Figure 1. Phytophthora pseudotsugae. A–H, Ovoid, papillate sporangia. I and J, Abnormally shaped sporangia. K, An intercalary sporangium. L and M, Germination of sporangia by production of zoospores. N, Oogonium formed adjacent to the mycelium. O, Oogonium on a stalk. P and Q, Oogonia containing oospores that, although aplerotic, nearly fill the oogonia; antheridia are paragynous. R, Oogonium containing an oospore; an amphigynous antheridium is attached. (Reproduced, by permission of the National Research Council Canada, from Hamm and Hansen, 1983) Click image to see larger view.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora pseudotsugae  Hamm & E. M. Hansen (1983)

 

Phytophthora pseudotsugae was first isolated from rotted roots of Douglas fir (Pseudotsuga menziesii) in forest tree nurseries but was identified only as Phytophthora species 1 (Pratt et al., 1976). P. pseudotsugae was described as a new species by Hamm and Hansen (1983) and is part of a complex that causes root rot on Douglas fir. It is phylogenetically close to P. cactorum (Kroon et al., 2004) and is a group I Phytophthora species (Hall, 1989; Stamps et al., 1990) (Fig. 1).

Cultural Characteristics

Growth occurs at 2–30°C, and the optimum temperature for growth is 20–25°C. Asexual structures are produced in distilled water and soil extract, while sexual structures are produced on cornmeal agar, lima bean agar, pea broth, and V-8 juice agar. 

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are either unbranched or long and irregularly branched (29–632 µm).

 

Sporangia:

Sporangia can be spherical, ovoid, obturbinate, or ampulliform. They are distinctly papillate and form when grown on pea broth and washed and incubated in distilled water or soil extract. Sporangia are noncaducous. Sporangia form at 10°C within 1–2 days and average 39 × 49 µm. Sporangia that form at 20°C are smaller, averaging 32 × 39 µm. The length–breadth ratios are less than 1.6. Intercalary sporangia may be formed on some isolates (Fig. 1A–K).

 

Chlamydospores:

Chlamydospores are not produced.

 

Hyphae:

Hyphae are aseptate and 3–7 µm in diameter (Waterhouse and Waterston, 1980). Irregular hyphal swellings have been observed (Hamm and Hansen, 1983). 

 

Sexual Structures

 

P. pseudotsugae is homothallic.

 

Antheridia:

Antheridia are hyaline and usually spherical or club shaped. They are 6–21 × 10–24 µm (average 11 × 15 µm). Antheridia are predominately paragynous but in some cases are amphigynous. Antheridia are attached to the oogonia at a point near the oogonial stalk.

 

Oogonia:

Size of oogonia vary with media on which P. pseudotsugae is grown. In V-8 juice agar, oogonia are 32–40 µm in diameter (average 35 µm), and on cornmeal agar and pea broth, the average diameter is 31 µm. Oogonia are spherical and smooth walled, with a wall thickness of 1.2 µm (Fig. 1N and O).

 

Oospores:

Oospores are pigmented, spherical, smooth walled, and aplerotic. On V-8 juice agar, oospores are 27–33 µm in diameter (average 24 µm). Most oospores are reported to be abortive (Hamm and Hansen, 1983) (Fig. 1P–R).

Host Range and Distribution

P. pseudotsugae is the causal organism of root rot of Pseudotsuga menziesii (Douglas fir). The pathogen infects roots and causes a rot of lateral roots. Infected nursery plants may become stunted over time.

Symptoms

During the first year of infection, roots of Douglas fir may become reddish brown, and during the second year, plants become stunted and roots become dark brown (Cooley et al., 1985; Erwin and Ribeiro, 1996).

References

Cooley, S. J., Hamm, P. B., and Hansen, E. M. 1985. Management guide to Phytophthora root rot in bareroot conifer nurseries of the Pacific Northwest. U.S. Dep. Agric. For. Serv. Pacific Northwest Region, Portland, OR.

 

Erwin, D. C., and Ribeiro, O. K. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society, St. Paul, MN.

 

Hall, G. 1989. Phytophthora pseudotsugae. CMI Descr. Pathog. Fungi Bact. 978.

 

Hamm, P. B., and Hansen, E. M. 1983. Phytophthora pseudotsugae, a new species causing root rot of Douglas-fir. Can. J. Bot. 61:2626-2631.

 

Kroon, L. P. N. M., Bakker, F. T., van den Bosch, G. B. M., Bonants, P. J. M., and Flier, W. G. 2004. Phylogenetic analysis of Phytophthora species based on mitochondrial and nuclear DNA sequences. Fungal Genet. Biol. 41:766-782.

 

Pratt, R. G., Roth, L. F., Hansen, E. M., and Ostrofsky, W. D. 1976. Identity and pathogenicity of species of Phytophthora causing root rot of Douglas-fir in the Pacific Northwest. Phytopathology 66:710-714.

 

Stamps, D. J., Waterhouse, G. M., Newhook, F. J., and Hall, G. S. 1990. Revised tabular key to the species of Phytophthora. Mycol. Pap. 162. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

 

Waterhouse, G. M., and Waterston, J. M. 1980. Phytophthora pseudotsugae. CMI Descr. Pathog. Fungi Bact. 978.