Introduction

Phytophthora vignae  Purss (1957)

Phytophthora vignae was first isolated from cowpea with root and stem rot in Queensland, Australia, and described by Purss in 1957 (Purss, 1957). There are no synonyms for P. vignae. P. vignae is pathogenic to cowpea (Vigna unguiculata [=Vigna sinensis]) and adzuki bean (Vigna angularis) and is host specific. Widespread disease outbreaks are limited geographically to Australia on cowpea (Purss, 1957; 1958) and to Japan on adzuki bean (Kitazawa et al., 1978; 1979). P. vignae is a group VI Phytophthora species (Stamps et al., 1990) (Fig. 1).

Cultural Characteristics

The minimum temperature for growth is 10–12°C, the maximum temperature for growth is 32°C, and the optimum temperature for growth is 28–30°C (Fig. 2A).

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are undifferentiated and produce sporangia in aqueous culture.

Sporangia:

Sporangia are ovoid to obpyriform and taper at the base. Sporangia are nonpapillate and noncaducous and proliferate externally and internally. They are 15–54 × 24–72 µm (average 27 × 48 µm) (Fig. 2B–E). Sporangia form in water or soil extract. Sporangia form on undifferentiated sporangiophores in aqueous culture, and on diseased tissue, they form on monochasial or compound sympodia.

Chlamydospores:

Chlamydospores are sparse, terminal or intercalary, and 12–21 µm in diameter (average 17 µm).

Hyphae:

Hyphal swellings are abundant, large, spherical or angular, intercalary, and rarely terminal. Knobby mycelial growth occurs (Gallegly and Hong, 2008).

Sexual Structures

P. vignae is homothallic.

Antheridia:

Antheridia are 15 × 16 µm, variable in shape from spherical to ovate, and amphigynous.

Oogonia:

Sex bodies are golden brown. Oogonia are smooth walled and 24–46 µm in diameter (average 32 µm).

Oospores:

Oospores form on host tissues and in axenic cultures. Oospores are aplerotic and have a wall thickness of 3 µm. Oospores are 18–32 µm in diameter (average 26 µm) (Fig. 2F–I). Oospores are produced on V-8 juice agar with sitosterol at 20°C and isolated free of mycelial debris. Oospores germinate on water agar and form nonpapillate sporangia at 25°C.

Host Range and Distribution

Symptoms

Stem Rot of Vigna unguiculata (Cowpea):

Stem rot occurs during moist weather and infected plants turn yellow. Sunken brown lesions begin at the ground level and extend upward on the stem. During moist weather, mycelium can be seen on the margins of lesions. Stems may become internally discolored and brown spots may develop on leaves (Purss, 1957). If dry weather occurs, infected plants wilt and die.

Stem Rot of Vigna angularis (Adzuki Bean) and Vigna sesquipedalis (Yard-Long Bean):

Reddish brown streaks appear on the stems, and after the stem is girdled, the plant wilts and dies (Kitazawa et al., 1978).

References