Figure 1. Morphology of Phytophthora inflata. Upper row, Ovate to obpyriform, sometimes irregular in shape, semipapillate sporangia. Inflated antheridia are the main diagnostic feature. Lower row, Oogonia with paragynous, often inflated and contorted antheridia. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

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

 

Figure 3. Semipapillate sporangia of Phytophthora inflata (×1,000). (Courtesy Jean B. Ristaino)

 

Figure 4. Oogonia and oospore of Phytophthora inflata with an inflated paragynous antheridium (×1,000). (Courtesy Jean B. Ristaino)

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora inflata  Caros. & Tucker (1949)

 

Phytophthora inflata was first described by Caroselli and Tucker in 1949 as causing pit canker of American and slippery elm (Ulmus americana and Ulmus fulva) (Caroselli and Tucker, 1949). The pathogen had been known since the turn of the century but was not described and characterized until 1949. The pathogen has a limited host range on elm in the eastern United States and Canada (Erwin and Ribeiro, 1996). It has also been isolated from Sambucus tenuifolium and Syringa vulgaris in the United Kingdom (Hall et al., 1992) and rhododendron in the United States (Testa et al., 2005). P. inflata was considered conspecific with P. citricola by Cooke et al. (2000), but Kroon et al. (2004) considered it a distinct species based on mitochondrial DNA. It is considered a group III species according to Stamps et al. (1990) because of its semipapillate sporangia and inflated antheridia (Cline et al., 2008; Waterhouse 1963) (Fig. 1). 

Cultural Characteristics

The optimum temperature for growth is 25–30°C, and the maximum temperature for growth is less than 35°C (Fig. 2). There is no available information on the minimum temperature for growth.

Reproductive Structures

Asexual Structures

Sporangiophores:
P. inflata forms sporangia on unbranched sporangiophores.

Sporangia:

Sporangia are noncaducous (persistent) on the stalk. Sporangia are semipapillate, broad, ovoid to obpyriform, limoniform to elongated, and 15–32 × 20–67 µm (Fig. 3). The length–breadth ratios are 1.3:1. Sporangia may be highly variable in size. Sporangia germinate by external proliferation.

 

Chlamydospores:

Chlamydospores are not formed.

 

Hyphae:

Thin-walled, hyaline, intercalary hyphal swellings form in aqueous cultures.

 

Sexual Structures

 

P. inflata is homothallic.

Antheridia:

Antheridia are paragynous. Antheridia are large, inflated, and contorted, and they twist around the stalk of the oogonia. They can be irregularly lobed or branched and up to 15 × 50 µm.

 

Oogonia:

Oogonia are 30–43 µm in diameter (average 34 µm).

 

Oospores:

Oospores are 26–39.3 µm in diameter (average 31.3 µm) with a thick wall (Fig. 4). Oospores are aplerotic.

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Ulmus americana

American elm

Pit canker

United States

Ulmus fulva

Slippery elm

 Pit canker

Canada, United States

Sambucus tenuifolium

 Elder

Root rot

United Kingdom

Syringa vulgaris

Common lilac

Root rot

United Kingdom

Rhododendron spp.

Rhododendron

Root rot

United States

Symptoms

Pit Canker of American Elm:

Concentric or zonate cankers and callus tissue form on the trunk or scaffold branches, followed by a general decline in the foliage and aboveground portions of the tree. A change in foliar color from green to yellow occurs. Cankers develop in a zonate fashion as callus forms and the fungus reinvades adjacent tissue. Cracks form around the canker margins and cankers exude a red to brown fluid. Rainy weather and high temperatures promote disease.

References

Caroselli, N. E., and Tucker, C. M. 1949. Pit canker of elm. Phytopathology 39:481-488.

 

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.

 

Cooke, D. E. L., Drenth, A., Duncan, J. M., Wagels, G., and Brasier, C. M. 2000. A molecular phylogeny of Phytophthora and related oomycetes. Fungal Genet. Biol. 30:17-32.

 

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

 

Hall, G., Dobson, S., and Nicholls, C. 1992. First record of Phytophthora inflata in the United Kingdom. Plant Pathol. 41:95-97.

 

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.

 

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.

 

Waterhouse, G. M. 1963. Key to the species of Phytophthora de Bary. Mycol. Pap. 92. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.