A new study shows how attempting to control banana Xanthomonas wilt (BXW) in Uganda, initially in the early 2000’s without requisite basic scientific information only escalated its spread to regional epidemic levels. Consequently, the negative impact of the disease eliminated the banana industry of East and Central Africa (ECA) worth US$4.3Bn, which is about 6% of the Gross Domestic Product (GDP) of ECA, enough to destabilize livelihoods of about 100 million people. BXW is known only in Africa, and today after more than two decades of pain, scientists show that it can be controlled effectively.
The study, titled “Changing Dynamics in the Spread and Management of Banana Xanthomonas Wilt Disease in Uganda Over Two Decades” by a team of scientists at the Department of Plant Sciences, Microbiology and Biotechnology at Makerere University and National Banana Research program of the NARO (Uganda) demonstrates the central significance of empirical scientific evidence prior to plant disease control.
How BXW begun in Uganda and caught us off-guard
In the year 2001 when a strange disease (later identified as BXW) was for the first time observed on Ugandan local banana in Kayunga district, none of us knew what the disease was at the time. The new disease had symptoms that were somewhat similar to another disease of bananas in central America, named “Moko disease”. The Ugandan scientists working on banana, thought that the disease control methods effective against Moko disease could also be effective in controlling the new disease in bananas in Uganda.
Unfortunately, this was not the case and methods for controlling Moko disease were completely ineffective against BXW in Uganda. Consequently, disease quickly spread to all major banana-growing regions in Uganda between 2001 and 2006. Within only nine (9) years from 2001-2009, the disease had spread to six (6) countries of the East and Central Africa and threatened to eliminate entire banana industry.
According to Prof. Arthur Tugume of the Department of Plant Sciences, Microbiology and Biotechnology, Makerere University, “The initial methods in controlling BXW were quite brave and necessary at the time; however, they were random, in a fire-fighting mode, not based on evidence, and themselves enhanced further spread of the disease.” He adds that, “Also, many of the methods were extremely cumbersome and simply became unsustainable that many abandoned banana farming.”
For example, farmers were required to cut down entire plantation(s) and further dig huge ditches where to burry colossal masses of banana stems, even when only a few plants were observed with BXW symptoms on the farm. The alternative was to burn the banana stems; however, high water-content of banana “pseudostem” made this practically unattainable.
How the picture changed with new scientific evidence
According to Dr. Jerome Kubiriba, the head of Banana research program at Kawanda then, and co-author to the current study, a sigh of relief came when new scientific information started trickling in – thanks to the efforts of scientist in Uganda who worked fast under the leadership of Prof. Wilberforce K. Tushemereirwe.
Dr. Kubiriba says, “First, we soon discovered that the disease was BXW, and was caused by a bacterium called Xanthomonas campestris pv. musacearum (Xcm). Recently this bacterium has been renamed Xanthomonas vascicola pv. musacearum (Xvm). We soon realized that Xcm and BXW had been endemic to Ethiopia on bananas and a cultivated relative of bananas called esent since 1968 and sudden appearance in Uganda was itself a surprise.”
Dr. Kubiriba adds: “Additional scientific work provided evidence that the bacterium has extremely poor survival in soil and rotting banana plant debris; but the bacteria are easily transmitted through contaminated tools used in husbandry. Bees and other pollinating agents that visit the male flowers in search for nectar and use of infected suckers as planting materials all transmit the disease. Our local banana varieties and clones grown in Uganda and elsewhere in the ECA region are all highly susceptible and easily succumb to BXW. Only a wild inedible ancestor of banana, named Musa balbisiana was resistant to BXW”
According to Dr. Kubiriba, “These sets of information and many others were essential because they became the basis of carefully designing specific control strategies against BXW in our local/regional context, quite different from that of Moko disease in central America. For example, there was no longer necessary to burn or burry colossal masses of banana tissues into the ground because it was now known that the bacterium does not survive beyond 35 days in the soil or plant debris.”
Also, transmission of the Xcm bacterium could be easily reduced by sterilizing farm tools with fire or jik or avoiding husbandry practices that involve cutting fresh tissues (except for harvesting) and ensuring disease free suckers for planting. Furthermore, male flowers (which attract pollinating agents) could be removed early using a forked stick to give no chance of insect-mediated transmission of bacteria within and between farms. “Although these did not completely eradicate the disease to zero, they were far more practical, relatively easier and effective than the earlier approaches”, Dr. Kubiriba said.
Bad “Top-down” approaches and weak extension systems
Earlier, an action plan of 2001 aiming at eradicating BXW around the initial outbreak areas was “top-down” in style. In this plan, a hierarchical system of government or management in which actions and policies are initiated at the higher level of government was used. Communities received “orders from above” and were given information via mass media, and threatened with sanctions if they did not act; however, but this was not helpful because Uganda was nearing food crisis. The situation was worsened by extremely weak and disjointed agricultural extension system.
