Fungal Rot Killing Coconut Trees in Hawaii

By: Janice Uchida and Jeri Ooka, 12 Aug 2009, Horticulture Digest.

Background
In the 1970’s, a new disease of coconut was discovered on Kauai, and subsequently found on Oahu, Maui, and Hawaii. The fungal pathogen, a Phytophthora species, has become a major factor in the destruction of mature coconut trees and has devastated coconut stands on Kauai and Oahu. Coconut plants of all ages succumb to this disease.

Symptoms
The early stage of this disease is characterized by the loss of young coconut fruits. The stem end of small, young fruits or the entire fruit is darkened and rotted. Large, immature or mature green fruits have brown, mottled, irregular to circular patterns distinctive of this disease. Heart rot of the plant (a rot of the growing point) follows and is evident as the youngest spear leaf dies. Eventually all leaves are killed, followed by tree loss which invariably follows heart rot.

Cause
The disease is caused by a fungal pathogen relatively new to Hawaii. It is similar to Phytophthora katsurae, a pathogen of chestnut in Japan. Morphologically, the coconut pathogen differs somewhat from the chestnut pathogen and thus, it may be a ne w, undescribed species. A coconut disease similar to the disease in Hawaii has been reported from Africa.

Spread
This disease can be spread by the movement of infected nuts and trees. The disease is slow in development, and early stages of infection may be difficult to detect. Since transplanting coconut is a common landscaping practice in Hawaii, symptomless but infected trees may be transported great distances. Tree pruning operations could also aid fungal spread by carrying fungal spores from diseased plants to healthy plant on contaminated trimming equipment and tools. Wounds provide excellent opportunities for fungal penetration and disease development. Insects and rodents could also aid movement of the fungus by carrying spores or infected host tissue containing spores. Wind-driven rain is known to spread other tropical Phytophthora diseases. Prolonged rainy periods favor disease development and spread. Phytophthora produces spores in abundance on infected coconuts in moist environments. Swimming spores (zo ospores) are produced by all Phytophthora species when water is available. These motile spores increase the potential for rapid disease spread.

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