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

 

Figure 2. Nonpapillate sporangium of Phytophthora multivesiculata (×1,000). (Courtesy Jean B. Ristaino)

 

Figure 3. Oogonium of Phytophthora multivesiculata displaying an aplerotic oospore and amphigynous antheridium (×1,000). (Courtesy Jean B. Ristaino)

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Introduction

Phytophthora multivesiculata  Ilieva, Man in 't Veld, W. Veenb.-Rijks & R. Pieters (1998)

 

Phytophthora multivesiculata was first described by Ilieva et al. in 1998 (Ilieva et al., 1998). This Phytophthora species causes a leaf blotch and a rot of cymbidium orchids in the North Island of New Zealand and in other regions, such as Northland, Auckland, Bay of Plenty, and Poverty Bay, where cymbidium orchids are commercially grown (Hill, 2004). P. multivesiculata vaguely resembles P. porri and P. megasperma. P. multivesiculata differs from P. porri and P. megasperma in morphology, pathogenicity, and disease symptoms. P. multivesiculata produces isozyme profiles that do not match the profiles of P. megasperma and P. porri types (Ilieva et al., 1998). P. multivesiculata is a group IV Phytophthora species (Stamps et al., 1990).

Cultural Characteristics

P. multivesiculata grows well on many culture media, including V-8 juice agar, potato dextrose agar, oatmeal agar, and carrot agar media. The culture is irregular and slightly fluffy (Ilieva et al., 1998) (Fig. 1). The species has a high growth rate at 20°C, with a maximum temperature for growth of 35°C (Erwin and Ribeiro, 1996).

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are long, slender (2–3 µm), and mainly twisted. Sporangia are produced singly on sporangiophores, but a sympodial arrangement of up to three sporangia has been reported (Ilieva et al., 1998).

 

Sporangia:

Sporangia are mainly ovoid and occasionally obpyriform. Sporangia may be nonpapillate and semipapillate. The majority of sporangia have a rounded base, but occasionally a tapered base may develop. Sporangia are 20–41 × 30–60 µm (average 33 × 45 µm) (Ilieva et al., 1998) (Fig. 2). Internal proliferation was observed in water cultures. While P. multivesiculata sporangia grow on both solid and liquid media, P. porri sparsely forms sporangia on solid media and never with internal proliferation. P. megasperma only forms sporangia in water culture and internal proliferation is more common than in P. multivesiculata (Ilieva et al., 1998).

 

Chlamydospores:

No chlamydospores have been observed.

 

Hyphae:

The development of catenulate hyphal swellings is characteristic. Hyphal swellings distinguish P. multivesiculata from P. porri and P. megasperma. Hyphal swellings form in colonies on liquid and solid media and are rounded, obpyriform, catenulate, and clustered. Swellings are 14–36 µm in diameter and new branches form at acute angles. P. porri hyphal swellings are mainly rounded with more radiating hyphae, while P. megasperma rarely forms hyphal swellings (Ilieva et al., 1998).

 

Sexual Structures

 

P. multivesiculata is homothallic.

 

Antheridia:

Antheridia are amphigynous and irregularly spherical or ellipsoidal. Antheridia of P. porri and P. megasperma are paragynous (Ilieva et al., 1998).

 

Oogonia:

Oogonia are spherical, smooth walled, and 28–50 µm in diameter on V-8 juice agar media. Oogonia are an average of 41 µm (Ilieva et al., 1998).

 

Oospores:

Oospores are spherical, smooth walled, and aplerotic. Oospore are 24–42 µm in diameter (average 33 µm) (Ilieva et al., 1998) (Fig. 3).

Host Range and Distribution

P. multivesiculata is responsible for a leaf blotch and a rot of cymbidium orchids.

Symptoms

In Cymbidium rot, the leafy parts of the plants and the pseudobulbs (modified stems) are affected. Infection tends to occur on leaves after exposure to water. Under humid conditions, infected mature leaves develop dark green lesions that expand into large irregular patches of water-soaked tissue. Tissues turn brown once dry conditions prevail, and horizontal zebralike stripes are visible (Ilieva et al., 1998). If the pathogen washes down to the base of the plant, immature leafy shoots may be attacked and severe internal rot rapidly develops. Leaves change color, and infected shoots become gray-green and rapidly loose turgor. Infection may spread from the attachment point of the shoot to the adjacent pseudobulb. Green parts of the leaf may also be affected. Infected young pseudobulb tissues develop a distinctive internal, blue-black or purplish brown discoloration and a sour odor. Roots are not infected.

References

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

 

Hill, C. F. 2004. First report of Phytophthora multivesiculata on cymbidium orchids in New Zealand. Australas. Plant Pathol. 33:603-604.

 

Ilieva, E., Man In 't Veld, W. A., Veenbaas-Rijks, W., and Pieters, R. 1998. Phytophthora multivesiculata, a new species causing rot in Cymbidium. Eur. J. Plant Pathol. 104:677-684.

 

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.