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
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
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 |
Yam |
Leaf blight |
|
Alocasia macrorrhiza |
Pai, taro, ape |
Leaf blight |
|
Amorphophallus campanulatus |
Elephant-foot yam |
Foliar blight |
|
Araceae |
Arum, aroids |
Foliar blight |
|
Catharanthus roseus |
Periwinkle |
Foliar blight |
|
Colocasia antiquorum |
Elephant’s ear, gabi |
Corm rot; leaf blight |
|
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 |
|
Piper betle |
Betel, pan |
Wilt |
|
Vinca rosea |
Periwinkle |
Blight |
|
Xanthosoma mafaffa |
Giant golden taro |
Foliar blight |
|
Xanthosoma sagittifolium |
Yautia, malanga, tannia, ocumo |
Foliar blight |
|
Xanthosoma violaceum |
Blue taro, blue ape |
Foliar blight |
|
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,
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,
Ko, W. H. 1979. Mating type distribution of
Phytophthora colocasiae on the
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.
Sawada, K. 1911. Infection of taro. Spec. Bull. Agric. Exp. Stn. Gov.
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,