Figure 1. Morphology of Phytophthora sojae. Upper row, Globose oogonium with a paragynous antheridium; ovoid and obpyriform sporangia. Lower row, Chlamydospore (note: Hildebrand [1959] is the only citation on chlamydospore formation); empty remnants of hyphal swellings. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

Figure 2. Culture of Phytophthora sojae grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

 

Figure 3. Sporangia of Phytophthora sojae (×1,000). (Courtesy Jean B. Ristaino)

 

Figure 4. Spherical oogonium of Phytophthora sojae with a plerotic oospore and a paragynous antheridium (×1,000). (Courtesy Jean B. Ristaino)

 

Figure 5. Postemergence damping-off, caused by Phytophthora sojae. (Courtesy Anne E. Dorrance; Reproduced from Dorrance and St. Martin, 2000)

 

Figure 6. Stem rot of soybean, caused by Phytophthora sojae. (Courtesy Anne E. Dorrance, The Ohio State University)

 

Figure 7. Soybean seedlings following inoculation with a race of Phytophthora sojae, the two seedlings on the left are resistant and the two seedlings on the right are susceptible. (Courtesy Anne E. Dorrance; Reproduced from Dorrance and St. Martin, 2000)

 
 
 
 
 
 
 
 

Introduction

Phytophthora sojae  Kaufm. & Gerd. (1958)
 

Phytophthora sojae is the causal organism of root rot of soybean. It was first identified as P. cactorum (Herr, 1957) and later redescribed as the new species P. sojae (Kaufmann and Gerdemann, 1958). P. sojae was isolated from immature seeds collected from pods that were produced on an infected stem (Klein, 1959). Synonyms include P. megasperma var. sojae A. A. Hildebr. (1959), P. megasperma f. sp. glycinea T. L. Kuan & Erwin (1980), and P. sojae f. sp. glycines Faris et al. (1989). P. sojae was placed into P. megasperma var. sojae by Hildebrand (1959) because the size of the oogonia was similar, but this species included isolates from several other hosts besides soybean. Hildebrand created the new variety P. megasperma var. sojae to accommodate isolates infecting soybean. Waterhouse (1963) separated P. megasperma into P. megasperma var. sojae and P. megasperma var. megasperma based on oogonium size (small vs. large, respectively). Kuan and Erwin (1980) questioned the separation of the species based on oogonium size since isolates from different hosts show a continuum of oogonial sizes and renamed the species P. megasperma f. sp. glycinea and P. megasperma f. sp. medicaginis for the soybean and alfalfa isolates, respectively. Others also revised the species (Faris et al., 1989). Faris et al. (1989) reinstated the name P. sojae and designated the soybean isolates P. sojae f. sp. glycines. Hansen and Maxwell (1991) and Erwin and Ribeiro (1996) also accepted the name P. sojae (Cline et al., 2008). The whole genome of this pathogen has been sequenced (Tyler et al., 2006). P. sojae is classified as a group V Phytophthora species (Stamps et al., 1990) (Fig. 1).

Cultural Characteristics

The optimum temperatures for growth are 25°C (Faris et al., 1989; Hildebrand, 1959) and 20°C (Hansen and Maxwell, 1991; Kaufmann and Gerdemann, 1958). The optimum temperature for growth is 25–28°C, and the maximum temperature for growth is 32–35 °C (Schmitthenner, 1989) (Fig. 2). P. sojae only grows on potato dextrose agar if sitosterol is added to the media at 30 µg/ml or if agar is diluted to 1:10 (Erwin et al., 1968).

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are simple and undifferentiated. Sporangia are formed sympodially and by internal and external proliferation (Hildebrand, 1959).

 

Sporangia:

Sporangia can be ovoid, ellipsoid, or obpyriform. Sporangia are noncaducous, nonpapillate, and 16.6–51.8 × 23.3–88.8 µm (average 38.3 × 58 µm (Kaufman and Gerdemann, 1958) (Fig. 3).

 

Chlamydospores:

Chlamydospores were not identified by Kaufmann and Gerdemann (1958), but Hildebrand (1959) showed that chlamydospores were present and that they were similar in size to oogonia. Hildebrand stated that they conformed to the definition of Blackwell (1949). Schmitthenner (1989) stated that chlamydospores occurred, but he did not give dimensions (Fig. 1).

 

Hyphae:

Intercalary hyphal swellings are spherical and irregular in shape and occur in aqueous culture (Fig. 1).

 

Sexual Structures

 

P. sojae is homothallic.

 

Antheridia:

Antheridia are mostly paragynous but can be amphigynous (Faris et al., 1989; Hildebrand, 1959). Many antheridia are perpendicular or oblique to the oogonial wall (Gallegly and Hong, 2008).

 

Oogonia:

Oogonia are spherical and subspherical and vary in size. Hildebrand (1959) described oogonia as 29.4–45.7 µm (average 36.3 µm). Kaufmann and Gerdemann (1958) described oogonia as 19.2–38.3 µm (average 31 µm). Faris et al. (1989) described oogonia as 32–38 µm.

 

Oospores:

Oospores form abundantly on lima bean agar. The oospore diameter is 19.2–38.3 µm (average 31.4 µm) (Kaufmann and Gerdemann, 1958) (Fig. 4). Oospores are plerotic and germinate at relatively high percentages (Bhat, 1991; Bhat and Schmitthenner, 1992; 1993a; 1993b; Bhat et al., 1990; 1992; 1993a; 1993b; Erwin and McCormick, 1971; Schechter and Gray, 1987).

Host Range and Distribution

P. sojae causes root and stem rot in soybean. P. sojae can infect different species of lupine. Jones and Johnson (1969) postulated that P. sojae might have existed on naturally occurring lupine plants prior to the introduction of soybean to North America. Others postulate an Asian origin for the pathogen.

