|Crop Knowledge Master||Fungi|
|Leaf spot and blight of coconut, Cocos nucifera|
TYPE: Kingdom: Fungus
Phylum: Ascomycota (teleomorph or sexual stage)
(Order: Dothideales; Family: Pleosporaceae;)
Traditional: Fungi Imperfecti: Deuteromycotina (anamorph or sexual stage)
TAXONOMY: Bipolaris incurvata was previously referred to as Helminthosporium incurvatum or Drechslera incurvata.
Leaf spot and blight of coconut, Cocos nucifera
Coconut and many palms such as Caryota mitis (clustered fishtail palm), Chamaedorea seifrizii (bamboo or reed palm), Chrysalidocarpus lutescens (golden palm; incorrectly called areca palm), Livistonia chinensis (Chinese fan palm), Phoenix roebelenii dwarf pygmy palm), Howea forsteriana (Forster sentry palm, incorrectly called Kentia palm), Rhapis species (lady palm), Ptychosperma elegans (Alexander palm), and Washingtonia robusta (Mexican fan palm).
In Hawaii, this pathogen has been found on Oahu and Hawaii but is likely to be present on all islands. It has been reported from many tropical and subtropical areas such as Florida, Jamaica, Asia, Australia, Oceania (French Polynesia, Fiji) Philippines, and the Seychelles.
Leaf spots of coconut caused by Bipolaris incurvata begin as tiny water soaked flecks. These expand into larger, tan to brown spots which are circular to oval with a darker brown border (Fig. 1). Slight yellowing of the area surrounding leaf spots also occurs. As larger areas of the leaf are diseased , the dead leaf tissue becomes tan and a more distinctly yellow area surrounds the lesions. Diseased tissue becomes brittle and parts of the infected area is lost giving the leaf its tattered appearance (Fig. 2).
Bipolaris incurvata produces microscopic spores that are long, curved, multicellular, and gray to pale olive-brown in color (Fig. 3). Pure cultures of Bipolaris incurvata are relatively easy to established from diseased plants but the sporulative phase is more difficult to maintain, as compared to other Bipolaris species. Spores of this fungus germinate by producing a germ tube (fungal thread) from both ends of the spore, that is, germination is bipolar. Germ tubes grow to establish a fungal colony or mass of fungal threads called mycelium. These colonies produce specialized fungal threads called conidiophores, which produce more spores or conidia.
This pathogen produces spores on the surface of blighted coconut leaves. Moisture favors the production of hundreds of conidiophores that emerge from the epidermis of the coconut leaves. Renewed high moisture levels or continuous moisture results in the formation of spores by the conidiophores. Lower temperatures (20 C or 68 F) favor spore formation while higher temperatures (31 C or 88 F) are not conducive.
Bipolaris spores are spread by wind and rain. Other species of Bipolaris are also spread by contact, so tree trimming operations, birds, and insects are also potential carriers of this pathogen. With moisture, spores germinate and germ tubes penetrate the host. The fungus grows within the host tissue, forming a colony inside the leaf. The leaf tissue dies and forms leaf spots or blights. Conidiophores produced by the colony emerge from the leaf and produce more spores.
Young coconut seedlings can be treated with fungicides such as Dithane M45 to reduce new infections; Severely diseased leaves should be removed. However, for mature coconut trees, the canopy is virtually inaccessible and little can be done. Keeping the older leaves trimmed will reduce spore levels.
1. Ellis, M. B. 1971. Dematiaceous Hyphomycetes. Commonwealth Agricultural Bureaux, Farnham Royal, Bucks., England
2. Farr, D. F., G. F. Bills, G. P. Chamuris, and A. Y. Rossman. 1989. Fungi on Plants and Plant Products in the United States. APS Press. St. Paul, Minnesota. 1252 pp
3. Uchida, J. Y. and M. Aragaki. 1991. Bipolaris and Exserohilum leaf spots. In: Diseases and Disorders of Ornamental Palms. Edited by Chase and Broschat. APS Press. St. Paul Minnesota. 55 pp. Note: photomicrograph of Bipolaris incurvata on page 6 is mislabeled as B. setariae.
COPYRIGHT: Janice Y. Uchida