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.^[1]

According to new terminology, the Sigatoka disease complex is a cluster of three closely related fungi—yellow sigatoka (Pseudocercospora musae), eumusae leaf spot (Ps. eumusae), and black sigatoka (Ps. fijiensis).^[2]

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.^[3]

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.^[3]

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.^[4] 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.^[5][6] 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.^[5]

Symptoms

Most infections start on the underside of the leaf.^[7] The symptoms start as small specks that become streaks running parallel to the leaf veins. These streaks aggregate and eventually form spots that coalesce, form a chlorotic halo, and eventually merge to cause extensive necrosis.

Commercial effect

The world-wide spread of the disease has been rapid, with its naming and first reported occurrence in 1963.^[8][9] The disease was reported from Honduras in 1972, from where it spread north and south from Mexico to Brazil and into the Caribbean islands,^[9] in 1991.^[10] The fungus arrived in Zambia in 1973 and spread to the banana-producing areas of Africa from that introduction.^[9] The first occurrence of black sigatoka in Florida was reported in 1999.^[11] As it spread, black sigatoka replaced the yellow form and has become the dominant disease of bananas worldwide.^[9]

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 in monoculture.^[9]

In commercial export plantations, the disease causes up to 50% loss of fruit and is controlled only by frequent applications of fungicides.^[3] 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 applications is required, increasing the impact on the environment and health of the banana workers. In regions where disease pressure is low and fungicide resistance has not been observed, it is possible to better time the application of systemic fungicides by using a biological forecasting system.^[12]

Small farmers growing bananas for local markets cannot afford expensive measures to fight the disease. However, some cultivars of bananas 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.^[13]

See also

• List of Mycosphaerella species

References

External links

• Bennett, R.S. and P.A. Arneson. 2003. Black Sigatoka, The Plant Health Instructor. doi:10.1094/PHI-I-2003-0905-01
• Genetic diversity of Mycosphaerella fijiensis project