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Symptoms are first visible as faint, minute, reddish-brown specks on the lower surface of the leaf. Specks elongate, becoming slightly wider, to form a characteristic narrow, reddish-brown streak with dimensions of 20 x 2 mm with the long axis parallel to leaf veins. Streaks frequently overlap to form compound streaks. The colour of streaks, which are now clearly visible on the upper leaf surface, changes to dark brown, almost black. The entire leaf can blacken at this stage if streaks are numerous. If less densely congregated, streaks broaden and become fusiform or elliptical spots. Water-soaked borders appear around spots and surrounding leaf tissue yellows slightly. The centres of spots become slightly depressed and dry out, becoming light grey or buff. Each spot has a well-defined, narrow dark brown or black border and surrounding tissue is often yellow. Whole sections of leaves can become necrotic as spots coalesce. After the leaf has withered, spots remain visible because of their light-coloured centres. Different stages of disease development have been identified (Meredith and Lawrence 1969; Fouré, 1987). Often, all stages of disease development can be seen on one leaf.
If inoculum pressure is high, leaves are rapidly destroyed. Often, fewer than six living leaves may be seen on a susceptible plant that is growing vegetatively. On resistant cultivars, symptoms are only usually seen on the older, lower leaves. The disease is more severe on plants with bunches because new leaves are no longer being produced to replace those lost due to disease. If disease pressure is great, it is not uncommon for a susceptible cultivar to have no viable leaves at harvest.
Removal and destruction of diseased leaves will reduce inoculum levels. If diseased leaves cannot be removed from the plot and burnt, they should be cut from plants and stacked on top of one another. This will prevent ascospores discharging effectively from the lower leaves in the pile. Overhead irrigation encourages disease and under-canopy micro-irrigation is preferable. Plants are also more prone to black leaf streak in sheltered areas where humidity levels are high. Good drainage systems that take surface water rapidly out of plantations can reduce humidity levels.
Genetic resistance to black leaf streak is clearly the best long-term goal for disease control, especially for smallholders who cannot afford to purchase chemicals. Cultivars with high levels of resistance include 'Yangambi Km 5' (AAA), 'Mysore' (AAB), 'Pelipita' (ABB), 'Saba' (ABB) and 'Pisang Awak' (ABB). However, these do not suit all local tastes and some are susceptible to Fusarium wilt (Fusarium oxysporum f.sp. cubense). Conventional banana breeding programmes utilize resistance to black leaf streak found in wild species of Musa, notably M. acuminata subsp. burmanicca, subsp. malaccensis and subsp. siamea, and in diploid cultivars such as 'Paka' (AA) and 'Pisang Lilin' (AA). Although a black leaf streak-resistant Cavendish-type dessert banana useful for the export trades has still to be developed, progress has been made towards breeding banana hybrids for local consumption. Plantain hybrids with good resistance have been bred by the International Institute for Tropical Agriculture (IITA), Nigeria, and by the Fundación Hondureña de Investigación Agricola (FHIA), Honduras. FHIA has also bred resistant dessert bananas hybrids which have fruit of a subacid or apple flavour and resistant robust cooking banana hybrids. Other breeding programmes include the Centre de coopération internationale en recherche agronomique pour le développement (CIRAD) in Guadeloupe and the Centre de recherches sur bananiers et plantains (CARBAP) in Cameroon Biotechnological techniques, such as exploiting somaclonal variation, gamma irradiation and genetic engineering, are also being utilised in attempts to produce banana cultivars resistant to black leaf streak (Jones et al., 2000).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Black leaf streak disease of banana has spread to all major banana-growing regions of the world since it was first recognised in Fiji in 1963. In the Pacific, it has been recorded in American Samoa, Australia (Torres Strait and Cape York Peninsula), the Cook Islands, Fiji, French Polynesia, Hawaii (USA), Micronesia, New Caledonia, Niue, Norfolk Island, Papua New Guinea, the Solomon Islands, Tonga, Vanuatu, Wallis and Fortuna Islands and Samoa. The disease has been found in many countries in Latin America namely Belize, Bolivia, Brazil (Amazonia), Colombia, Costa Rica, Ecuador, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Panama and Venezuela. In the Caribbean region, black leaf streak has been identified in Florida (USA), Cuba, Jamaica, the Dominican Republic and now poses a threat to the export industry based in the Windward Islands. West African countries with the disease are Benin, Cameroon, the Central African Republic, Congo, Côte d'Ivoire, the Democratic Republic of Congo, Gabon, Ghana, Nigeria, S¦o Tomé and Togo. In the East African region, black leaf streak has been found in Burundi, Comoros, Kenya, Madagascar, Malawi, Mayotte, Rwanda, Tanzania (including Zanzibar), Uganda and possibly Zambia. Bhutan, China (Hainan, Guangdong and Yunnan), Indonesia (Halmahera, Java, Kalimantan and Sumatra), Malaysia (Johore, Langkawi Island and Sarawak), the Philippines, Singapore, Taiwan, Thailand and Vietnam have been found with the disease in Asia. However, the situation is more confused in this region because M. fijiensis is not common at all locations. Competition from other leaf spots and the diversity of banana germplasm with varying degrees of disease resistance may account for its erratic distribution (Carlier et al., 2000; Jones, 2003).
