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Introduction
Phytophthora tropicalis Aragaki
& J. Y. Uchida (2001)
Phytophthora tropicalis was previously identified as part of the species P. capsici. Several isolates classified as P. capsici differed morphologically from the species and were referred to as P. palmivora MF4. The isolates were obtained from cacao and a wide variety of other tropical plants. The morphological and pathological distinctions between P. capsici and P. palmivora MF4 led to the creation of a new taxon, P. tropicalis. The most notable difference between P. tropicalis and P. capsici is the narrow sporangia with mostly tapered bases of P. tropicalis compared with the broad sporangia with mostly rounded bases of P. capsici (Aragaki and Uchida, 2001). P. mexicana is listed as a synonym of P. tropicalis by Mchau and Coffey (1995), while P. palmivora is a partial synonym of P. tropicalis (Aragaki and Uchida, 2001; Cline et al., 2008). Zhang et al. (2004) used DNA sequencing from the internal transcribed spacer (ITS) regions to separate the species and suggested that P. capsici and P. tropicalis are in the same clade but distinct species. Gallegly and Hong’s DNA fingerprint data also support the two species as unique (Gallegly and Hong, 2008). P. tropicalis is a Waterhouse group II Phytophthora species (Stamps et al., 1990).
Cultural Characteristics
P. tropicalis can be grown on lima bean agar, carrot agar, and V-8 juice agar (Fig. 1). The optimum temperature for growth is 24–28°C (Aragaki and Uchida, 2001). P. tropicalis has poor or no growth at 35°C.
Reproductive Structures
Asexual Structures
Sporangia are produced in umbellate sympodia (Fig. 2). It also produces simple sympodia in water.
Sporangia:
Sporangia can be uniform, obovoid, pyriform, or ellipsoidal (Aragaki and Uchida, 2001). Sporangial bases are tapered, have a single papillae and long pedicels, and are caducous in water (Figs. 2 and 3) (Alizadeh and Tsao, 1985; Zhang et al., 2004). Sporangia have conspicuous papillae and are 19–27 × 40–55 µm (Bush et al., 2006). The length–breadth ratios are greater than 1.8.
Chlamydospores:
Chlamydospores are 27–33 µm in diameter with walls up to 3 µm thick (Aragaki and Uchida, 2001) (Fig. 4).
Hyphae:
Hyphae are 3–8 µm in diameter, with a common diameter of 4–5 µm (Aragaki and Uchida, 2001).
Sexual Structures
P. tropicalis is a heterothallic species.
Antheridia:
Antheridia are amphigynous and 13.0 ±2 × 14.8 ±1 µm (Aragaki and Uchida, 2001).
Oogonia:
Oogonia are spherical to subspherical and smooth walled and have an average diameter of 28.0 ±1.7 µm) (Aragaki and Uchida, 2001).
Oospores:
Oospores are spherical, plerotic, hyaline, and thick walled (Fig. 5). Oospores average 25.1 ±1.7 µm in diameter (Aragaki and Uchida, 2001).
Host Range and Distribution
|
Host |
Common Name |
Disease |
Geographical Distribution |
|
Theobroma cacao |
|
Black pod; stem canker; chupon wilt |
|
|
Piper nigrum |
Black pepper |
Leaf blight; stem canker |
|
|
Macadamia integrifolia |
Macadamia |
Nut blight |
|
This species may have been reported before 2001 as P. capsici; therefore, the full geographic distribution is unclear. The pathogen infects 14 genera in 12 families (Cline et al., 2008).
Symptoms
The pathogen was first isolated from cacao and black pepper and was originally designated as P. palmivora MF4 and then subsequently assigned to P. capsici (Alizadeh and Tsao, 1985; Mchau and Coffey, 1995). The pathogen infects stems and pods of cacao and leaves and stems of black pepper. Aragaki and Uchida (2001) separated the isolates from macadamia, black pepper, and cacao into a new species called P. tropicalis.
