Figure 1. Morphology of Phytophthora hibernalis. Upper left, Ellipsoid, caducous, papillate sporangia; upper right, sporangia borne on an irregular sympodium; and lower left, sex organs with amphigynous and paragynous antheridia. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

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

 

Figure 3. Sporangia and zoospores of Phytophthora hibernalis from cultures grown on dilute V-8 medium and then incubated in soil water for 2–3 days at 18°C with a 16-h light cycle. (Courtesy Cheryl Blomquist; Reproduced from Blomquist et al., 2005) Click image to see larger view.

 

Figure 4. Oogonium of Phytophthora hibernalis with an amphigynous antheridum and plerotic oospore. (Courtesy Paul Reeser, Oregon State University)

 

Figure 5. Rhododendron infected with Phytophthora hibernalis from a Del Norte County, California nursery. (Courtesy Cheryl Blomquist; Reproduced from Blomquist et al., 2005)

 

Figure 6. Lesion caused by Phytophthora hibernalis on rhododendron. (Courtesy Cheryl Blomquist; Reproduced from Blomquist et al., 2005)

 

Introduction

Phytophthora hibernalis  Carne (1925)

 

Phytophthora hibernalis was first isolated from diseased citrus in Western Australia by Carne in 1925 (Carne, 1925). The pathogen infects fruit and sometimes leaves during cool months. P. hibernalis was considered by Tucker (1931) to be synonymous with P. syringae, but they are now considered distinct species (Cline et al., 2008; Erwin and Ribeiro, 1996). Ho and Jong (1993) compared P. hibernalis and P. syringae, which were previously described by Tucker to be conspecific. P. hibernalis can be distinguished from P. syringae by elongate sporangia with long pedicels and mostly amphigynous antheridia. P. hibernalis is described by Waterhouse and Waterston (1964) and is a group IV species (Stamps et al., 1990; Waterhouse, 1963) (Fig. 1).

Cultural Characteristics

The minimum temperature for growth is less than 5°C, the optimum temperature for growth is 15°C, and the maximum temperature for growth is 25°C. The colony grows in a rosette pattern (Fig. 2).

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are unbranched or can form as long, lateral branches in an irregular sympodium.

 

Sporangia:

Sporangia are semipapillate and caducous with a long pedicel (23–73 µm) (Blomquist et al., 2005) (Fig. 3). Sporangia are elongated, ellipsoid, ovoid, or obovoid and tapered at the base; most of the time the broadest point is at the apex of the sporangia. Sporangia are 14–22 × 29–53 µm (average 19 × 40 µm).

 

Chlamydospores and Hyphal Swellings:

Chlamydospores and hyphal swellings are not formed.

 

Sexual Structures

 

P. hibernalis is homothallic.

 

Antheridia:

Antheridia are mostly amphigynous but occasionally paragynous.

 

Oogonia:

Oogonia are 22–56 µm in diameter (average 35 µm) (Fig. 4).

 

Oospores:

Oospores are plerotic and 22–45.6 µm in diameter (average 30 µm). 

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Agathis australis

Kauri pine

Canker

New Zealand

Carthamus tinctorius

Safflower

Root rot

Venezuela

Chamaecyparis lawsoniana

Port-Orford white cedar

Canker

United Kingdom

Citrus spp.

Citrus

Leaf blight; brown rot of fruit; defoliation; twig blight; fruit decay; twig dieback

Australia; Portugal; Israel; Fiji Islands; Turkey; South Africa; United States; Italy; France

Rhododendron spp.

Rhododendron

Leaf blight

United States

Lycopersicon esculentum

Tomato

Root rot

Israel

Malus pumila

Apple

Root rot

Israel

Sesamum indicum

Sesame, gingelly

Root rot; stem canker

Venezuela

Solanum melongena

Eggplant, brinjal

Root rot

Israel

Symptoms

Symptoms of brown rot on citrus caused by P. hibernalis are similar to those caused by P. citrophthora, P. nicotianae, and P. syringae. The pathogen infects citrus fruit on the bottom end from splash dispersal from the soil. Infected areas on the fruit coalesce. The diseased fruit is brown and leathery and can mummify on the tree and eventually drop to the ground. The pathogen can sporulate on the fruit surface. The pathogen infects during the cool season and disease is more severe with rain and wind (Erwin and Ribeiro, 1996). Caducous sporangia can infect the upper parts of the tree.

 

The pathogen has also been isolated from symptomatic rhododendrons and causes leaf lesions (Figs. 5 and 6). P. hibernalis could potentially be misdiagnosed by polymerase chain reaction (PCR) as P. ramorum (Blomquist et al., 2005) since internal transcribed spacer (ITS) DNA sequences are very similar.

 

Disease develops rapidly under conditions of high humidity and soil moisture. The greatest damage occurs after rainfall and wind during autumn and winter.

References

Blomquist, C., Irving, T., Osterbauer, N., and Reeser, P. 2005. Phytophthora hibernalis: A new pathogen on Rhododendron and evidence of cross amplification with two PCR detection assays for Phytophthora ramorum. Plant Health Progress doi:10.1094/PHP-2005-0728-01-HN.

 

Carne, W. M. 1925. A brown rot of citrus in Australia (Phytophthora hibernalis n. sp.). J. R. Soc. West. Aust. 12:13-41.

 

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., and Jong, S. C. 1993. Phytophthora hibernalis and Phytophthora syringae. Mycotaxon 47:439-460.

 

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.

 

Tucker, C. M. 1931. Taxonomy of the genus Phytophthora de Bary. Univ. Mo. Agric. Exp. Stn. Bull. 153:1-208.

 

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