Black leaf streak is a major constraint to banana production in most countries where it occurs (Stover, 1983, 1986; Fouré, 1985; Fullerton, 1987; Stover and Simmonds, 1987). After the first occurrence of black leaf streak in an area, the disease usually builds up and often reaches an epidemic level in a few years (Fullerton and Stover, 1990; Belalcazar, 1991). Chemical control costs and crop losses are well documented for industrially produced bananas (Stover, 1986, 1990). Losses to smallholders' crops that are consumed locally are harder to estimate.
Black leaf streak does not kill plants immediately, but crop losses increase gradually with the age of plantings. The decrease in functional leaf area caused by the disease results in a reduction in the quality and quantity of fruit (Stover, 1983; Stover and Simmonds, 1987; Pasberg-Gauhl, 1989; Mobambo et al., 1993, 1996b). Fruit from infected plants ripens prematurely and does not properly fill. Bananas for export are sometimes harvested at a lower grade (younger age) in order to reduce the risks of premature ripening in transit to overseas markets (Stover and Simmonds, 1987).
Until the 1970s, the common leaf diseases of plantain were not considered economically important. This changed when black leaf streak spread to areas where the crop was extensively grown. All over the tropics, plantain is cultivated and fruit consumed by smallholders. In many areas, black leaf streak has caused a considerable decrease in the availability of fruit for local consumption and this has resulted in a substantial increase in their market price. Smallholders growing plantain in the Americas either go out of business, because they cannot cover the high costs of chemical control, or form cooperatives so that their limited resources can be pooled to fight the disease.
Black leaf streak is endangering the food security of resource-poor people. Africa alone contributes about 50% of the world plantain production and the demand for plantain is steadily increasing (Wilson, 1987). All known plantain cultivars (Fouré, 1985; Mobambo et al., 1996a) are susceptible to black leaf streak and are severely defoliated by the disease. Plants in the ratoon crop are weaker than in the first cycle and thus more affected by wind damage. On poor sandy soils in West Africa, Mobambo et al. (1996b) estimated that yield losses due to black leaf streak are 33% and 76% during the first and second cropping cycle respectively. However, in intensively cropped backyard or home garden systems, cultivation is not so seriously affected (Mobambo et al., 1994). Under marginal conditions, plantain production is often abandoned due to low yields.
Plantain is not the only smallholder banana to be affected in Africa. The disease also causes serious damage to East African highland cultivars in the Lujugira-Mutika subgroup (AAA). In Uganda, Tushemereirwe (1996) reported yield losses of 37% due to the effects of a leaf spot complex consisting mainly of black leaf streak and Cladosporium leaf speckle.
Black leaf streak has had a devastating effect on the production of export bananas in the South Pacific. Firman (1972) noted that only 49% of unsprayed Cavendish cultivars produced fruit that reached the export quality standard. Fiji ceased exporting bananas in 1974 and Samoa in 1984. Exports also dropped in Tonga and the Cook Islands, because producers had problems maintaining fruit quality standards for their markets in New Zealand. Black leaf streak control has become the single largest production cost (Fullerton, 1987).
