Figure 1. Culture of Phytophthora pseudosyringae grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

 

Figure 2. Phytophthora pseudosyringae. Top row left, Semipapillate sporangia (isolate PSEU 16) borne in a regular compact sympodium. Top row middle to Bottom row left, Semipapillate terminal sporangia in water culture. Bars = 20 µm. Top row middle, Ovoid empty sporangia with a conspicuous basal plug and an externally proliferating mature sporangium with a short pedicel (isolate PSEU 1). Top row right and Middle row left, Limoniform to ellipsoid, mature sporangia, some of them caducous with a short pedicel (isolates PSEU 16 and 6). Middle row middle and right, Ovoid mature sporangia, one of them releasing zoospores (isolate PSEU 25). Bottom row left, Obovoid mature sporangium (isolate PSEU 25). Bottom row middle, Chains of spherical to subspherical hyphal swellings (isolate PSEU 1) in water culture. Bar = 25 µm. Bottom row right, Spherical hyphal swelling with radiating hyphae in water culture (isolate PSEU 12). Bar = 20 µm. (Reproduced, by permission of The British Mycology Society, from Jung et al., 2003) Click image to see larger view.

 

Figure 3. Semipapillate terminal sporangia of Phytophthora pseudosyringae (×10). (Courtesy David Rizzo, University of California-Davis)

 

Figure 4. Culture of Phytophthora pseudosyringae with chains of inflated, spherical hyphal swellings. (Courtesy David Rizzo, University of California-Davis)

 

Figure 5. Oospores of Phytophthora pseudosyringae (×10) with paragynous antheridia (×20). (Courtesy David Rizzo, University of California-Davis)

 

Figure 6. Globose to subglobose oogonia and oospores of Phytophthora pseudosyringae on V-8 juice agar. Bars = 20 µm. Top row left to Middle row left, Oogonia with paragynous antheridia (isolates PSEU 12, 8, 6, 14). Middle row middle and right, Oogonia with amphigynous antheridia (isolate PSEU 8). Bottom row left, Oogonium with aborted oospore (isolate PSEU 8). Bottom row middle, High proportion of oogonia with aborted oospores on V-8 juice agar (isolate PSEU 8). Bottom row right, Irregular, coralloid hyphae of P. pseudosyringae (isolate PSEU 16) on V-8 juice agar. Bar = 25 μm. (Reproduced, by permission of The British Mycology Society, from Jung et al., 2003) Click image to see larger view.

 

Introduction

Phytophthora pseudosyringae  T. Jung & Delatour (2003)

Phytophthora pseudosyringae is a soilborne pathogen of oak and beech trees. Because of its morphological characteristics, P. pseudosyringae was first identified as P. syringae. RFLP analysis of the internal transcribed spacer (ITS)-DNA sequence data revealed that P. pseudosyringae was more closely related to P. ilicis and P. psychrophila than to P. syringae, and the species name was changed to P. pseudosyringae (Gallegly and Hong, 2008). P. ilicis is the closest relative to P. pseudosyringae. P. pseudosyringae is a group III Phytophthora species (Erwin and Ribeiro, 1996; Waterhouse, 1963) and a member of clade 3 according to Cooke et al. (2000).

Cultural Characteristics

Cultures show a stellate growth pattern on V-8 juice agar and carrot agar (CA) (Fig. 1). On malt extract agar and cornmeal agar (CMA) amended with beta-sitosterol, cultures show a stellate to rosaceous growth pattern. Cultures have an appressed to limited aerial growth on CA and CMA media and limited aerial to fluffy growth on V-8 juice agar. On potato dextrose agar, cultures have appressed and stellate to rosaceous patterns. Not all isolates show the same growth patterns. P. pseudosyringae's temperature range for growth is 5–25°C, and the optimum temperature for growth is 20°C. The maximum temperature for growth is slightly higher for P. pseudosyringae (25°C) than for P. syringae (23°C).

Reproductive Structures 

Asexual Structures 

Sporangiophores:
Sporangiophores are unbranched. Sympodium are simple, lax, or compact and sporangia are terminal.

