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June 17, 2009

Banana blight puts livelihoods at risk

The bacterial banana Xanthomonas wilt (BXW) disease will endanger the livelihoods of millions of East African farmers if left uncontrolled, according to specialists. First reported about 40 years ago in Ethiopia, BXW is endemic in most of Uganda, and has been reported in Burundi, the Democratic Republic of the Congo (DRC), Kenya and Rwanda.

“BXW is the most serious threat to banana production in East Africa,” said Wafa Khoury, a plant pathologist and agricultural officer in the Plant Production and Protection Division of the UN Food and Agriculture Organization (FAO). “BXW is very serious because it could wipe out all cultivars planted in the continent, with almost no resistance detected...

"The plants wilt and eventually die. They either do not produce fruits, and when they do, they are hard and inedible to either humans or animals and they cannot be processed even," Khoury said.

Bananas and plantains are the world’s fourth most important food crop after rice, wheat, and maize.

BXW symptoms include premature ripening of fruits, pale yellow ooze from cut surfaces, wilting of bracts and male buds, and progressive yellowing leading to complete wilting. Plants generally show symptoms within three weeks of infection.

Fields infested with the bacteria cannot be replanted with banana for at least six months due to carry-over of soil-borne inoculum, according to a report titled Xanthomonas wilt, a threat to banana production in East and Central Africa, by the
International Institute of Tropical Agriculture (IITA).

Once BXW occurs in a field, there is no remedy other than to cut down all infected plants, completely dig out the rhizomes, and place the field under at least a six-month fallow period or a prolonged crop-rotation regime.

"... If uncontrolled, BXW would spread at a rate of 8 percent per annum in cooking banana plantations, causing an estimated production loss of about 53 percent over a 10-year period," stated the report.

BXW has devastated plantations in the central, western and southern regions of Uganda, said Wilberforce Tushemereirwe, head of banana research at Uganda’s National Agriculture Research Organization (NARO).

At least 50 percent of plantations in the affected districts have been wiped out, threatening the food security of up to 14 million people, according to NARO. Uganda is the world’s second-largest banana producer after India.

In 2005, Uganda produced 650,000 metric tonnes of bananas; however, output is estimated to have dropped to about 400,000MT in 2008, according to the agriculture ministry. East Africa is the largest banana-producing and consuming region in Africa.

NARO predicts land under banana cultivation could drop another 20 percent in 2010 without adequate funding for disease control.

The potential economic impact of BXW is high. "Based on estimates of the Ugandan government, BXW caused yield losses of up to US$75 million in 2006 with a projected overall economic loss of $2-$8 billion in the next 10 years," said FAO's Khoury. However, no other impact studies have been done.

“Luckily, and despite the fact that the disease can spread like fire... BXW could be contained using proper field management practices,” he said.

These include removing male buds, which are the entryway for new infections, destroying infected plants, using clean field tools and planting material, as well as not using banana remains from unknown sources as mulches.

“Wherever these simple management recommendations have been practised by farmers in a consistent way, the disease has been completely eliminated from their fields.”

Local government authorities have passed by-laws including fines and penalties for farmers who do not heed similar directives in Uganda.

In Rwanda, BXW is devastating plantations, especially in the north. According to Rwanda’s institute of statistics, banana production has maintained an average growth of around 5 percent in the last three years, but this may be affected by the disease.

Kagera region, northwest Tanzania, is among areas affected, said Khadija Rajab, research coordinator and deputy head of the plant protection division in Zanzibar's Ministry of Agriculture.

"Unlike black sigatoka, which affects mostly the Cavendish and dwarf Cavendish varieties, and Panama disease, which affects the Bluggoe and Silk varieties, Xanthomonas can affect all the banana varieties in the region."

Leena Tripathi, a biotechnologist with IITA, said that in the short term, decapitating male buds and cutting and burying infected banana plants, are recommended.

"But most of the farmers are reluctant to apply these as they are labour-intensive," Tripathi said. "Farmers prefer resistant varieties. So developing resistant varieties is [the] long-term solution," she said.

However, conventional breeding of bananas is a difficult and lengthy process due to the sterility of most cultivars, coupled with long generation times. "To circumvent these difficulties, transgenic technologies may provide a cost-effective alternative solution to the BXW pandemic," she said.

In 2008, Uganda announced plans to start testing wilt-resistant genetically modified bananas, using a protein gene from sweet pepper to counter the disease.

According to NARO, preliminary laboratory tests have indicated that transgenic banana plants appear to be resistant to the wilt but these efforts have been hampered by inadequate funding.

Most of the bananas grown in the world are consumed locally with less than 10 percent sold commercially, making the risk of reduced exports because of anti-GM policies low, say researchers

In Burundi, Rwanda and Uganda, bananas constitute more than 30 percent of the daily per capita calorie intake.

Meanwhile, according to FAO's Khoury, BXW management through field practice remains important. "Of course, resistant varieties are the easiest, quickest and most economical control methods for almost all diseases, including BXW. The reliance on resistant varieties however, tends to lead to relaxation of the field management practices.

"How safe are the transgenic bananas is a question that is difficult to answer, as in all cases of genetically modified crops."

He noted that the development of a resistant variety or transgenic type and its widespread adoption would take several years. "In the meantime, BXW could wipe out all bananas," he said.

In the coming months, FAO will be undertaking an analysis of the present, and potential, risk to banana production from BXW and Banana Bunchy Top Virus in sub-Saharan Africa.


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