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
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.
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 |
|
Medicago sativa |
Alfalfa, lucerne |
Root rot |
|
Onobrychis viciifolia |
Sainfoin |
Root rot |
|
Prunus mahaleb |
Cherry |
Root rot |
(artificially
inoculated) |
Sesbania
spp. |
Sesbania |
Root rot |
|
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,
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of
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Erwin, D. C.
1954. Root rot of alfalfa caused by
Phytophthora cryptogea.
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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.
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alfalfa. (Abstr.) Phytopathology 47:520.
Erwin, D. C., and Ribeiro, O. K. 1996.
Phytophthora
Diseases
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Förster, H., Kinscherf, T. G., Leong, S. A., and Maxwell, D. P. 1989.
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