Sigatoka leaf spot diseases

Two forms of Sigatoka leaf spots affect bananas - yellow Sigatoka, caused by the fungus Mycosphaerella musicola (Zimm.) Deighton (anamorph Pseudocercospora musae), and black Sigatoka, caused by M. fijiensis (Morelet) Deighton (anamorph Paracercospora fijiensis) (Stover, 1980). Both leaf spot diseases are of economic importance, but black Sigatoka develops more rapidly, causes more severe defoliation, and is more difficult to control than yellow Sigatoka (Stover & Dickson, 1976). In addition to being more virulent on its traditional banana hosts, black Sigatoka also attacks plantains, which are not normally susceptible to yellow Sigatoka (Stover, 1980).

Although it has been reported, anecdotally, that wherever it has been introduced, black Sigatoka has replaced yellow Sigatoka within two years, this has not been proven, and is now under some doubt (Jones, 1990). Since c. 1930 the banana industry has been afflicted with the original leaf spot disease, yellow Sigatoka (then known simply as Sigatoka) for which control treatments were needed only for export production. Control of black Sigatoka, however, is practiced with fungicides (usually triazoles in rotation with other compounds) which have to be applied more frequently than for yellow Sigatoka. The economic consequences of this disease are serious for export producers, and can be catastrophic for non-export and subsistence growers. Environmentally, the consequences of frequent aerial fungicide applications are of great concern to governments in Central and South America.

All of the Cavendish varieties (AAA) are highly susceptible to black Sigatoka, as are dessert bananas of the 'Silk' and 'Pome' clones (AAB), most plantain cultivars (AAB) and the AAB and ABB cooking bananas (Stover & Simmonds, 1987). For export producers the need to develop a dessert banana with resistance to black Sigatoka has never been greater; for subsistence farmers and non-export commercial growers in Africa and South America the presence or imminent arrival of black Sigatoka is of equal concern. These producers do not have the chemical control option, economically or practically. If major shortages or shifts to other crops are to be avoided, new resistant cultivars will have to be introduced, and this is a primary focus of the banana breeding programmes.

	Figure 1 Typical symptoms of yellow Sigatoka on banana leaf.
	Figure 2 Typical symptoms of black Sigatoka on banana leaf.

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