Black Sigatoka is a leaf spot disease of banana plants caused by ascomycete fungus Mycosphaerella fijiensis (Morelet). Black Sigatoka, also known as black leaf streak, 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 fifty sprays a year.
M. fijiensis reproduces both sexually and asexually, and both conidia and ascospores are important in its dispersal. The conidia are mainly waterborne to short distances, while ascospores are carried by wind to more remote places (the distances being limited by their susceptibility to ultraviolet light). Over sixty distinct strains with different pathogenetic potentials have been isolated. In order 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 humidity is high or a film of water 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 via 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.
The world-wide spread of the disease has been rapid, with its naming and first reported occurrence in 1963. The disease was reported from Honduras in 1972 from which it spread north and south from Mexico to Brazil and into the Caribbean islands, in 1991. The fungus arrived in Zambia in 1973 and spread to the banana producing areas of Africa from that introduction. The first occurrence of Black Sigatoka in Florida was reported in 1999. As it spread, Black Sigatoka replaced the yellow form and has become the dominant disease of bananas worldwide.
The most likely route of infection is through the importation of infected plant material and infection can spread rapidly in commercial areas where bananas are farmed as a monoculture.
In commercial export plantations, Black Sigatoka causes up to 50% loss of fruit and is controlled by frequent applications of fungicides. Removal of affected leaves, good drainage, and sufficient spacing also help to fight the disease. Although fungicides improved over the years, the pathogen developed resistance. Therefore higher frequency of application is required, increasing the impact on the environment and health of the banana workers.
Small farmers growing bananas for local market cannot afford expensive measures to fight the disease. However, some cultivars of banana are resistant to the disease. Research is done to improve productivity and fruit properties of these cultivars. A genetically modified banana variety made more resistant to the fungus was developed and was field tested in Uganda in the late 2000s.
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- Genetic diversity of Mycosphaerella fijiensis project