Phytophthora alni

Name:   Phytophthora alni
Pest Authorities:  Brasier et al.
Taxonomic Position:  Stramenopila: Pythiales: Pythiaceae
Sub-specific Taxon:  
Pest Type:   Fungus or fungus-like
Common Name(s):
   Phytophthora disease of alders
Numerical Score:  6
Relative Risk Rating:  High Risk
Uncertainty:   Very Uncertain
Uncertainty in this assessment results from: The susceptibility of North American Alnus species is unknown.

Establishment Potential Is Moderate Risk
The relevant criteria chosen for this organism are:  
  • Suitable climatic conditions and suitable host material coincide with ports of entry or major destinations.
  • Organism has high inoculum potential or high likelihood of reproducing after entry.
Justification: Potential host are abundant throughout much of North America particularly in regions that are climatically similar to the host range in Europe.

Spread Potential Is Moderate Risk
The relevant criteria chosen for this organism are:  
  • Organism is capable of dispersing more than several km per year through its own movement or by abiotic factors (such as wind, water or vectors).
  • Newly established populations may go undetected for many years due to cryptic nature, concealed activity, slow development of damage symptoms, or misdiagnosis.
  • Eradication techniques are unknown, infeasible, or expected to be ineffective.
Justification: Like many other species of Phytophthora, spores are soil borne and could be transported both long and short distances by infested soil adhering to tires of vehicles and feet of animals, by windblown soil and by movement of water.

Economic Potential Is High Risk
The relevant criteria chosen for this organism are:  
  • Organism directly causes tree mortality or predisposes host to mortality by other organisms.
  • Damage by organism causes a decrease in value of the host affected, for instance, by lowering its market price, increasing cost of production, maintenance, or mitigation, or reducing value of property where it is located.
  • Organism may cause loss of markets (domestic or foreign) due to presence and quarantine significant status.
  • No effective control measure exists.
Justification: This pest is apparently capable of killing healthy mature trees. If red alder (Alnus rubra) is a susceptible host, the economic impact could be locally significant since red alder is one of the few commercial hardwood species in the western US.

Environmental Potential Is High Risk
The relevant criteria chosen for this organism are:  
  • Organism is expected to cause significant direct environmental effects, such as extensive ecological disruption or large scale reduction of biodiversity.
  • Introduction of the organism would likely result in control/eradication programs that may have potential adverse environmental affects.
Justification: Alnus species play an important ecological role in forest development as pioneer species and in soil building as a nitrogen-fixer. The loss of these species could alter the nutrient status of some soils that could influence species composition. Loss of alder in the riparian zone could result in disruption of fish and wildlife habitats, and affect stream flow or water quality.

The disease has been reported on Alnus glutinosa, A. incana and A. cordata (Brasier et al. 1995, Gibbs et al. 1999, Santini et al. 2001). North American red alder (Alnus rubra) was shown to be susceptible to the fungus in inoculation experiments, but was rather more resistant to infection than was A. glutinosa (Gibbs and Lonsdale 1998).

      Austria, Denmark, France, Germany, Hungary (Ilona et al. 2000), Italy (Santini et al. 2001) Netherlands, Sweden, Englnad, Scotland and Wales (Gibbs and Lonsdale 1998, Gibbs et al. 1999), Belgium (Debruxelles and de Merlier, 2002)

The exact taxonomic position of the fungus causing Phytophthora disease in alders has not been determined. It has variously been reported as a cross between Phytophthora cambivora and P. fragariae (Hawkes 1999); a cross between P. cambivora and a second species close to, but not identical to, P. fragariae (Pain 1999); a unique species close to P. cambivora (Brasier et al. 1995), or an unusual form of Phytophthora cambivora (Gibbs et al. 1999). A "standard" hybrid alder Phytophthora thought to be a hybrid between P. cambivora and a Phytophthora close to P. fragariae has been named Phytophthora alni (Brasier et al. 2004). Several other regional variants are described and named in the same publication.

Little is known about the biology of the pathogen under field conditions. Most species of Phytophthora infect their hosts mainly by motile spores (zoospores) that are dispersed through water. This could explain the high incidence of the disease on alder in the riparian zone, although the disease may also occur in sites remote from waterways (Gibbs and Lonsdale 1998). Zoospores of the fungus have been shown to be attracted to fine roots of alder, but it not known whether such roots are infected in the field. On the contrary, it appears that infection may take place through the bark near the root collar (Gibbs and Lonsdale 1998), and that mechanical injuries may not be necessary for infection (Koltay 2003). Nothing is known about the role of oospores in the biology of the fungus. The alder Phytophthora is homothallic, and produces oospores in culture, but viability, as determined by the tetrazolium bromide method, is very low, and no germination was observed (Delcan & Brasier 2001).

Optimum temperature of the pathogen in culture is 22.5-25.0 C, with the upper temperature limit about 30 C (Brasier et al. 1995, Santini et al. 2001).

