|Crop Knowledge Master|
|Heart rot of
Fruit rot of coconut
TYPE: Kingdom: Chromista
The Phytophthora species that has been isolated from coconut in Hawaii, resembles P. katsurae. Although this name is used here, the coconut pathogen may be a new species. Phytophthora katsurae was first isolated from chestnut in Japan. It was named P. castaneae and later renamed P. katsurae. Both the coconut pathogen and P. katsurae have similar morphological characteristic but the coconut pathogen has larger sporangia, less oogonial protuberances and longer, tapered, or funnel-shaped oogonial bases.
The causal organism of diseased coconut on the Ivory Coast was called P. heveae but is now believed to be misidentified. This Ivory Coast Phytophthora has oogonial protuberances like the coconut pathogen in Hawaii. Phytophthora heveae has a long tapered oogonial base, like P. katsurae but does not have oogonial protuberances.
Phytophthora katsurae has been reported from Japan, Taiwan, Australia and Papua New Guinea.
Coconut (Cocos nucifera)
This coconut disease was first noticed on Kauai in 1971. It is now also present on the islands of Hawaii, Maui, and Oahu. It is most serious in wet locations such as Hilo, Hawaii and Waimanalo, Oahu. This disease has been also reported from the Ivory Coast on coconut.
Death of young coconut fronds is the first common symptom of this disease (Fig. 1 and 2). The young leaves wilt, dry, and are often bent or drooping into the tree canopy. As more young leaves are lost, trees are left with only a skirt of older leaves (Fig. 3 and 4). Eventually all leaves are killed and only trunks remain.
By the time young leaves are killed, the heart of the tree is already in advanced stages of decay (Fig. 5, 6, and 7). Trees with this symptom never recover. Occasionally trunk rots are also found on the lower stem areas (Fig. 8). These trunk lesions are associated with dead petioles and sheaths that are left on trees.
Premature loss of fruits is an early sign of disease. In later stages, fruit rots become more common. Infected green fruits have characteristic black to brown rots that expand irregularly and frequently form green areas surrounded by darkened diseased tissue (Fig 9). Internally the infected husk is dark brown (Fig. 10). Inoculations of healthy green fruits show that the initial fruit rot symptoms are irregular, dark green, water-soaked areas (Fig. 11). The fungus grows rapidly through the skin or epidermis of the coconut fruit to cause disease.
This fungus produces lemon-shaped spores called sporangia. The sporangia are microscopic, 22-41 X 30-49 um, hyaline, and deciduous (fall-off from the colony). In water or with high moisture, sporangia produce numerous swimming spores called zoospores. Without free water the sporangia will germinate directly and penetrate host tissue. Sporangia are produced on the surface of infected tissue. Phytophthora katsurae is homothallic or self-fertile. It readily produces large numbers of sexual spores or oospores in the infected host tissue. These oospores are relatively easy to find in coconut fruit and because the sexual stage is distinctive, the pathogen is readily identified (Fig. 12 and 13). It is very difficult to find oospores in the large rancid heart or in infected trunk lesions.
The oogonium or mother spore is a structure that produces the oospore within it. This oogonium has lumps or wall protuberances on its surface and a long, tapered base that resembles a funnel (Fig. 14). These characteristics are unique and aid pathogen identification. However, identification is best made after the fungus has been cultured and grown on agar under standard conditions. Oogonial protuberances are difficult to find within host tissue and if not found the pathogen may be incorrectly identified as P. heveae(see taxonomy).
Moist environments are conducive to Phytophthora species. Growth, sporulation, dissemination, and disease development are all favored by moisture. The fungus grows well at 68 to 80 F.
Little is known about the mode of spread and early stages of mature tree infection. Hundreds of trees have been killed on Kauai and Oahu. Wind damage of the canopy and distribution of the fungus by wind driven rain has been suggested. Tree trimming operations could also serve as a source of the pathogen and aid disease spread. Inoculations of non-wounded trunks show that the pathogen does not penetrate the dry bark and is only able to kill the plant if the white inner bark is exposed.
Fruits are easily penetrated by the fungus and this may be a route for pathogen movement into the heart. The role of insects also needs to be considered.
Movement of infected trees and nuts has probably moved the pathogen throughout the State.
Infected trees which are identified by dead young leaves cannot be saved. Application of systemic fungicides has not prevented loss of infected trees. Rapid removal and destruction of infected trees is highly recommended. In groves of infected trees, every effort is needed to increase air circulation and reduce humidity in the field. Old leaves and petioles (leaf stems) need to be removed. By removing all infected trees and increasing the air circulation, healthy trees may be protected.
To date no other palm has been found with this pathogen. Thus a diversified planting of several palm species would be a practical alternative to landscaping needs. If coconut fruit are needed, select new trees carefully, avoid planting trees close together and monitor for early signs of this disease.
1. Katsura, K. 1976. Two new species of Phytophthora causing damping-off of cucumber and trunk rot of chestnut. Trans. Mycol. Soc. Japan. 17:238-242.
2. Ko, W. H. and H. S. Chang, 1979. Phytophthora katsurae, a new name for P. castaneae. Mycologia 71:840-844.
3. Quillec, G. and J. L. Renard. 1984. La pourriture a Phytophthora du cocotier. Oleagineux 39:143-147.
4. Quillec, G., J. L. Renard, and H. Ghesquiere. 1984. Le Phytophthora heveae du cocotier: Son role dans la pourriture du coeur et dans la chute des noix. Oleagineux 39:477-485
5. Uchida, J. Y. , J. J. Ooka, N. M. Nagata and C. Y. Kadooka. 1992. A new Phytophthora fruit and heart rot of coconut. Research Ext. series 138. CTAHR publication, Univ. of Hawaii.
6. Uchida, J. Y., M. Aragaki, J. J. Ooka, N. M. Nagata. 1992. Phytophthora fruit and heart rots of coconut in Hawaii. Plant Disease 76:925-927.
7. Uchida, K. 1967. Phytophthora disease of chestnut. Plant Prot. 21:383-387.
COPYRIGHT: Janice Y. Uchida