Symptoms

Root and Stem Rot of Glycine max (Soybean):

Preemergence damping-off occurs in seedlings, and at the postemergence stage, seedlings may be killed quickly (Fig. 5). On older plants, symptoms include the wilting of leaves, the development of chlorotic areas between the veins and along margins of the leaves, and the extension of a brown girdling rot up the stem (Fig. 6). Once the roots are infected, lateral and branch roots turn brown and the cortex and vascular tissues become discolored. P. sojae can persist as oospores in infected tissue or soil. Root and stem rot develops rapidly at 25–30°C and under moist soil conditions. In susceptible cultivars, P. sojae infects lateral roots and progresses into the taproots and hypocotyls (Fig. 7). In susceptible cultivars, the pathogen infects the cortex and stele of the lateral roots (Beagle-Ristaino and Rissler, 1983).

References

Bhat, R. G. 1991. Genetics of virulence in Phytophthora megasperma f. sp. glycinea. Ph.D. dissertation. Department of Plant Pathology, Ohio State University, Columbus, OH.

 

Bhat, R. G., and Schmitthenner, A. F. 1992. Virulence evaluation of single-oospore cultures of various physiologic races of Phytophthora sojae. (Abstr.) Phytopathology 82:1082.

 

Bhat, R. G., and Schmitthenner, A. F. 1993a. Selection and characterization of inhibitor-resistant mutants of Phytophthora sojae. Exp. Mycol. 17:109-121.

 

Bhat, R. G., and Schmitthenner, A. F. 1993b. Genetic crosses between physiologic races of Phytophthora sojae. Exp. Mycol. 17:122-129.

 

Bhat, R. G., Schmitthenner, A. F., and McBlain, B. A. 1990. Genetic analysis of virulence in Phytophthora megasperma f. sp. glycinea. (Abstr.) Phytopathology 80:968.

 

Bhat, R. G., Olah, A. F., and Schmitthenner, A. F. 1992. Characterization of universally avirulent strains of Phytophthora sojae. Can. J. Bot. 70:1175-1185.

 

Bhat, R. G., McBlain, B. A., and Schmitthenner, A. F. 1993a. The inheritance of resistance to metalaxyl and to fluorophenylalanine in matings of homothallic Phytophthora sojae. Mycol. Res. 97:865-870.

 

Bhat, R. G., McBlain, B. A., and Schmitthenner, A. F. 1993b. Development of pure lines of Phytophthora sojae races. Phytopathology 83:473-477.

 

Beagle-Ristaino, J. E., and Rissler, J. F. 1983. Histopathology of susceptible and resistant soybean roots inoculated with zoospores of Phytophthora megasperma f. sp. glycinea. Phytopathology 73:590-595.

 

Blackwell, E. 1949. Terminology in Phytophthora. Mycol. Pap. 30. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

 

Cline, E. T., Farr, D. F., and Rossman, A. Y. 2008. A synopsis of Phytophthora with accurate scientific names, host range, and geographic distribution. Plant Health Progress doi:10.1094/PHP-2008-0318-01-RS.

 

Dorrance, A. E., and St. Martin, S. 2000. Phytophthora sojae: Is it time for a new approach? APSnet Feature. American Phytopathological Society, St. Paul, MN. www.apsnet.org/publications/apsnetfeatures/Pages/PhytophthoraSojae.aspx.

 

Erwin, D. C., and McCormick, W. H. 1971. Germination of oospores produced by Phytophthora megasperma var. sojae. Mycologia 63:972-977.

 

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

 

Erwin, D. C., Partridge, J. E., and McCormick, W. H. 1968. Mycelial growth and production of oospores as affected by ß–sitosterol, and the germination of oospores of Phytophthora megasperma var. sojae. (Abstr.) Phytopathology 58:1049.

 

Faris, M. A., Sabo, F. E., Barr, D. J. S., and Lin, C. S. 1989. The systematics of Phytophthora sojae and P. megasperma. Can. J. Bot. 67:1442-1447.

 

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

 

Hansen, E. M., and Maxwell, D. P. 1991. Species of the Phytophthora megasperma complex. Mycologia 83:376-381.

 

Herr, L. J. 1957. Factors affecting a root rot of soybeans incited by Phytophthora cactorum. (Abstr.) Phytopathology 47:15-16.

 

Hildebrand, A. A. 1959. A root and stalk rot of soybeans caused by Phytophthora megasperma Drechsler var. sojae var. nov. Can. J. Bot. 37:927-957.

 

Jones, J. P., and Johnson, H. W. 1969. Lupine, a new host for Phytophthora megasperma var. sojae. Phytopathology 59:504-507.

 

Kaufmann, M. J., and Gerdemann, J. W. 1958. Root and stem rot of soybean caused by Phytophthora sojae n. sp. Phytopathology 48:201-208.

 

Klein, H. H. 1959. Etiology of the Phytophthora disease of soybeans. Phytopathology 49:380-383.

 

Kuan, T.-L., and Erwin, D. C. 1980. Formae speciales differentiation of Phytophthora megasperma isolates from soybean and alfalfa. Phytopathology 70:333-338.

 

Schechter, S. E., and Gray, L. E. 1987. Oospore germination in Phytophthora megasperma f. sp. glycinea. Can. J. Bot. 65:1465-1467.

 

Schmitthenner, A. F. 1989. Phytophthora rot. Pages 35-38 in: Compendium of Soybean Diseases, 3rd ed. J. B. Sinclair and P. A. Backman, eds. American Phytopathological Society, St. Paul, MN.

 

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.

 

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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.