Introduction
Phytophthora clandestina P. A.Taylor, Pascoe & F. C. Greenh. (1985)
Phytophthora clandestina
Cultural Characteristics
P. clandestina is a slow-growing species. The minimum temperature for growth is less than 5°C, the optimum temperature for growth is 25°C, and the maximum temperature for growth is 31°C. Growth is at a maximum at a pH of 6.0 at 25°C. Survival in soil is best at 5–10°C (Wong et al., 1986).
Reproductive Structures
Asexual Structures
Sporangiophores:
Sporangia form on simple sympodia on sporangiophores. Sympodia form in water.
Sporangia:
Sporangia are caducous with short pedicels. Sporangia are papillate with multiple apices and form readily in infected roots and on media. Apical thickening is about 4–6 µm deep, with an exit pore about 6–7 µm wide. Sporangia are broadly ellipsoid to ovoid, sometimes globose, and 15–44 × 16–66 µm (Fig. 3B). Sporangia have a length–breadth ratio of 1.35:1 (range of 1.2 to 1.4:1) (Erwin and Ribeiro, 1996). A conspicuous cone-shaped basal plug protrudes into the sporangium.
Chlamydospores:
Chlamydospores have not been seen.
Hyphae:
Hyphal swellings are present on V-8 juice agar (Fig. 3A).
Sexual Structures
P. clandestina is homothallic.
Antheridia:
Antheridia are usually terminal but can be subterminal or occasionally intercalary. They are delimited by a thick septum and variable in shape, measuring 4–17 × 9–45 µm (average 11 × 18 µm). Digitate appendages are characteristic on antheridia. Antheridia are predominately paragynous in host tissue and can be amphigynous on agar media.
Oogonia:
Oogonia are usually terminal with a diameter of 21–36 µm (average 30 µm).
Oospores:
Oospores are markedly aplerotic with diameters of 18–31 µm (average 25 µm) and a wall thickness of 0.3–3.6 µm (average 1.4 µm) (Fig. 3C and D).
Host Range and Distribution
The pathogen infects
subterranean clover (Trifolium
subterraneum) and is weakly pathogenic on several alfalfa species, including
Medicago truncatula, Medicago rugosa, and
Medicago scutellata.
Symptoms
The pathogen causes orange-brown to dark brown lesions and discoloration of the
cortical and stellar tissue and can kill the taproot of subterranean clover
(Fig. 4).
In contrast, Pythium irregulare
generally infects the lateral roots and
Fusarium
avenaceum
causes a black discoloration of the taproot.
Disease is most severe at low temperatures of 15
and 20°C (Wong et al., 1986).
Diagnostics:
Distinguishing characters are slow growth at low temperatures and subterminal or digitate antheridia.
References
Erwin, D. C., and Ribeiro, O. K.
1996.
Phytophthora Diseases Worldwide. American Phytopathological Society,
Kroon, L. P. N. M., Bakker, F. T., van den Bosch, G. B. M., Bonants, P. J. M.,
and Flier, W. G.
2004.
Phylogenetic analysis
of
Phytophthora species based on mitochondrial and nuclear DNA
sequences. Fungal Genet. Biol. 41:766-782.
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,
Taylor, P. A., Pascoe,
Wong, D. H., Sivasithamparam,
K., and Baretti, M. J. 1986. Influence of environmental factors on the growth
and survival of
Phytophthora clandestina.