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black sigatoka
Black sigatoka is a leaf-spot disease of banana plants caused by the ascomycete fungus ''Mycosphaerella fijiensis'' (Morelet). Also known as black leaf streak, it was discovered in 1963 and named for its similarities with the yellow sigatoka, which is caused by ''Mycosphaerella musicola'' (Mulder), which was itself named after the Sigatoka Valley in Fiji, where an outbreak of this disease reached epidemic proportions from 1912 to 1923. Plants with leaves damaged by the disease may have up to 50% lower yield of fruit and control can take up to 50 sprays a year.
== Life history ==
''M. fijiensis'' reproduces both sexually and asexually, and both conidia and ascospores are important in its dispersal. The conidia are mainly waterborne for short distances, while ascospores are carried by wind to more remote places (the distances being limited by their susceptibility to ultraviolet light). Over 60 distinct strains with different pathogenetic potentials have been isolated. To better understand the mechanisms of its variability, projects to understand the genetic diversity of ''M. fijiensis'' have been initiated.〔
When spores of ''M. fijiensis'' are deposited on a susceptible banana leaf, they germinate within three hours if the humidity is high or a film of water is present. The optimal temperature for germination of the conidia is 27 °C (81 °F). The germ tube grows epiphytically over the epidermis for two to three days before penetrating the leaf by a stoma. Once inside the leaf, the invasive hypha forms a vesicle and fine hyphae grow through the mesophyll layers into an air chamber. More hyphae then grow into the palisade tissue and continue on into other air chambers, eventually emerging through stomata in the streak that has developed. Further epiphytic growth occurs before the re-entry of the hypha into the leaf through another stoma repeats the process. The optimal conditions for ''M. fijiensis'' as compared with ''M. musicola'' are a higher temperatures and higher relative humidity, and the whole disease cycle is much faster in ''M. fijiensis''.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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