In 1974, the production of dessert bananas and plantains in Central America was seriously affected by hurricane Fifi, which was also thought to be responsible for the wind borne spread of black leaf streak to new areas. In many countries, production subsequently dropped substantially. Before 1974, Honduras exported 500,000 boxes of plantain each year, but afterwards exports dropped to below 1,000 boxes (Stover, 1983). In 1978, the export of plantain from Honduras to the USA was curtailed because of the shortage of fruit with the required quality (Bustamente, 1983). Plantain exports only resumed in 1985, when black leaf streak was controlled by the aerial application of fungicides (Stover, 1987).
After the identification of black leaf streak in Costa Rica in 1979, the government initiated a quarantine programme (Woods, 1980). This programme consisted of the destruction of host plants in the affected area and the establishment of roadside quarantine stations strategically located to stop movements of banana and plantain leaves which were used for padding and shading fruit. About 3,000 hectares of plantain were destroyed in 1979 and early 1980. The Costa Rican government paid about US$ 3 million for the eradication program, but the spread of the disease could not be stopped (Woods, 1980). By 1982, the Ministry of Agriculture in Costa Rica estimated that black leaf streak alone reduced plantain production by 40% (Romero, 1986). In general, yields of plantains in well maintained fields on rich fertile soils in Central America may have fallen by 20-50% (Stover, 1983a; Pasberg-Gauhl, 1989).
Black leaf streak was first detected in Panama in 1980. Bureau (1990) estimated that plantain production in Panama decreased by 69% between 1979 (100,910 t) and 1984 (31,134 t). During this period, the price of plantains rose by up to 50% in local markets. Jaramillo (1987) reported that between 1982 and 1985, the area planted with plantain decreased by 22% from 7432 ha to 5800 ha. About 34% of growers were believed to have abandoned their holdings leading to a decrease in production of 47%.
Colombia is one of the largest plantain producers in Latin America with 400,000 ha under cultivation and an estimated yearly production of 2.5 million t. About 96% of plantains are consumed locally, the remainder being exported (Belalcazar, 1991). Smallholders grow about 88% of the plantain in association with coffee. Only 12% of the crop is grown in monoculture in larger plantations. After the introduction of black leaf streak, this staple food became scarce and much higher prices were demanded in the market (Belalcazar, 1991). Due to the high cost of plantain, consumers changed to other, cheaper food crops. This in turn had a negative effect on plantain production. Black leaf streak has thus had a significant impact on Colombian agriculture and the eating habits of a nation.
Black leaf streak can be chemically controlled on plantations, but the cost is substantial. Up to 36 spray cycles per year may be required for plantations growing dessert bananas for export and up to 19 cycles for commercial plantings of plantain (Fouré, 1983, 1988a, b; Stover, 1980b, 1990; Belalcazar, 1991; Gauhl, 1994; Romero and Sutton, 1997b). Stover and Simmonds (1987) reported that 27% of production costs in dessert banana plantations was spent controlling black leaf streak. From 1972 until 1985, the estimated cost of black leaf streak control in Central America, Colombia and Mexico was more than US$ 350 million (Stover and Simmonds, 1987). The cost of chemical control measures has been calculated to be US$ 400-1,400 per ha per year (Anon., 1993). During the 1980s, the cost of black leaf streak control in export banana crops in Costa Rica was estimated at approximately US$ 17.5 million per year (Stover and Simmonds, 1987). Between 1985 and 1994, the area under banana cultivation increased from approximately 21,000 ha to 52,737 ha (Serrano and Marín, 1998). As a consequence, the cost of black leaf streak control in 1995 was estimated to have increased to US$ 49 million per year (Romero and Sutton, 1997b). Recent information from Venezuela indicates that the cost of black leaf streak control in plantain farms south of Lake Maracaibo amounts to almost 50% of production costs. However, even with control measures being taken, yields were still 25% below those before the appearance of the disease in that country (Zabala and Bermudez, 1999).