Figure 1. Morphology of Phytophthora insolita. A, Nonpapillate, ovoid sporangia: an empty sporangium producing secondary sporangia (internal proliferation), an ungerminated sporangium, and a sporangium giving rise to a germ tube emanating from within an empty sporangium (external proliferation). B, Globose chlamydospores with thin walls. C, Hyphal swellings. D, Globose, thick-walled oogonia without antheridia. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

Figure 2. Culture of Phytophthora insolita grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

 

Figure 3. Phytophthora insolita. A, Colony. B–D, Hyphal swellings. E–H, Sporangia. I, Empty cyst after repeated emergence of zoospores. J–M, Oogonia with oospores. Bar = 20 µm. All are the same magnification except A. (Courtesy Hon H. Ho; Reproduced, by permission of the Institute of Plant and Microbial Biology, from Ho et al., 1995) Click image to see larger view.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora insolita  Ann & W. H. Ko (1980)

 

Phytophthora insolita was first described by Ann and Ko in 1980 (Ann and Ko, 1980). The pathogen was isolated from soil using citrus leaf baits in Hawaii. Ko named the species insolita since it produces unusual (Latin insolita) oogonia that are devoid of antheridia. The pathogen has been reported to cause foliar necrotic lesions and twig dieback (Cline et al., 2008; Testa et al., 2005). P. insolita is a group V species (Stamps et al., 1990) (Fig. 1).

Cultural Characteristics

The minimum temperature for growth is greater than 9°C, the optimum temperature for growth is 32°C, and the maximum temperature for growth is 38°C (Figs. 2 and 3A).

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangia are borne terminally on the sporangiophores and proliferate both internally and externally.

 

Sporangia:

Sporangia are noncaducous, nonpapillate, and terminal. Sporangia are ovoid and 29–39 × 38–68 µm (Ho et al., 1995) (Fig. 3E–H). New sporangia can proliferate both externally and internally from old sporangia. They form readily in water.

 

Chlamydospores:

Chlamydospores are 22–48 µm and thin walled. They form terminally and are globose (Fig. 1B).

 

Hyphae:

Irregularly shaped hyphal swellings occur on V-8 agar media (Figs. 1C and 3B–D).

 

Sexual Structures

 

P. insolita is homothallic.

 

Antheridia:

Antheridia are not formed in this species.

 

Oogonia:

Oogonia are 29–36 µm and spherical. Oogonial stalks branch sympodially beneath previous oogonium.

 

Oospores:

Oospores are 27–31 µm in diameter and globose and nearly fill the oogonium (Fig. 3J–M).

Host Range and Distribution

Few natural hosts have been reported. P. insolita was isolated from soil in Taiwan. The pathogen is able to infect Malus domestica (apple) and Cucumis sativus (cucumber) fruits when inoculated (Erwin and Ribeiro, 1996; Ko and Ann, 1985). It has been reported on rhododendron in Ohio (Testa et al., 2005).

Symptoms

P. insolita causes foliar necrotic lesions and twig dieback in rhododendron.

References

Ann, P. J., and Ko, W. H. 1980. Phytophthora insolita, a new species from Taiwan. Mycologia 72:1180-1185.

 

Cline, E. T., Farr, D. F., and Rossman, A. Y. 2008. A synopsis of Phytophthora with accurate scientific names, host range, and geographic distribution. Plant Health Progress doi:10.1094/PHP-2008-0318-01-RS.

 

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

 

Ho, H. H., Ann, P. J., and Chang, H. S. 1995. The genus Phytophthora in Taiwan. Inst. Bot. Acad. Sinica Monogr. Ser. 15.

 

Ko, W. H., and Ann, P. J. 1985. Phytophthora humicola, a new species from soil of a citrus orchard in Taiwan. Mycologia 77:631-636.

 

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.

 

Testa, A., Schilb, M., Lehman, J. S., Cristinzio, G., and Bonello, P. 2005. First report of Phytophthora insolita and P. inflata on rhododendron in Ohio. Plant Dis. 89:1128.