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 yellow Sigatoka, which is caused by Mycosphaerella musicola (Mulder), which was itself named after the Sigatoka Valley in Fiji. In the same valley an outbreak of this disease reached epidemic proportions from 1912 to 1923.

According to new terminology, the Sigatoka disease complex is a cluster of three closely related fungi: Black Sigatoka and its congeners Yellow Sigatoka (Ps. musae) and eumusae leaf spot (Ps. eumusae).

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 pathogenic potentials have been isolated. To better understand the mechanisms of its variability, projects to understand the genetic diversity of M. fijiensis have been initiated. 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. Tropical weather is the preferred climate for banana cultivation, but it is also the environment where the pathogen thrives: hot and humid, with plenty of rainfall to aid in dispersal. The optimal environment of the pathogen is similar to that of the banana tree. The fungus infects mature banana leaves and will continue to cause infection without proper control.

In the early stages of the infection of the plant, the lesions have a rusty brown appearance and appear to be faint, paint-like specks on the leaves. They become more visible on the undersides of the banana leaf as the lesions and leaves grow. The spots on the undersides of leaf are the fungus itself. The sign of the pathogen consists of the ascocarp which holds the ascospores used for dissemination to infect healthy new plants when the environment is conducive. The pathogen then survives on dead plant tissue as mycelium. The dimensions of the lesions are characteristically with a well defined wall surrounding it. After further development, they become darker, sink into the leaf, and turn into depressions. The depressions themselves and the chlorosis surrounding them are the visible symptoms of the plant pathogen. They eventually will merge, causing the rapid decline of plant morphological and physiological function. Leaves with large infectious lesions will start to degrade and collapse because the leaf spots interrupt the plant's ability to perform photosynthesis, leading to the ultimate death of the plant. Leaves that have already been infected must be removed mechanically to save the rest of the tree. Methyl benzimidazole carbamates are also used and there is also resistance known to these. Although fitness penalties are common in resistance evolution many MBC resistant pathogens do not suffer any penalty and a few have the opposite effect. Furthermore, the search for genetic resistance shows promise with the discovery of a protein that can produce a hypersensitive response to control M. fijiensis that is being introduced into banana trees. This may lead to the identification of a resistance gene that could be transferred to banana trees.

Importance

The worldwide spread of the disease has been rapid, with its naming and first reported occurrence in 1963. The disease was reported in 1972 in Honduras, from where it spread north to central Mexico and south to Brazil and into the Caribbean islands The fungus arrived in Zambia in 1973 and spread to the banana-producing areas of Africa from that introduction. 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 via the importation of infected plant material, and infection can spread rapidly in commercial areas where bananas are farmed in monoculture.

Bananas are a principal crop for people with limited access to other resources, and the decrease in production of the fruit can limit their diet. There is also the possibility that the cost of bananas will rise with the substantial loss of bananas, leading to unaffordability.

See also

  • List of Mycosphaerella species

References

  • Bennett, R.S. and P.A. Arneson. 2003. Black Sigatoka, The Plant Health Instructor.
  • Genetic diversity of Mycosphaerella fijiensis project