Figure 1. Morphology of Phytophthora colocasiae. A–E, Semipapillate ellipsoid sporangia. F, Intercalary chlamydospore. G, Terminal chlamydospore. H, Globose oogonium with an amphigynous antheridium. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

Figure 2. The disease cycle of taro leaf blight, caused by Phytophthora colocasiae. (Courtesy Vickie Brewster; Reproduced from Brooks, 2005) Click image to see larger view.

 

Figure 3. Sporangium of Phytophthora colocasiae with an apical papillum (arrow) and basal, intermediate-length pedicel (sporangium 26 × 35 µm). (Courtesy Fred Brooks; Reproduced from Brooks, 2005)

 

Figure 4. Oogonium (A) with an amphigynous male antheridium (B) and aplerotic oospore (C) of Phytophthora colocasiae. (Courtesy R. Fullerton; Reproduced from Brooks, 2005)

 

Figure 5. Taro leaf blight, caused by Phytophthora colocasiae, appears as small, dark brown flecks or light brown spots on the upper leaf surface and is spread by rain splash and windblown rain between plants. (Courtesy Fred Brooks; Reproduced from Brooks, 2005) Click image to see larger view.

 

Figure 6. Taro leaf blight, caused by Phytophthora colocasiae. Note: As spots increase in size, they coalesce and quickly destroy the leaf. (Courtesy Fred Brooks; Reproduced from Brooks, 2005) Click image to see larger view.

 

Figure 7. Taro leaf blight, caused by Phytophthora colocasiae. Note: Spots enlarge rapidly, becoming circular, zonate, and purplish brown to brown. (Courtesy Fred Brooks; Reproduced from Brooks, 2005) Click image to see larger view.

 

Introduction

Phytophthora colocasiae  Racib. (1900)

 

Phytophthora colocasiae was first isolated and described by Raciborski in 1900 as the causal organism of leaf spot of taro (Colocasia esculenta) (Raciborski, 1900). Synonyms include Kawakamia colocasiae (Racib.) Sawada (1911) and P. parasitica var. colocasiae (Racib.) Sarej. (1936). The pathogen is the major limiting factor for taro production in the developing world (Erwin and Ribeiro, 1996). Taro is a major food crop for subsistence farmers in Southeast Asia, where yield reductions of 50% can occur. P. colocasiae is a group IV species (Stamps et al., 1990) (Figs. 1 and 2).

Cultural Characteristics

The minimum temperature for growth is less than 10°C, the optimum temperature for growth is 27–30°C, and the maximum temperature for growth is slightly less than 35°C.

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores in culture are irregularly branched with basal swellings, but on leaf surfaces, they are usually unbranched.

 

Sporangia:

Sporangia are semipapillate and caducous with a medium pedicel length of 3.5–10 µm (Brooks, 2005) (Fig. 3). They are ovoid or ellipsoid and 17–28 × 40–70 µm. They have a length–breath ratio of 1.6–2.6 and a tapered base with an occasional lateral or intercalary attachment. Sporangia have a conspicuous basal plug at the point of attachment of the sporangia to the sporangiophore.

 

Chlamydospores:

Chlamydospores are abundant in some isolates and rare in others. They are 17–38 µm in diameter (average 27 µm) and the wall is 2–3 µm thick. Formation is either intercalary or terminal (Fig. 1F and G).

 

Sexual Structures

 

P. colocasiae is heterothallic, although some isolates produce oogonia in single cultures. Ko (1979) reported only the A1 mating type on the island of Hawaii, while elsewhere, the A2 is distributed.

 

Antheridia:

Antheridia are amphigynous and subterminal.

 

Oogonia:

Oogonia are 20–35 µm in diameter (average 29 µm).

 

Oospores:

Oospores are 18–30 µm in diameter (average 23 µm) and aplerotic (Fig. 4).

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Alocasia spp.

Yam

Leaf blight

Ceylon

Alocasia macrorrhiza

Pai, taro, ape

Leaf blight

Solomon Islands

Amorphophallus campanulatus

Elephant-foot yam

Foliar blight

India

Araceae family members

Arum, aroids

Foliar blight

Italian East Africa

Catharanthus roseus

Periwinkle

Foliar blight

Hawaii

Colocasia antiquorum

Elephant’s ear, gabi

Corm rot; leaf blight

India, Java, Philippines, China

Colocasia esculenta

Taro, dasheen, gabi, dalo

Leaf spot; wet rot of corms; storage rot of corms

Java, India, Taiwan, Ceylon, Guam, Hawaii, Burma, China, Malaysia, Fernando Po, Fiji, British Solomon Islands, Hong Kong, Nepal, Thailand, Ethiopia, Papua New Guinea

Hevea brasiliensis

Rubber

Leaf fall; stem canker; black stripe

China

Piper betle

Betel, pan

Wilt

Malaysia, India, Burma

Vinca rosea

Periwinkle

Blight

Hawaii, Argentina

Xanthosoma mafaffa

Giant golden taro

Foliar blight

Fernando Po

Xanthosoma sagittifolium

Yautia, malanga, tannia, ocumo

Foliar blight

Dominican Republic

Xanthosoma violaceum

Blue taro, blue ape

Foliar blight

Fernando Po

Symptoms

The pathogen produces both chlamydospores and oospores and can survive in the rhizomes of taro. It is splash dispersed with rainfall. The pathogen causes a foliar blight of taro (Fig. 5). It has been distributed by vegetative propagation and infested soil (Aggarwal et al., 1990). The early symptoms of blight are characterized by water-soaked, purple or brown circular lesions on the leaf that vary in size up to 2 cm (Fig. 6). Infection occurs in areas of the leaf where water accumulates (Erwin and Ribeiro, 1996). The lesions become dark brown with yellow margins, and white mycelium can be seen around the lesion (Fig. 7). A diagnostic feature is the yellow to red fluid that exudes from the center of the spots. Irregular lesions eventually rot but do not drop from the plant. Petioles can become infected. A postharvest rot of the corms can occur and the corms become leathery and firm in contrast to the soft rot caused by other fungi, such as Phyllosticta colocasiae.

 

Diagnostics:

The pathogen can be readily isolated from diseased leaf lesions onto antibiotic-amended media (Ko, 1979).

References

Aggarwal, A., Narula, K. L., Gurinderjitt, K., Mehrotra, R. S., and Kaur, G. 1990. Phytophthora colocasiae Racib. Its taxonomy, physiology, pathology, and control. Pages 105-134 in: Perspectives in Mycological Research, Vol. II. S. K. Hasija and K. S. Bilgrami, eds. Int. Biosci. Ser. Vol. 15, Today and Tomorrow's Printers, New Delhi, India.

 

Brooks, F. E. 2005. Taro leaf blight. Plant Health Instructor doi:10.1094/PHI-I-2005-0531-01.

 

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

 

Ko, W. H. 1979. Mating type distribution of Phytophthora colocasiae on the island of Hawaii. Mycologia 71:434-437.

 

Raciborski, M. 1900. Page 9 in: Java’s parasitic algae and fungi. Part I. Batavia.

 

Sarejanni, J. A. 1936. A collar rot of cultivated Solanum and the classification of the genus Phytophthora. Ann. Inst. Phytopathol. Benaki 2:35-52.

 

Sawada, K. 1911. Infection of taro. Spec. Bull. Agric. Exp. Stn. Gov. Formosa 2:75-84.

 

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.