Figure 1. Morphology of Phytophthora ilicis. Left, Globose, smooth-walled oogonium containing a plerotic oospore; antheridium is amphigynous. Center, Ovoid, semipapillate sporangium. Right, Ovoid sporangium in which zoospores have formed immediately before emission. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996)
 

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

Figure 3. Oogonium of Phytophthora ilicis with an amphigynous antheridium and plerotic oospore (×400). (Courtesy Jean B. Ristaino)
 

Figure 4. Japanese holly plants infected with Phytophthora ilicis. (Courtesy Lyle J. Feilmeier; Reproduced, by permission, from Pscheidt, 2010)
 

Figure 5. Shoot dieback and necrotic berries on English holly infected with Phytophthora ilicis. (Courtesy Jay W. Pscheidt; Reproduced, by permission, from Pscheidt, 2010)
 

Introduction

Phytophthora ilicis  Buddenh. & Roy A. Young (1957)

 

Phytophthora ilicis was first described by Buddenhagen and Young in 1957 on English holly in the western United States (Buddenhagen and Young, 1957). It was also reported in Canada on the same species (Punja and Omrod, 1979). According to Cline et al. (2008), Waterhouse (1970) erroneously listed this as an invalid name, lacking citation of a type specimen, but the Code does not require citation of the type in publications prior to 1958. Stamps et al. (1990) lists P. ilicis as a valid species in their tabular key. Hall summarized the disease symptoms on holly (Hall, 1991). It is classified as a group IV species (Stamps et al., 1990; Waterhouse, 1963; Waterhouse and Waterston, 1964) (Fig. 1).

Cultural Characteristics

The minimum temperature for growth is 5°C, the optimum temperature for growth is 20°C, and the maximum temperature for growth is 25°C (Fig. 2). 

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are sympodial and form in water.

 

Sporangia:

Sporangia are obpyriform, ovoid to spherical, semipapillate, and caducous with a medium pedicel length of 5–15 µm. Sporangia are small, 18–30 × 30–50 µm (average 24 × 39 µm on holly tissue). Sporangial length–breadth ratios are 1.3:1–1.6:1 (Fig. 1).

 

Chlamydospores and Hyphal Swellings:

Chlamydospores are rare and hyphal swellings are not produced.

 

Sexual Structures

 

P. ilicis is homothallic.

 

Antheridia:

Antheridia are amphigynous.

 

Oogonia:

Oogonia have a tapered base and are small, 25–32 µm in diameter (average 27 μm) in leaves and 17–28 µm in diameter (average 25 µm) in pea broth.

 

Oospores:

Oospores are plerotic and 18–26 µm in diameter (average 21 µm) in leaves and 15–23 µm (average 20 µm ) in pea broth (Fig. 3).

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Ilex aquifolium

English holly

Leaf and twig blight

United States, Canada

Ilex crenata

Japanese holly

Leaf and twig blight

United States

Symptoms

The appearance of black spots on the periphery leaves of the lower part of the holly tree is the first observable symptom (Fig. 4). This happens during cool, wet weather in the fall. Defoliation occurs after the leaf spots. Cankers can occur on the twigs and stems and do not exceed 12 cm in length (Erwin and Ribeiro, 1996). Berries are also susceptible to infection and the twig beneath the berry can become girdled (Fig. 5). The disease can progress up the tree and eventually defoliate the entire tree. The pathogen sporulates on the underside of the leaf surfaces and oospores form in the tissue.

References

Buddenhagen, I. W., and Young, R. A. 1957. A leaf and twig disease of English holly caused by Phytophthora ilicis n. sp. Phytopathology 47:95-101.

 

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.

 

Hall, G. 1991. Phytophthora ilicis. Mycopathologia 115:225-226.

 

Pscheidt, J. W., ed. 2010. Holly—Phytophthora Leaf and Twig Blight. Online Guide to Plant Disease Control. Oregon State University Extension, Corvallis, OR.

 

Punja, Z. K., and Omrod, D. J. 1979. New or noteworthy plant diseases in coastal British Columbia 1975-1977. Can. Plant Dis. Surv. 59:22-32.

 

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.

 

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.

 

Waterhouse, G. M. 1970. Taxonomy in Phytophthora . Phytopathology 60:1141-1143.

 

Waterhouse, G. M., and Waterston, J. M. 1964. Phytophthora hibernalis. CMI Descr. Pathog. Fungi Bact. 31:1-2.