Figure 1. Morphology of Phytophthora medicaginis. Nonpapillate sporangia, hyphal swellings, and paragynous and amphigynous antheridia on globose oogonia. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

Figure 2. Sporangia of Phytophthora medicaginis with motile zoospores from a germinated oospore. (Courtesy Deborah A. Samac, USDA-ARS, St. Paul, MN)

 

Figure 3. Common leaf spot of alfalfa caused by Phytophthora medicaginis. (Courtesy W. G. Willis; Reproduced from APS Digital Image Collections, Diseases of Legumes, American Phytopathological Society, St. Paul, MN)

 

Figure 4. Early symptoms of yellow leaf blotch on alfalfa caused by Phytophthora medicaginis. (Courtesy W. G. Willis; Reproduced from APS Digital Image Collections, Diseases of Legumes, American Phytopathological Society, St. Paul, MN)

 

Figure 5. Seedlings with reddish leaves, brown discoloration of roots caused by Phytophthora medicaginis on the left. (Courtesy Deborah A. Samac, USDA-ARS, St. Paul, MN)

 

Figure 6. Soft rot of taproot caused by Phytophthora medicaginis. (Courtesy Deborah A. Samac, USDA-ARS, St. Paul, MN)

 

Figure 7. Missing taproot, dark lesion caused by Phytophthora medicaginis. (Courtesy Deborah A. Samac, USDA-ARS, St. Paul, MN)

 

Introduction

Phytophthora medicaginis  E. M. Hansen & D. P. Maxwell (1991)

 

Phytophthora medicaginis has the synonym P. cryptogea (Erwin, 1954). The pathogen was first found to be the causal organism of a root rot of alfalfa (Medicago sativa) in California, but it probably existed long before this first discovery. P. medicaginis is responsible for root rot of alfalfa and affects all regions in the world where alfalfa is grown. Erwin reclassified the pathogen as P. megasperma in 1965 because of the discovery of both paragynous and amphigynous antheridia (Erwin, 1965). Waterhouse (1963) separated P. megasperma into P. megasperma var. sojae and P. megasperma var. megasperma based on oogonium size. Kuan and Erwin (1980) questioned the separation of the species based on oogonium size since isolates from different hosts show a continuum of oogonium sizes. Narrow host range isolates from alfalfa were named P. megasperma f. sp. medicaginis and those from soybean were named P. megasperma f. sp. glycinea (Kuan and Erwin, 1980). Faris et al. (1989) moved the soybean isolates from P. megasperma f. sp. sojae to P. sojae f. sp. glycines and the alfalfa isolates from P. megasperma f. sp. medicaginis to P. sojae f. sp. medicaginis. Molecular evidence indicates the two species are distinct from each other and from true P. megasperma isolates (Förster et al., 1989). Hansen and Maxwell named the alfalfa pathogen P. medicaginis in 1991 (Hansen and Maxwell, 1991). See Erwin and Ribeiro (1996) and Cline et al. (2008) for a complete description of the historical nomenclature of this pathogen. P. megasperma is a group V Phytophthora species (Stamps et al., 1990) (Fig. 1). 

Cultural Characteristics

On V-8 juice agar, cornmeal agar, and synthetic agar, there is little growth pattern, but on potato dextrose agar, radial growth is somewhat petallate (Erwin, 1966; Kuan and Erwin, 1980). P. medicaginis growth is optimal at 25–26°C but is relatively slow.

Reproductive Structures

Asexual Structures

 

Sporangia:

Sporangia in aqueous solutions are ovoid to ellipsoid, nonpapillate, and noncaducous and can vary in size, 24–30 × 40–60 µm (Fig. 2).

 

Chlamydospores:

Chlamydospores have not been readily producible in soil or media cultures (Erwin and Ribeiro, 1996), but they have been described in cultures that were incubated in soil and have been found among hyphal swellings. Chlamydospores are thin walled and 6–16 µm in diameter. They can occur in globs, singly, in chains, or in clusters on affected roots of alfalfa plants.

 

Hyphae:

Hyphal swellings are present in P. medicaginis, and they can be found in aqueous cultures on infected plant tissue or on vegetable juice agar plugs incubated in water (Erwin, 1954; 1965) (Fig. 1).

 

Sexual Structures

 

P. medicaginis is homothallic.

 

Antheridia:

Antheridia formed by single-zoospore cultures can be amphigynous and paragynous. In some cultures, paragynous antheridia are dominant, while in others, amphigynous antheridia are dominant. Paragynous antheridia tend to be smaller (7–13 µm) than amphigynous antheridia (10–15 µm).

 

Oogonia:

Oogonia are very small, 31–38 µm in diameter (average 33 µm).

 

Oospores:

Oospores are produced by single-zoospore isolates on suitable media and on infected roots of alfalfa. Oospores average 23–34 µm in diameter and have walls that are approximately 2–4 µm thick.

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Cicer arietinum

Chickpea, gram, garbanzo bean

Root rot

United States (artificially inoculated), Australia

Medicago sativa

Alfalfa, lucerne

Root rot

United States, Australia, Canada, Greece, Israel, Japan, Mexico, South Africa

Onobrychis viciifolia

Sainfoin

Root rot

United States

Prunus mahaleb

Cherry

Root rot

(artificially inoculated)

Sesbania spp.

Sesbania

Root rot

United States (artificially inoculated)

Symptoms

The first symptoms of root rot are the yellow and reddish brown discoloration of lower leaves and the wilting of the foliage (Figs. 3–5). Lesions occur on taproots and are tan and brown with diffuse margins and are usually initiated where lateral roots emerge from the taproot (Figs. 6 and 7). The internal, infected tissue usually turns yellow. Some favorable infection courts for the fungus include the nodules caused by the bacterium Rhizobium meliloti and the stomata on stems of chickpea at the soil line. Infection of alfalfa plants by the root-knot nematode Meloidogyne hapla leads to more severe cases of root rot (Erwin and Ribeiro, 1996). The pathogen can also cause a root rot on sesbania (Erwin and Kennedy, 1957).

References

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.

 

Erwin, D. C. 1954. Root rot of alfalfa caused by Phytophthora cryptogea. Phytopathology 44:700-704.

 

Erwin, D. C. 1965. Reclassification of the causal agent of the root rot of alfalfa from Phytophthora cryptogea to P. megasperma. Phytopathology 55:1139-1143.

 

Erwin, D. C. 1966. Varietal reaction of alfalfa to Phytophthora megasperma and variation in virulence of the causal fungus. Phytopathology 56:653-657.

 

Erwin, D. C., and Kennedy, B. W. 1957. Studies on the Phytophthora root rot of alfalfa. (Abstr.) Phytopathology 47:520.

 

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

 

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.

 

Förster, H., Kinscherf, T. G., Leong, S. A., and Maxwell, D. P. 1989. Restriction fragment length polymorphisms of the mitochondrial DNA of Phytophthora megasperma isolated from soybean, alfalfa and fruit trees. Can. J. Bot. 67:529-537.

 

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

 

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

 

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

 

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.