Dr. Kubiriba says “Even though information from government reached more than 85% of the banana farming communities in Uganda, this did not mobilize the communities into action; hence, the impact was too small that by 2005, BXW was still ravaging the crop. Clearly, the disease persisted, not because there was no information but deliberate community actionable tasks were lacking.”
At this point, we realized that instead of the top-down approach, a carefully crafted community-based participatory approach was needed. This involved different stakeholders with banana farmers in the center together with NARO, local government leaders, religious leaders, etc with the aim of developing and implementing an action plan to solve the problem. Somehow, localized judicial systems named “community by-laws” were initiated, designed and administered farming communities themselves to enforce effective BXW control that saw reduction of a disease in Uganda to less than 10% by 2012.
New scientific information provide further control options against BXW
While it became possible to manage the disease, it was still burdensome that in 2010, a farmer has to cut-down all the plants on banana stool, even when only one (1) plant on that single family had BXW. Consequently, scientists discovered that this again is not needed, instead, farmers were encouraged to adopt an approach called “single diseased stem removal” (SDSR) in which only the plant with symptoms is cut with a clean tool at the soil level while leaving all the rest that are disease-free. This method became “gold-standard” in Uganda and other parts of ECA in controlling BXW; however, the famer must police own plantation to ensure timely observation of symptoms immediately they appear.
Prof. Tugume states, “SDSR was based on the possibility that Xcm bacteria could be slow in colonizing many tissues, and given the anatomical differences between aerial and underground banana organs. SDSR could allow farmers to continue harvesting banana bunches from survivor plants as opposed to uprooting everything unnecessarily. By 2013, it was discovered that the Xcm bacterium often faces difficulties infecting the entire plant easily. Consequently, by 2022, it was known that the banana’s large plant body size an advantage to BXW disease control.
What can be done to effectively manage BXW?
To effectively manage and control BXW on a banana farm, one must regularly inspect own farm at least twice per week seeking possible disease symptoms but also ensure to remove male flowers that attract pollinators whether the symptoms have been observed or not. Use of contaminated tools must be avoided completely on the farm. If you must cut anything at all, it is advisable to cut only dry tissues or only the plant with harvested bunch. Immediately symptoms are observed, practice SDSR in less than two-days after this observation. No uprooting of partial or whole corms or application of ash or urine are needed whatsoever.
Dr. Kubiriba says, “While the aforementioned approaches are effective against BXW, it is clear that there must be deliberate efforts by the community and institutions to sustain these gains in BXW control. Sustaining these interventions is not automatic, but is only possible in light of deliberate active systems of continuous monitoring and appraisal by functional agricultural extension systems, outside of which BXW re-emergences cannot be ruled out completely.”
There are more scientific research gaps going forward
According to Prof. Tugume, “The hypothesis that an unknown disease [BXW] could be controlled using methods used against another known disease [Moko disease] was both a technical and scientific error, seasoned by unpreparedness on our side because as a country, we do not invest resources in basic scientific research in advance.”
Therefore, he highlights some of the existing gaps that may help further sustaining the gains made in BXW control. They are as follows:
- Xcm bacterial isolates in Uganda and ECA where BXW is need to be extensively analyzed for their demographic and population biology because this will allow us tell whether they are homogeneous or heterogeneous, and if a threat of re-emergence is an issue needing serious considerations.
- The mechanism by which the plants slow down movement of bacteria to allow us practice SDSR is unknown.
- The link between the stage of plant growth and rate of BXW disease development is not well unknown although the assumption is that disease progresses faster in younger than old plants.
- Is it possible to consider biocontrol methods using antagonistic microbes to control BXW? For example, some soil-borne viruses called “bacteriophages” have been shown to control bacteria that are related to Xcm. One wonders if there might be some bacteriophages in our soils that destroy Xcm. If this was the case, it wouldn’t surprise us because Xcm is already unable to survive beyond 35 days in soil or plant debris. This possibility is worth a scientific study.
- Healthy plants as well as those infected with diseases are a home of a diverse array of microbial communities, known as the “microbiome”. Usually, these communities are too diverse and studying them would help in discovering appropriate members of those communities that promote plant growth and/or resistance or some antagonism to plant disease. However, these communities for the banana-BXW pathosystem are not studied.
- Finally, only the wild inedible Musa balbisiana has resistance against BXW, and due to complex sexual reproductive biology of bananas, this resistance is not easily transferrable to commercial banana varieties. However, using transgenic technologies, we have developed bananas that are resistant to BXW but these are still restricted in confined-field trails in various parts of Uganda. These can be released to farmers only after after appropriate legislative frameworks are in force: this is the part for government of Uganda on whether or not these should be added onto the existing tools against BXW.
This study has been published by the American Phytopathological Society (APS) in an open access “Phytobiomes Journal”, and is freely accessible on:
For more details, contact;
- Prof. Arthur Tugume
College of Natural Sciences
- Dr. Jerome Kubiriba
National Banana Research Program
- Ms. Hasifa Kabejja
Principal Communication Officer
College of Natural Sciences