Economic Impact:    The organism causes a serious disease of alder, including lower stem bark lesions, root and collar necrosis and crown dieback typical of some other Phytophthora diseases (Brasier et al. 1995, Gibbs et al. 1999). Many infected trees die relatively quickly once symptoms develop, but others may recover. Disease severity varies considerably from place to place. In southern England and Wales, the disease has killed around 10 percent of the alders and up to 2 percent of the remaining population are dying yearly (Gibbs et al. 1999). In Hungary, average disease incidence was 1-5 percent, but reached 30-60 percent in some cases (Koltay 2003). In the Netherlands damage is much less apparent and the fungus has been isolated from trees that remain symptom free ( Its presence in the Tuscany region of Italy indicates that it has potential to cause significant loses in the drier and warmer climates of southern Europe than was previously supposed (Santini et al. 2001).

Environmental Impact:   Alders are an important species in substantial areas of natural woodland throughout Europe. Its nitrogen-fixing ability results in greatly improved soil quality, particularly following regeneration in disturbed areas ( Heavy loss of alders due to Phytophtora infection could result in significant ecological effects including changes in forest composition, wildlife food and habitat, increased soil erosion and changes in soil composition.

Control:    No control measures are reported.

Symptoms:    In mid to late summer diseased alder exhibit thin crowns and small, pale leaves. Leaves frequently fall prematurely, leaving the tree bare. Foliar symptoms do not occur until the stem has been largely girdled. On severely affected trees, tarry or rusty spots are present on the bark at the base of the tree. These spots indicate that the underlying bark is dead (Brasier et al. 1995; Gibbs and Lonsdale, 1998). The fungus may completely girdle the tree, resulting in death; or narrow strips of bark may remain alive and support limited growth (Gibbs and Lonsdale, 1998).

Morphology:    Colony characteristics and gametangial morphology are described in detail by Brasier et al. (1995).

Testing Methods for Identification:    The pathogen can be isolated from bark lesions or from soil around symptomatic trees (Brasier et al. 1995). Samples of inner bark (phloem) tissue from outer edges of necrotic lesions may be washed in running water and plated directly onto a selective medium, and incubated at 20 C (Brasier et al. 1995). An improved DAS ELISA technique suitable for mass testing of plant material has been developed for diagnosing the alder Phytophthora in Sweden (Olsson 1999). The sensitivity of the method was found to be comparable to that of the DNA-based methods using PCR. The pathogen can be isolated from water using alder twigs as bait, but the efficiency of this method is low (Streito, et al. 2002).

Nursery stock, i.e., rooted plants, and contaminated soil are the most likely means of long-distance transport of the pathogen. Wood with bark is another possible means of man-made spread. Infested soil also can be moved by vehicles or animals. Because of the common occurrence of the disease along streams, zoospores of the fungus are likely to be dispersed by flowing water. Dispersal could also take place by wind-blown infested soil.

Brasier, C.M.; Kirk, S.A.; Delcan, J.; Cooke, E.L. Jung, T.; Man In't Veld, W.A. 2004. Phytophthora alni sp. nov. and its variants: designation of emerging heteroploid pathogens spreading on Alnus trees. Mycological Research 108 (10): 1172-1184
Brasier, C.M.; Rose, J.; Gibbs, J.N. 1995. An unusual Phytophthora associated with widespread alder mortality in Britain. Plant Pathology 44: 999 - 1007.
Delcan, J.; Brasier, C.M. 2001. Oospore viability and variation in zoospore and hyphal tip derivatives of the hybrid alder Phytophthoras. Forest Pathology 31: 65-83
EPPO. 2001. First report of the Phytophthora disease of alder in Hungary. EPPO Reporting Service 2001, NO. 2. Paris, France, 2001-02-01. #2001/033.
Erwin, D.C.; Ribeiro, O.K. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society Press, St. Paul, Minnesota, USA. 562 pp.
Gibbs, J.; Lonsdale, D. 1998.. Phytophthora Disease of Alder. Information Note. July 1998. The Forestry Authority, Forestry Commission, Edinburgh, Scotland. 6 pp.
Gibbs, J.N.; Lipscombe, M.A.; Peace, A.J. 1999. The impact of Phytophthora disease on riparian populations of common alder (Alnus glutinosa) inSouthern Britain. European Journal of Forestry 29(1): 1-88.
Hawkes, N. 1999. Double fungus trouble. London Times 5.11.9.
Ilona, S.; Nagy, Z.; bakonyi, J.; Ersek, K. 2000. Firest report of Phytophthora root and collar rot of alder in Hungary. Plant Disease 84(11): 1251
Olsson, C.H.B. 1999.. Diagnosis of root-infecting Phytophthora spp. Acta Universitatis Agriculturae Suecia Agraria 161: 100 pp. [CABI WEB-SPIRS abstract].
Pain, S. 1999. Fiendish fungus. A hybrid blight is running amok in Europe's woodlands. New Scientist Vol. 162 (No. 2186) 15 May 1999, p. 7.
Santini, A.; Barzanti, G.P.; Capretti, P. 2001.. A new Phytophthora root disease of alder in Italy. Plant Disease 85: 560
Lesley Cree
Name and Address of the First Author:
Lesley Cree
Canadian Food Inspection Agency
3851 Fallowfield Rd.
Nepean, ON
Canada K0A 2T0
CREATION DATE:        07/16/99
MODIFICATION DATE:        08/09/06

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River Creedy, England
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