Black Pod of
Theobroma cacao (
Pods or cherelles (immature pods) may be infected at any place on the surface, but infection is most often initiated at the tip or stem end. The first symptom is the development of a brown to black spot on the pod, which spreads rapidly in all directions, eventually covering the entire pod. Under humid conditions, a white bloom of mycelium and sporangia forms on the surface of the diseased pods. As the disease progresses to advanced stages, P. tropicalis invades the internal tissue and causes discoloration and shriveling of the bean. Diseased pods eventually turn black and mummify. Black pod may create a distinct seaweedlike odor (Sreenivasan and Quesnel, 1977). Contact of diseased pods with healthy ones allows the pathogen to spread.
Stem Canker
of Theobroma cacao
(
The first visible symptom of stem canker is defoliation, which is caused by coalescence of the cankers that girdle the stem or fan branches. In early stages of canker development, the bark can be peeled to reveal wood of a watery gray color, often with reddish streaks that intensify in color after exposure to air. In advanced stages, a reddish fluid exudes from cankers and dries into a rusty deposit. The wood turns brown, and blackish streaks can often be seen. A protuberance or “bottom bulge” may be observed on the surface of diseased trunks that exudes a reddish brown substance. The disease is spread during heavy rainfalls and with physical contact between diseased and healthy plants.
Chupon Wilt
of Theobroma cacao (
The chupon is a soft tissue or sucker that emerges at the base of the cocoa tree. Usually, the axil of a young leaf is infected first, and then an angular, necrotic lesion forms. The young shoots are then rapidly girdled, resulting in wilting of the shoot.
Blight of Macadamia:
P. tropicalis can infect the flower and the nut of macadamia. P. tropicalis initially causes a dark brown to black discoloration on the nut; sometimes it can penetrate under the husk of the nut and kill the kernel.
Leaf and
Fruit Blight of Breadfruit:
The pathogen has also been reported to cause a leaf blight and fruit blight of
breadfruit in
References
Alizadeh, A. P., and Tsao, P. H. 1985. Effect of light on
sporangium formation, morphology, ontogeny, and caducity of
Phytophthora capsici and Phytophthora
palmivora MF4 isolates from black pepper and other hosts. Trans. Br. Mycol.
Soc. 85:47-49.
Aragaki, M., and Uchida, J. Y. 2001. Morphological distinctions between
Phytophthora capsici and P.
tropicalis sp. nov.
Mycologia
93:137-145.
Bush, E. A., Stromberg, E. L., Hong, C., Richardson, P. A., and Kong, P. 2006.
Illustration of key morphological characteristics of Phytophthora species
identified in
Cerqueira, A. O., Luz, E. D. M. N., and de Souza, J. T. 2005.
First record of Phytophthora tropicalis causing leaf blight and fruit rot of
breadfruit in
Cline, E. T., Farr,
D. F., and Rossman, A. Y.
2008. A
synopsis of Phytophthora with accurate
scientific names, host range, and geographic distribution. Plant Health Progress
doi:10.1094/PHP-2008-0318-01-RS.
Gallegly, M., and Hong, C. 2008. Phytophthora: Identifying Species by Morphology
and DNA Fingerprints. American Phytopathological Society,
Mchau, G. R. A., and Coffey, M. D.
1995. Evidence for the existence of two subpopulations in
Phytophthora capsici and a redescription of the species.
Mycol. Res. 99:89-102.
Sreenivasan, T. N., and Quesnel, V. C. 1977. Field differentiation of black pod
from other cacao pod diseases. Trop. Agric. 54:371-372.
Stamps, D. J., Waterhouse, G. M., Newhook, F. J., and Hall, G. S. 1990. Revised tabular key to the species of Phytophthora. Mycol. Pap. 162. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.
Zhang, Z. G., Zhang, J. Y., Zhen, X. B., Yang, Y. W., and Ko, W. H. 2004.
Molecular distinctions between
Phytophthora capsici and Phytophthora
tropicalis based on ITS sequences of ribosomal DNA.
J. Phytopathol.
152:358-364.