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

 

Figure 2. Phytophthora nemorosa. a, Ovoid, semipapillate sporangia formed in water and attached to sporangiophore displaying close sympodia, giving the appearance of terminal clusters. b, Individual sporangia formed in water displaying various pedicel lengths. c, Small hyphal swellings formed in cornmeal agar displaying various sizes and small blisters or fingerlike projections. d–f, Oogonia formed in cornmeal agar displaying aplerotic oospores and amphigynous antheridia. Bar = 10 µm. (Courtesy Paul W. Reeser; Reproduced, by permission of Mycotaxon, Ltd., from Hansen et al., 2003) Click image to see larger view.

 

Figure 3. Symptoms caused by Phytophthora nemorosa on camellia. (Courtesy Nik Grunwald, USDA-ARS, Corvallis, OR)

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora nemorosa  E. M. Hansen & Reeser (2003)
 

Phytophthora nemorosa is a foliar and bole pathogen of various evergreen hardwood trees (Hansen et al., 2003) and exhibits characteristics similar to those of P. ramorum, P. ilicis, and P. psychrophilia. P. nemorosa and P. ramorum can be found in similar geographical areas, but P. nemorosa grows slower in culture than does P. ramorum. The two species can also be distinguished based on optimum growing temperature. P. nemorosa has an optimum temperature for growth of 15°C, and P. ramorum has an optimum temperature for growth of 20°C (Hansen et al., 2003). P. nemorosa differs from P. ilicis in that P. ilicis has smaller oogonia (27 µm) (Hansen et al., 2003). P. nemorosa differs from P. psychrophilia in that P. nemorosa has smaller oogonia, slightly rounder sporangia, and distinctly patterned colony growth on agar media (Hansen et al., 2003). P. psychrophilia grows without pattern (Hansen et al., 2003). P. nemorosa is a group IV Phytophthora species (Erwin and Ribeiro, 1996; Waterhouse, 1963).

Cultural Characteristics

Isolates of P. nemorosa can be grown on Difco cornmeal agar, fresh potato dextrose agar, and V-8 juice agar (Fig. 1). Sporangia can be produced by transferring disks of mycelium from V-8 juice agar media into natural stream water. Sexual reproductive structures can be grown on cornmeal agar with beta-sitosterol. Isolates grow optimally at 15°C, with a maximum temperature for growth of 20°C (Hansen et al., 2003).

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are sympodially branched and sporangia appear as terminal clusters (Hansen et al., 2003).

 

Sporangia:

Sporangia are ovoid and scarcely semipapillate. Caducous sporangia are 28–50 × 38–71 µm (average 37 × 51 µm) (Hansen et al., 2003). Sporangia have medium pedicels of less than 5 to 20 µm (Hansen et al., 2003) (Fig. 2a and b).

 

Chlamydospores:

Chlamydospores have not been found.

 

Hyphae:

Small, blistered hyphal swellings have been found on cornmeal agar with beta-sitosterol (Fig. 2c).

 

 

Sexual Structures

 

P. nemorosa is homothallic.

 

Antheridia:

Antheridia are amphigynous and about 13 × 13 µm. Their size may be 10–15 × 9–19 µm (Hansen et al., 2003).

 

Oogonia:

Oogonia are terminal and 23–40 µm (average 33 µm) (Hansen et al., 2003).

 

Oospores:

Oospores are slightly aplerotic and 19–35 µm (average 29 µm) (Hansen et al., 2003) (Fig. 2d–f).

Host Range and Distribution

P. nemorosa has most commonly been isolated from necrotic leaf tips of myrtlewood or California bay (Umbellularia californica). It has been found on leaves of manzanita (Arctostaphylos spp.) and coast redwood (Sequoia sempervirens) and causes bark cankers on tan oak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia). P. nemorosa has been found in Oregon and California.

Symptoms

P. nemorosa symptoms are indistinguishable from P. ramorum symptoms. P. nemorosa causes leaf spots and twig cankers (Fig. 3). P. nemorosa is seldom associated with lethal trunk cankers on mature trees, but when it does cause lethal cankers, P. nemorosa is usually associated with single dead trees rather than patches of dead trees (Martin and Tooley, 2003). P. nemorosa is believed to be an endemic pathogen versus an introduced pest (Hansen et al., 2003).

References

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

 

Hansen, E. M., Reeser, P. W., Davidson, J. M., and Garbelotto, M. 2003. Phytophthora nemorosa, a new species causing cankers and leaf blight of forest trees in California and Oregon, U.S.A. Mycotaxon 88:129-138.

 

Martin, F. N., and Tooley, P. W. 2003. Phylogenetic relationships of Phytophthora ramorum, P. nemorosa, and P. pseudosyringae, three species recovered from areas in California with sudden oak death. Mycol. Res. 107(12):1379-1391.

 

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.