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

 
Figure 2. Sporangium of Phytophthora foliorum born terminally on a sporangiophore are ovoid and semipapillate with short pedicels. (Courtesy Ryan Donahoo; Reproduced, by permission, from Donahoo et al., 2006)
Figure 3. Oospore of Phytophthora foliorum with paragynous antheridia. Occasionally amphygynous. (Courtesy Samantha Thomas; Reproduced, by permission, from Donahoo et al., 2006)

 
Figure 4. Leaf lesions caused by Phytophthora foliorum on azalea 'Pink Ruffles'. A, Three days after wounding and B, not wounded. (Courtesy Ryan Donahoo; Reproduced, by permission, from Donahoo et al., 2006)

 

Introduction

Phytophthora foliorum sp. nov.  Donahoo & Lamour (2006)

 

Phytophthora foliorum sp. nov. was first isolated from evergreen hybrid azalea leaves during surveys for P. ramorum in 2004 and 2005 (Donahoo et al., 2006). P. foliorum causes a leaf blight on azalea. The pathogen is not widespread and its importance to the horticultural industry needs further evaluation. During a nationwide survey of nursery stock in response to potential spread of the sudden oak death pathogen P. ramorum, an internal transcribed spacer (ITS)-based nested polymerase chain reaction (PCR) was used for the detection of P. ramorum. However, several different Phytophthora species had been identified and showed false-positive reactions in the ITS detection assay for P. ramorum. Sequences from ITS regions were similar, leading to coamplification of P. foliorum and P. hibernalis with P. ramorum in nested PCR reactions (Donahoo et al., 2006). Although the pathogen produced false-positive reactions in PCR tests for P. ramorum, P. foliorum has different morphological characteristics.

 

P. foliorum differs from P. ramorum in that it is homothallic and rarely if ever produces sporangia in culture. It differs from P. lateralis in that it has semipapillate sporangia. Unlike P. lateralis and P. ramorum, P. foliorum has not been found to produce chlamydospores.

 

Phylogenetic analysis revealed that P. foliorum is basal in the clade and shares a common ancestor with P. hibernalis, P. ramorum, and P. lateralis. The isolates of this species have limited genotypic diversity with amplified fragment length polymorphism (AFLP) analysis. P. foliorum is a group III Phytophthora species (Erwin and Ribeiro, 1996; Waterhouse, 1963).

Cultural Characteristics

Isolates of P. foliorum can be grown on cornmeal agar between 3 and 28°C. The optimum temperature for growth is 18–22°C, at a rate of 3 mm/day on V-8 juice agar (Fig. 1).

Reproductive Structures

Asexual Structures

 

Sporangia:

Sporangia may be caducous, ovoid, semipapillate, and average 34 × 51 µm. Sporangia are deciduous with short pedicels (<5 to 20 µm). Sporangia were only produced in soil extract water and rarely, if ever, are produced in culture (Fig. 2).

 

Chlamydospores:

Chlamydospores have not been found.

 

Sexual Structures

 

P. foliorum is homothallic.

 

Antheridia:

Antheridia are mostly paragynous and usually attached to the oogonia next to the oogonial stalk.

 

Oogonia:

Oogonia are not ornamented and are 32–43 µm (average 37 µm) (Donahoo et al., 2006).

 

Oospores:

Oospores are plerotic, spherical, and abundantly produced in culture. Average oospore size is 33 µm, with a range from 28 to 38 µm (Donahoo et al., 2006) (Fig. 3). Oogonia are mostly paragynous but are occasionally amphigynous.

Host Range and Distribution

P. foliorum has been isolated from azalea and found in California and Tennessee (Donahoo et al., 2006).

Symptoms

P. foliorum causes leaf spot on azalea (Fig. 4).

References

Donahoo, R., Blomquist, C. L., Thomas, S. L., Moulton, J. K., Cooke, D. E. L., and Lamour, K. H. 2006. Phytophthora foliorum sp. nov., a new species causing leaf blight of azalea. Mycol. Res. 110:1309-1322.

 

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

 

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.