Black sigatoka

Black tsigatoka 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 Tsigatoka 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

Black Sigatoka lesions on mature banana leaf.

Black Sigatoka is also known as Black Leaf Streak, and is found mostly on the banana crop. The pathogen Mycosphaerella fijiensis causes streaks that run parallel to the leaves. It is an ascomycete fungus that affects banana trees specifically in tropical climates; including Asia, West Africa, China, and South America.^[7] Tropical weather is the preferred climate to grow banana trees, but it is also the environment that the pathogen thrives in, hot, humid, with plenty of rainfall to aid in dispersal. The optimal environment of the pathogen would be similar to that of the banana tree. The fungus infects mature banana leaves and will continue to cause infection without proper control.^[8]

In the early stages of the plant’s parthenogenesis, the lesions start off looking rusty brown and they 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.^[9] The dimensions of the lesions are characteristically 20 x 2mm with a well defined wall surrounding it.^[10] 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, and causing the rapid decline of plant morphological and physiological functions.^[11] Leaves with a large infectious lesions will start to degrade and collapse off the branches because the leaf spots interrupt the plant’s ability to perform photosynthesis, leading to the ultimate death of the plant.^[7]

Yellow Leaf Streak is in the same genus as Black Leaf Streak that affects banana trees as well. Yellow Leaf Streak in contrast has smaller, yellow-green lesions, and appear on the top of the leaves.^[7]

Management

There are several ways to control for Black Sigatoka, either by way of cultural and chemical means or genetic engineering. Cultural control includes the destruction of the leaves that have been infected with M. fijiensis. This will help reduce the initial (ascospores) and secondary (conidia) spread of inoculum of new plant leaves and help upset the pathogen’s poly-cyclic disease cycle. Another way of reducing primary/secondary inoculums would be to have a proper drainage and irrigation system set in place. The technique of keeping the environment around the plants at low humidity helps the ascospores/conidia produced by the pathogen from being dispersed in the water draining towards other healthy, susceptible hosts.^[10] Other techniques include planting the banana trees more than 1,000 meters above sea level and practicing multi-cropping, mixing the banana tree with other trees or vegetation.^[8]

One form of chemical control is applying fungicides. This is a preemptive control used on banana trees in order to protect them from primary inoculum. The fungicide does not kill the pathogen itself, but only works on the pre-necrotic spots on the leaves, stopping the secondary spores from inoculating new, healthy plant tissue. The best time to apply, due to it being a protective fungicide, would be in the beginning of the season in order to stop any initial infection.^[11] The class of fungicides widely used to control for Black Leaf Streak are the triazole fungicides.^[9] This class of fungicide is a demethylation inhibitor and should be rotated with other modes of action to best lower the risk of resistance.^[12] The leaves that have already been infected must be removed mechanically to save the rest of the tree.^[11] Research has shown that there may be fungicide resistance developing for M. fijiensis. It has been observed that following the application of intense chemicals, the fungus persisted and spread. The same observations were found in fields with no chemical interference; the belief now being that the untreated fields are “breeding grounds for (the) development of resistant strains”.^[13] Research today shows continuous action towards reinventing banana breeding programs. 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.^[14] 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 to the banana trees. This in turn will hopefully show that there is a resistant gene available to be manufactured and injected to make new genetically modified, pathogen-resistant banana trees.^[15]

Importance

The world-wide spread of the disease has been rapid, with its naming and first reported occurrence in 1963.^[16][17] The disease was reported from Honduras in 1972, from where it spread north and south from Mexico to Brazil and into the Caribbean islands,^[17] in 1991.^[18] The fungus arrived in Zambia in 1973 and spread to the banana-producing areas of Africa from that introduction.^[17] The first occurrence of Black Sigatoka in Florida was reported in 1999.^[19] As it spread, Black Sigatoka replaced the yellow form and has become the dominant disease of bananas worldwide.^[17]

Black Sigatoka macroscopic view of lesions. This is the severe stage of the disease.The banana tree has lost a majority of the leaf surface due to the pathogen, reducing the photosynthetic ability of the plant.

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 mono-culture.^[17] 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.^[20]

Bananas are a principal crop for people with limited access to other resources, and the decrease in production of the fruit can encroach on their current diet, being that bananas are staple crop in some countries. There is also the possibility that the costs of bananas will rise with the substantial loss of bananas; this will result in a lot of people not being able to afford them.^[10] M. fijiensis has been found in all regions of the world that are major producers of bananas and is a constraint for these countries; specifically, Africa, Asia, and South America. Black Sigatoka is a very destructive disease to the foliage of banana trees. The disruption in photosynthesis is so severe that it can cause the farmer to lose up to 50% of their fruit yield.^[8] The fruits yielded from banana trees infected with Black Streak Leaf can interrupt the ripening of the banana, specifically causing it to “ripen prematurely and unevenly, and as a result becoming unsuitable for export”.^[11] A disruption in the maturation of the fruit can lead to a major shift in the economy of the international commerce. 10% of the bananas that are grown are sold to other countries while the other 90% is consumed by the farmers and local communities. Small farmers growing bananas for local markets cannot afford expensive measures to fight the disease. Black Sigatoka of Bananas threatens the fruit’s economy and the lives of the people who depend on the fruit for subsistence.^[21]

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