Sporangia:
Sporangia are semipapillate and very rarely bipapillate. Sporangia are usually limoniform, ellipsoid, or ovoid. Sporangia are mostly noncaducous, but the caducous sporangia that occur have medium pedicels (<5 to 20 μm). Sporangia are 15–48 × 20–98 μm (average 29.5 ±3.2 × 47.5 ±6.3 μm) (Figs. 2.10–2.16 and 3). The length–breadth ratio is 1.61 ±0.13 μm (Jung et al., 2003).

Chlamydospores:
No chlamydospores have been observed.

Hyphae:
On V-8 juice agar, primary Hyphae are 4–9 μm wide (average 6.2 μm). Hyphae do not branch sympodially. Most isolates produce irregular to coralloid lateral hyphae (Jung et al., 2003) (Fig. 6.9). Chains of inflated, spherical to deltoid hyphal swellings are produced sometimes with radiating hyphae in water culture in isolates from oak and beech and the hyphal swellings average 15.5 μm in diameter (Figs. 2.17 and 2.18 and 4). Isolates from alder form small numbers of single spherical hyphal swellings.

Sexual Structures

P. pseudosyringae is homothallic.

Antheridia:
Antheridia are hyaline, single, terminal, and spherical or club shaped to irregular in shape. Antheridia are predominately paragynous and 6.5–15 × 8.5–26 μm (average 9.5 × 14.8 μm) (Jung et al., 2003).

Oogonia:
Oogonia are smooth walled and spherical. Oogonia were borne terminally. Oogonia are 16.5–42.5 μm in diameter (average 29 ±2.9 μm) (Jung et al., 2003).

Oospores:
Oospores are spherical and usually plerotic (Fig. 5). Oospore are 15–36.5 μm in diameter (average 26.5 ±2.8 μm) (Fig. 6.1–6.8). Oospore walls are 0.5–2.5 μm (average 1.35 ±0.22 μm) (Jung et al., 2003).

Host Range and Distribution

P. pseudosyringae has been found in forest soils in Bavaria, northern Germany, northeastern and southern France, and northern Italy. P. pseudosyringae has been found to be pathogenic to oak (Quercus petraea, Quercus robur, and Quercus cerris), beech (Fagus sylvatica), alder (Alnus glutinosa), and holly (Ilex aquifolium). P. pseudosyringae has also been found to cause severe fruit rot on apples of the cultivars Cox’s Orange Pippin and Bonapfel (Jung et al., 2003).

Symptoms

Symptoms of P. pseudosyringae are similar to the symptoms of P. ramorum. 

Quercus robur and Quercus petraea (Oak):
 P. pseudosyringae causes root rot in Quercus robur and Quercus petraea.

Fagus sylvatica (Beech):
P. pseudosyringae has been found in the soil surrounding beech trees and has been isolated from necrotic roots of beech trees. P. pseudosyringae also causes large necrotic bark lesions on the lower stems of Fagus sylvatica. 

Alnus glutinosa (Alder):
P. pseudosyringae is pathogenic to the bark of alder trees.

Ilex aquifolium (Holly):
 P. pseudosyringae causes necrosis on holly leaves.

Apple:
 P. pseudosyringae causes severe fruit rot on apples.

References

Cooke, D. E. L., Drenth, A., Duncan, J. M., Wagels, G., and Brasier, C. M. 2000. A molecular phylogeny of Phytophthora and related oomycetes. Fungal Genet. Biol. 30:17-32. 

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

Gallegly, M., and Hong, C. 2008. Phytophthora: Identifying Species by Morphology and DNA Fingerprints. American Phytopathological Society, St. Paul, MN.

Jung, T., Nechwatal, J., Cooke, D. E., Hartmann, G., Blaschke, M., Osswald, W. F., Duncan, J. M., and Delatour, C. 2003. Phytophthora pseudosyringae sp. nov., a new species causing root and collar rot of deciduous tree species in Europe. Mycol. Res. 107:772-789. 

Waterhouse, G. M. 1963. Key to the species of Phytophthora de Bary. Mycol. Pap. 92. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.