Tropical Research and Education Center,
University of Florida, IFAS, Homestead
Origins and importance of banana as a food crop
Despite the current, clear understanding of its ancestry, the edible bananas’ origins are often confused in the literature. Almost all of the 300 or more cultivars that are known arose from two seeded, diploid species, Musa acuminata Colla and M. balbisiana Colla; they are diploid, triploid and tetraploid hybrids among subspecies of M. acuminata, and between M. acuminata and M. balbisiana.
Conventionally, the haploid contributions of the respective species to the cultivars are noted with an A and B. For example, the Cavendish cultivars that are the mainstays of the export trades are pure triploid acuminata and, thus, AAA. The Linnaean species M. paradisiaca (the AAB plantains) and M. sapientum (the sweet dessert bananas, of which Silk AAB is the type cultivar) are invalid and no longer used.
Banana is now one of the most popular of all fruits. Although it is viewed as only a dessert or an addition to breakfast cereal in most developed countries, it is actually a very important agricultural product. After rice, wheat and milk, it is the fourth most valuable food. In export, it ranks fourth among all agricultural commodities and is the most significant of all fruits, with world trade totaling $2.5 billion annually. Yet, only 10% of the annual global output of 86 million tons enters international commerce. Much of the remaining harvest is consumed by poor subsistence farmers in tropical Africa, America and Asia. For most of the latter producers, banana and plantain (which is a type of banana) are staple foods that represent major dietary sources of carbohydrates, fiber, vitamins A, B6 and C, and potassium, phosphorus and calcium.
Impact of banana diseases
A leaf spot disease is the most important of these problems. Black Sigatoka, which is also known as black leaf streak, causes significant reductions in leaf area, yield losses of 50% or more, and premature ripening, a serious defect in exported fruit. It is more damaging and difficult to control than the related yellow Sigatoka disease, and has a wider host range that includes the plantains and dessert and ABB cooking bananas that are usually not affected by yellow Sigatoka.
In export plantations, Black Sigatoka is controlled with frequent applications of fungicides and cultural practices, such as the removal of affected leaves, and adequate spacing of plants and efficient drainage within plantation. In total, these are very expensive practices. For example, fungicide application includes the use of airplanes or helicopters, permanent landing strips and facilities for mixing and loading the fungicides, and the high recurring expense of the spray materials themselves. In total, it has been estimated that the costs of control are ultimately responsible for 15-20% of the final retail price of these fruit in the importing countries. Their great expense makes them essentially unavailable to small-holder farmers who grow this crop, it is these producers who are affected most by this important disease.
Distribution, etiology and epidemiology of black Sigatoka
Black Sigatoka is caused by the ascomycete, Mycosphaerella fijiensis Morelet [anamorph: Paracercospora fijiensis (Morelet) Deighton] (a variant of the pathogen, M. fijiensis var. difformis, that was previously reported in tropical America, is no longer recognized). The pathogen produces conidia and ascospores, both of which are infective. They are formed under high moisture conditions, and are disseminated by wind, and in the case of conidia, also by rain and irrigation water. Due to their greater abundance and small size, ascospores are more important than conidia in spreading the disease within plants and plantations. In contrast, infected planting material and leaves, which are used often in the developing world as packing materials, are usually responsible for the long-distance spread of the disease. The recent outbreak of black Sigatoka in South Florida almost certainly resulted from the importation of infected germplasm by local growers (see Plant Disease note D-1998-1217-03N).
Since there is a tendency for resistance or tolerance to develop in M. fijiensis towards the systemic fungicides, they are usually applied in combination or alternation with broad-spectrum, protectant fungicides, such as the dithiocarbamates and chlorothalonil. With the exception of chlorothalonil, these fungicides are usually mixed with petroleum-based spray oils. The oils themselves are fungistatic and retard the development of the pathogen in the infected leaf. When they are mixed in water emulsions with fungicides, the resulting “cocktails” provide superior disease control.
The export plantations in the Philippines and Central and South America that produce fruit for the developed world are vast monocultures of Cavendish cultivars, usually of Grand Nain but also of Williams and Valery. In order to treat these large areas with fungicides, helicopters or fixed wing aircraft are used. Application schedules in the plantations are routinely determined with disease-forecast systems that incorporate data on disease severity within the plantation and environmental factors that are known to affect infection and disease development. These epidemiological tools enabled producers in Central America to substantially reduce the number of fungicide applications that were needed for control. However, increased tolerance in the pathogen to the DMI fungicides has made it necessary to increase applications in several countries in the region to previous frequencies of 25 - 40 per year.
The annual cost of fungicide applications in export plantations is about $1,000 per hectare. Although the international trades can add this expense to the price they charge for fruit, this is not an option for subsistence farmers. Thus, the latter producers must use different strategies to manage black Sigatoka. These include the removal of older leaves to reduce inoculum levels in a plantation, interplanting with other nonsusceptible crops, and planting in partial shade which results in less severe disease development.
The potential for bred bananas
Unfortunately, resistance to black Sigatoka among pre-existing banana genotypes is poor. The Cavendish cultivars that are used for export are so susceptible that nothing short of intensive fungicide application will control the disease in most areas. Resistant cultivars that could be used in subsistence situations are available, but they are usually less productive or desirable than those that are susceptible. This situation has begun to change as a result of new, resistant hybrids that are being developed by the banana breeding programs (http://www.promusa.org ).
The first program to make significant progress in improving this crop was that of the Fundación Hondureña de Investigación Agrícola (FHIA http://www.honduras.com/fhia/espamenu.htm) in La Lima, Honduras. It was begun by the United
In the future, products of the breeding programs will play increasingly important roles in subsistence agriculture. Whether new hybrids are used eventually to replace the Cavendish cultivars that are used by the export trades, however, remains to be seen. The very substantial infrastructure that characterizes export production is focussed on producing only these cultivars. Converting these operations to the production and handling of another type of banana would be an expensive proposition. Moreover, the currently available hybrids do not meet the very high standards for fruit quality and post-harvest shelflife that are demanded by the trades. Yet, as fungicides continue to lose their effectiveness against black Sigatoka, and as the practice of fungicidal disease control becomes more expensive and less appealing to consumers in the importing countries, the trades may eventually be forced into making the difficult transition away from the Cavendish clones.
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