Figure 1. Morphology of Phytophthora clandestina. A, Sporangia. B, Zoospore and encysted zoospores. C, Hyphal swellings on V-8 juice agar. Bar = 10 μm. (Reproduced, by permission of Mycotaxon Ltd., from Taylor et al., 1985) Click image to see larger view.

 

Figure 2. Morphology of Phytophthora clandestina gametangia in roots of subterranean clover seedlings. A, Oogonia and paragynous antheridia. B, Oogonia and amphigynous antheridia. Note the digitate protuberances of antheridia and aplerotic oospores. (Reproduced, by permission of Mycotaxon Ltd., from Taylor et al., 1985) Click image to see larger view.

 

Figure 3. Morphology of Phytophthora clandestina. A, Hyphal swellings. B, Papillate sporangia. C, Gametangia in roots. D, Gametangia in lima bean agar. Bar = 20 µm. (Reproduced, by permission of Mycotaxon Ltd., from Taylor et al., 1985) Click image to see larger view.

 

Figure 4. Seedlings affected by root rot, caused by Phytophthora clandestina, often die of extensive rotting of the taproot. (© NSW Department of Primary Industries; Photographer Peter Cregan; Reproduced from Diseases of Sub Clover, Part B of Agfact P2.5.16, 4th ed.) Click image to see larger view.

 

Introduction

Phytophthora clandestina  P. A.Taylor, Pascoe & F. C. Greenh. (1985)

 

Phytophthora clandestina was first isolated in Victoria, Australia, from taproots of subterranean clover, Trifolium subterraneum, a pasture legume (Taylor et al., 1985). It is a group I Phytophthora species (Stamps et al., 1990). Because of inadequate isolation methods, Pythium irregulare Buisman and Fusarium avenaceum (Corda ex. Fr.) Sacc. had been previously thought to be the major pathogens in roots of this species. The species designation has been confirmed by Kroon et al. (2004) (Figs. 1 and 2).

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, St. Paul, MN.

 

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, Surrey, England.

 

Taylor, P. A., Pascoe, I. G., and Greenhalgh, F. H. 1985. Phytophthora clandestina sp. nov. in roots of subterranean clover. Mycotaxon 22:77-85.

 

Wong, D. H., Sivasithamparam, K., and Baretti, M. J. 1986. Influence of environmental factors on the growth and survival of Phytophthora clandestina. Can. J. Microbiol. 32:553-556.