Black sigatoka
Black Sigatoka is a leaf spot disease of banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....

 plants caused by ascomycete fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

Mycosphaerella fijiensis (Morelet). Plants with leaves damaged by the disease may have up to 50% lower yield of fruit. Black Sigatoka, also known as black leaf streak, was named for its similarities with the Yellow Sigatoka caused by Mycosphaerella musicola
Mycosphaerella musicola
Mycosphaerella musicola is a fungal plant pathogen, which is the causal agent of Yellow Sigatoka leaf spot disease on banana plants.-Characteristics:...

(Mulder), after the Sigatoka Valley in Fiji
Fiji , officially the Republic of Fiji , is an island nation in Melanesia in the South Pacific Ocean about northeast of New Zealand's North Island...

 where an outbreak of this disease reached epidemic proportions from 1912 to 1923.

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
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light). Over sixty distinct strains
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...

 with different pathogenetic potentials have been isolated. In order to better understand the mechanisms of its variability, the "Genetic diversity of Mycosphaerella fijiensis Project" has been initiated.

When spores of M. fijiensis are deposited on a susceptible banana leaf they germinate within three hours if there is a film of water present or if the humidity is very high. The optimal temperature for germination of the conidia is 27°C. The germ tube grows epiphytically
An epiphyte is a plant that grows upon another plant non-parasitically or sometimes upon some other object , derives its moisture and nutrients from the air and rain and sometimes from debris accumulating around it, and is found in the temperate zone and in the...

 over the epidermis for two to three days before penetrating the leaf via a stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

. 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.

In commercial export plantations, Black Sigatoka is controlled by frequent applications of fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...

s. 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 has recently been developed and will be soon field tested in Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...

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