 0 0 0 0
All stems dead/diseased 8 7 3.6 5.7 2.9
Some stems healthy, others diseased/dead 15 13 5.2 13.9 5.2
All stems healthy 20 19 6.1 12.8 7.2
Missing trees/stools 3 5

Biology of the Alder Phytophthoras (PB8832)

Influence of Nitrogen Levels on Disease Development
Following work which showed that an increased incidence of alder disease associated with raised nitrate levels in rivers, experiments were set up to study this finding. Young alder were exposed to different nitrogen regimes (using KNO3), inoculated with the alder Phytophthora and the lesion development measured. Results are shown below in Figure 1.

Overall, the results were largely inconclusive, except that the higher levels of nitrate seemed to supress Phytophthora development. In some cases, the infections were extensive and even near-girdling before any signs of damage were visible on infected plants. This mirrors the findings in riverside trees which can have extensive lesion development in the collar region before any crown symtoms are apparent. The experiment also confirmed that individual isolates of the alder Phytophthora vary significantly in their aggressiveness to alder.

Population Biology of the Alder Phytophthoras
The alder Phytophthoras comprise a standard hybrid type and a range of variant hybrid types, each virtually equivalent to a distinct species in properties. In order to (1) distinguish between the standard hybrid and variant types and (2) to develop a method suitable for discriminating different genotypes in other Phytophthora species, such as the oak Phytophthoras, a RAPD/PCR based protocol has been developed. The approach is an extension of the method used previously (in collaboration with Imperial College) to discriminate Ophiostoma species.

Initially, a range of c. 20 RAPD (random oligonucleotide) primers were used to screen a sample of alder Phytophthora isolates. From these, three primers have been found which clearly discriminate between the standard and variant hybrids types. These primers have been tested for practical use on a wider sample of alder Phytophthora isolates although work is not completed on their ability to discriminate within and between Phytophthora species. This method provides a quick method of diagnosis for this disease and the ability to distinguish between the different variants in relatively quickly and on a consistent basis.

Biology of Phytophthoras (PB8833)

Pathogenicity and host range studies with Phytophthoras from trees planned for 2002 were deferred because of tests on P. ramorum (see Report: New and Exotic Pathogens) and also because of reductions in staff numbers. Papers on Phytophthora taxonomy (outputs 33 and 43) were written and submitted for publication and a presentation made at the Phytophthora Workshop at the International Plant Pathology Congress during February 2003.

Report prepared by J F Webber
27 March 2003