Black Sigatoka

Black Sigatoka is a leaf spot disease of banana plants caused by ascomycete fungus 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 (Mulder), after the Sigatoka Valley in Fiji 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 light). Over sixty distinct strains 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 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.

In commercial export plantations, Black Sigatoka 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 has recently been developed and will be soon field tested in Uganda.