Figure 1. Culture of Phytophthora mirabilis grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

 

Figure 2. Sporangia of Phytophthora mirabilis (×1,000). (Courtesy Jean B. Ristaino)

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora mirabilis  Galindo & H. R. Hohl (1985)

 

Phytophthora mirabilis causes leaf blight of Mirabilis jalapa, known as four-o’clock or marvel of Peru (Galindo and Hohl, 1985). The pathogen was first classified as P. infestans var. mirabilis by Servin (1958) and others (Erwin and Ribeiro, 1996; Romero and Fourton, 1962). Galindo and Hohl (1985) found differences in nutritional requirements, morphological characters, and host specificity and renamed the species P. mirabilis. P. infestans and P. mirabilis are closely related and almost morphologically indistinguishable. The main differentiating characters are that P. mirabilis has slightly larger sporangia and slightly smaller oogonia and oospores compared with those of P. infestans (Galindo and Hohl, 1985). Moller et al. (1993) named the pathogen P. infestans f. sp. mirabilis because of mtDNA restriction fragment patterns and hybridization to a nuclear DNA probe. Tooley et al. (1996) could not separate the two species based on internal transcribed spacer region 2. Host specialization as well as post-zygotic reproductive isolation mechanisms were suggested (Goodwin and Fry, 1994). Goodwin et al. (1999) examined isozymes, mtDNA, and random amplified polymorphic DNA (RAPD) markers, and their data suggest little gene flow between P. infestans and P. mirabilis. The data supported the name P. mirabilis. The two taxa are reproductively isolated in Mexico, and the pathogen does not infect potato plants. P. mirabilis is a group IV Phytophthora species (Stamps et al., 1990).

Cultural Characteristics

P. mirabilis grows well on synthetic media containing nitrate as the nitrogen source (Fig. 1). P. infestans does not grow on this media (Hohl, 1975; 1983). The pathogen grows on a wide variety of media (Goodwin et al., 1999).

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are compound sympodial with indeterminate branches.

 

Sporangia:

Sporangia are ellipsoid, caducous with short pedicels, and semipapillate. Sporangia are 26.4–38.5 µm (average 33.7 µm). The length–breadth ratio is 1.9:1 (Fig. 2).

 

Chlamydospores:

No chlamydospores have been reported for P. mirabilis.

  

Sexual Structures

 

P. mirabilis is heterothallic.

 

Antheridia:

Antheridia are amphigynous and 11.8 × 13.8 µm.

 

Oogonia:

Oogonia are globose, smooth walled, and 26.4 µm in diameter.

 

Oospores:

Oospores are smooth walled, colorless, and 21.3 µm in diameter. 

Host Range and Distribution

Host

Common Name

Diseases

Geographical Distribution

Mirabilis jalapa

Four-o’clock

Leaf blight

Highlands of Mexico

Symptoms

P. mirabilis causes leaf blight on four-o’clock. This is the only reported host for P. mirabilis.

References

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

 

Galindo, A. J., and Hohl, H. R. 1985. Phytophthora mirabilis, a new species of Phytophthora. Sydowia Ann. Mycol. Ser. II 38:87-96.

 

Goodwin, S. B., and Fry, W. E. 1994. Genetic analysis of interspecific hybrids between Phytophthora infestans and Phytophthora mirabilis. Exp. Mycol. 18:20-32.

 

Goodwin, S. B., Legard, D. E., Smart, C. D., Levy, M., and Fry, W. E. 1999. Gene flow analysis of molecular markers confirms that Phytophthora mirabilis and P. infestans are separate species. Mycologia 91:796-810.

 

Hohl, H. R. 1975. Levels of nutritional complexity in Phytophthora: Lipids, nitrogen sources and growth factors. Phytopathol. Z. 84:18-33.

 

Hohl, H. R. 1983. Nutrition of Phytophthora. Pages 41-54 in: Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology. D. C. Erwin, S. Bartnicki-Garcia, and P. H. Tsao, eds. American Phytopathological Society, St. Paul, MN.

 

Moller, E. M., de Cock, A. W. A. M., and Prell, H. H. 1993. Mitochondrial and nuclear DNA restriction enzyme analysis of the closely related Phytophthora species P. infestans, P. mirabilis, and P. phaseoli. J. Phytopathol. 139:309-321.

 

Romero, S., and Fourton, H. 1963. Leaf blight of Mirabilis jalapa in Mexico caused by a specialized form of Phytophthora infestans. (Abstr.) Phytopathology 53:25.

 

Servin, L. 1958. Especie de Phytophthora atacando Mirabilis jalapa (Species of Phytophthora that attack Mirabilis jalapa). Pages 491-500 in: Memoria del Primer Congreso Nacional de Entomologia y Fitopatologia, Mexico. Escuela Nacional de Agricultura.

 

Stamps, D. J., Waterhouse, G. M., Newhook, F. J., 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.

 

Tooley, P. W., Carras, M. M., and Falkenstein, K. F. 1996. Relationships among group IV Phytophthora species inferred by restriction analysis of the ITS2 region. J. Phytopathol. 144:363-369.