September 18, 2017 -- GAINESVILLE -- The banana crop is an important commodity in Tanzania and much of East Africa, with many in the region eating up to 400 kilograms of bananas per person per year. Plant diseases are a major threat to the sustainability of the crop, and over the past decade, a bacterial infection once found only in Ethiopia has risen in prominence in all countries around Lake Victoria, including Tanzania.
The disease, called Banana Xanthomonas Wilt, or BXW, is caused by the Xanthomonas campestris pv. musacearum bacterium, which induces wilting in banana plants and renders banana fruits unpalatable. Since government officials first identified BXW in Tanzania’s Kagera region in 2006, the pathogen has spread all over Kagera, threatening the livelihoods of subsistence farmers throughout the countryside in the nation’s largest banana-producing region. Plant pathologists at the Emerging Pathogens Institute have studied the disease and the factors that encourage transmission, and have identified methods to mitigate its spread.
“Other than wilting and necrosis of leaves, one of the main symptoms of Banana Xanthomonas Wilt is a premature and uneven ripening of the fruits,” said Dr. Mpoki Shimwela, a former graduate student in the department of plant pathology and an associate member of UF’s Emerging Pathogens Institute. Shimwela lives in the Kagera region, where he is a senior research scientist and the national lead scientist on banana research.
“Yellow ooze secrets from a cut on any part of an infected plant,” Shimwela said. This ooze contains many millions of the Xanthomonas bacteria per droplet. Eventually, the pathogen kills the infected plant.
In order to contain the spread of disease, the government mandated that farmers cut down infected banana plants. Infection rates continued to rise, however, especially in the rainy season. Thus, the results of the eradication approach are less than optimal.
“There was a reduction in the number of infected banana plants right after cutting the plants down, but during the subsequent years the number of infected trees expanded again”, said Dr. Ariena van Bruggen, a professor of plant pathology and a member of the Emerging Pathogens Institute.
Using geographic information systems and mathematical modelling software, Shimwela and van Bruggen were able to determine where in Kagera BXW was most concentrated, along with the speed and route of transmission. Findings published in the journal “Plant Pathology” indicate that rainfall patterns were integral to the spread of the disease.
Van Bruggen suggested that some of the government’s management strategies actually contributed to the spread of the disease during the rainy season.
The banana plant, often called a “tree,” can grow up to 30 feet tall and its mat extends underground. Because the government issues up to $10 fines for leaving infected plants in the field – a steep fee in a region where the average person earns less than a dollar a day – people cut down as many infected trees as they can find. They often leave the infected mat and stalk in the ground, and when it rains, rainwater can carry the bacteria to other plants.
Also, when farmers cut down infected plants, the bacteria continue to live on their tools and the infection can spread to other plants through contaminated tools.
The banana leaves are thought to play a role in the long-distance spread of the disease, since people use them to cover banana bunches that are transported to other parts of the region.
This map highlights the areas of Tanzania's Kagera region affected by the Xanthomonas pathogen.
Given the increased threat of transmission during rain, both van Bruggen and Shimwela recommend not cutting down plants during the rainy season. According to van Bruggen, threat of transmission is actually minimal while the plants are intact. Van Bruggen also suggested that packing the stumps of cut-down banana plants with dirt could act as a deterrent to spread during the rain. Farmers in Kagera oftentimes lack the resources to combat the spread of BXW.
“Some of these farmers don’t have money for bleach, which is recommended to sterilize field tools,” Van Bruggen said. She recommends farmerscreate baking ovens in the field to help them sterilize their machetes and other tools, thus reducing the spread of the disease.