Figure 1. Morphology of Phytophthora cinnamomi. Nonpapillate sporangia, proliferation of sporangia, globose oogonia and oospores, globose chlamydospores, and numerous hyphal swellings. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

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

 

Figure 3. Sporangia of Phytophthora cinnamomi (×1,000). (Courtesy Mike Benson, North Carolina State University)

 

Figure 4. Chlamydospore of Phytophthora cinnamomi (×400). (Courtesy Jean B. Ristaino)

 

Figure 5. Coralloid hyphal swellings of Phytophthora cinnamomi (×400). (Courtesy Jean B. Ristaino)

 

Figure 6. Oospore of Phytophthora cinnamomi (×1,000). (Courtesy Mike Benson, North Carolina State University)

 

Figure 7. Wilt caused by Phytophthora cinnamomi in the top of a Fraser fir. (Courtesy Mike Benson, North Carolina State University)

 

Figure 8. Fraser fir branch flagging caused by Phytophthora cinnamomi. (Courtesy Mike Benson, North Carolina State University)

 

Figure 9. Fraser fir killed by Phytophthora cinnamomi. (Courtesy Mike Benson, North Carolina State University)

 

Introduction

Phytophthora cinnamomi  Rands (1922)

 

Phytophthora cinnamomi was first described on Cinnamomum burmannii by Rands in Sumatra (Indonesia) in 1922 (Rands, 1922). The only known synonym is an undescribed pathogen called Pseudopythium phytophthoron by Sideris in 1929 on pineapple (Sideris, 1929). This pathogen was subsequently called P. cinnamomi by Mehrlich in 1932 (Mehrlich, 1932) and Sideris withdrew his description. The pathogen has been confused in the past with P. cambivora. Waterhouse named P. cinnamomi as a valid species distinct from P. cambivora in 1963 (Waterhouse, 1963). A description by Waterhouse and Waterston (1966) and a monograph by Zentmyer (1980) are useful references. P. cinnamomi is a group VI species (Stamps et al., 1990) (Fig. 1). 

Cultural Characteristics

Cultures show profuse tough aerial mycelium that sometimes grows in a rosette pattern (Fig. 2). The minimum temperature for growth is 5–6°C, the optimum temperature for growth is 24–28°C, and the maximum temperature for growth is 32–34°C. There is no colony growth above 35°C. 

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are thin (3 µm in diameter). They exhibit simple or sympodial branching and typically proliferate through the empty sporangium.

 

Sporangia:

Sporangia form only in aqueous solutions. They are nonpapillate and ovoid, obpyriform, or broadly ellipsoid. Sporangia are an average of 40 × 75 µm (Fig. 3). Apical thickening is slight and noncaducous.

 

Chlamydospores:

There are grapelike clusters of thin-walled chlamydospores (Fig. 4).

 

Hyphae:

Hyphae on malt agar are coralloid (i.e., with frequent, rounded nodules), becoming broad (8 µm wide) and very tough (Fig. 5). Hyphal swellings are typically spherical (average 42 µm, maximum 60 µm in diameter) and in clusters, and the wall is not very thick.

  

Sexual Structures

 

P. cinnamomi is heterothallic. Oospores form in intraspecific matings and in crosses with P. cryptogea.

 

Antheridia:

Antheridia are amphigynous and long and average 17 × 21–23 µm.

 

Oogonia:

Oogonia are round and smooth walled with a tapered base. They are hyaline to yellow-brown with age and 21–28 µm in diameter.

 

Oospores:

Oospores nearly fill the oogonia and are round and plerotic. The average diameter is 35 µm. The wall is colorless and 2 µm thick (Fig. 6).

Host Range and Distribution

The pathogen causes disease on pineapple (Ananas comosus); Lawson cyprus (Chamaecyparis lawsoniana); chestnut (Castanea dentata); quinine (Cinchona spp.); Cinnamomum spp.; species in the family Ericaceae, including Rhododendron spp.; Eucalyptus spp., especially jarrah (Eucalyptus marginata); walnut (Juglans spp.); pine (Pinus spp.); almond, cherry, peach, and plum (Prunus spp.); oak (Quercus spp.); avocado (Persea spp.); cranberry (Vaccinium macrocarpon); and more than 100 other host species, including English walnut, Irish yew, and Douglas fir. The pathogen causes jarrah dieback and has devastated Australian forests. See Zentmyer (1980) and Erwin and Ribeiro (1996) for further host lists.

Symptoms

The pathogen has a saprophytic phase and can survive in dead plant materials for years. Sporangia release zoospores into the soil that can infect roots. Propagules can also be splash dispersed to the aerial parts of plants. The pathogen can also infect leaves, stems, fruits, and pods.

 

Symptoms vary with the host infected, but in general, the pathogen can cause a root rot that leads to wilting and dieback of the plant (Figs. 7–9). P. cinnamomi causes a rot of fine feeder roots that leads to dieback. Wilt, stem cankers, decline in yield, decreased fruit size, gum exudation, collar rot near graft lines, and heart rot are also symptoms.

 

In pineapple, heart rot can be detected when the crown leaves turn from their normal green to a yellow or reddish color; also, these leaves can be easily removed when pulled. The pathogen infects the roots of kiwifruit trees, causing bark sloughing and exposing black-stained vascular tissues. When camellia plants become infected, depending on the severity of infection, leaves can become yellowish green and plants become stunted, gradually wilt, and eventually die.

 

Diagnostics:

Plant foliage becomes chlorotic, wilts, and dies back rapidly when root rot occurs. Occasionally, the infected plant suddenly collapses, but the plant can live for several years, especially in cooler and damper environments. Movement of symptomless plants is often the major source of spread of P. cinnamomi.

References

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

Mehrlich, F. P. 1932. Pseudopythium phytophthoron, a synonym of Phytophthora cinnomomi. Mycologia 24:453-454.

Rands, R. D. 1922. Streepkanker van Kaneel, veroorzaakt door Phytophthora cinnamomi n. sp. (Stripe canker of cinnamon caused by Phytophthora cinnamomi n. sp.), Meded. Inst. Plantenziekten 54. (In Dutch)

Sideris, C. P. 1929. Stem rot of pineapple plants. (Abstr.) Phytopathology 19:1146.

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., and Waterston, J. M. 1966. Phytophthora cinnamomi. CMI Descr. Pathog. Fungi Bact. 113:1-2.

Zentmyer, G. A. 1980. Phytophthora cinnamomi and the diseases it causes. Monogr. 10. American Phytopathological Society, St. Paul, MN.