By Walter Ocimati
Photo 1: A farmer washing with soap a machete previously used on a Xanthomonas wilt infected banana plant
Like Covid-19, the bacteria that causes Xanthomonas wilt (aka BXW) in bananas can spread quickly through a population of susceptible plants, albeit with a more devastating impact since not only will the infected plants succumb to the disease, all the cultivars that have been exposed to the bacteria so far turned out to be susceptible. But just as several countries managed from the start to protect their populations from the worst effects of the virus by introducing measures to break its cycle of transmission, banana farmers can do the same for their banana plants by applying preventive cultural practices. Moreover, even if worse comes to worst, infested fields can be rehabilitated to grow bananas again in a few months.
In both cases, the degree of disease control achieved depends on people adopting the measures recommended by health authorities or extension services. In the case of BXW, one of the main modes of transmission is when cutting tools that have come into contact with the bacterial ooze produced by infected plants are used on healthy plants. Hence the recommendation, early in the BXW epidemic, to disinfect cutting tools by cleaning the blade with household bleach or putting it in a fire for 20 to 30 seconds. While these methods are effective, most farmers cannot afford household bleach, which is also not readily available in remote rural areas. Farmers also reported that the flame and the heat damage the blade. Moreover, farmers are reluctant to start a fire during the dry season, whereas fires are hard to start during the wet season. Their best access to fire is when the banana field is near the house, where a fire already on the way can often be taken advantage of.
To help farmers adopt the habit of disinfecting tools, we looked for alternatives. Among the options we tested, we found that the one that was as effective as the recommended practices had been close all along, even more so since the beginning of the Covid-19 pandemic: washing with soap.
We tested the effectiveness of these alternative methods by first disinfecting the blade of the cutting tools and then smearing them with a uniform quantity of bacterial ooze. We waited 30 minutes for the ooze to set on the blades before applying the sterilization methods. We then recovered the bacteria left on the blades and estimated their number.
Applying soap to a sponge and scrubbing the blade with it, or using powder detergent, were as effective as household bleach and fire in eliminating bacteria. Soap and detergents act by dissolving the bacterial ooze and sap from the plant, allowing for the bacteria to be easily washed off. Both are cheaper than household bleach and readily available, even in the most remote communities. However, bars of soap are more commonly found in farming households than detergents since they are cheaper and used for everything including cleaning utensils and clothes.
Photo 2: A) A plant showing yellow leaf symptoms, B) a farming cutting a diseased plant; C) bacterial ooze on the surface of a cut banana stem and D) discoloured pulp of infected banana fingers (Credit: Bioversity International/A. F. Tazuba)
Immersing tools in boiling water for about a minute was also as effective as household bleach. It could be an option when the banana field is close to the house or other methods are not feasible. We also found that putting the blade in a fire only eliminated the bacteria after a one-minute exposure, suggesting that the previous recommended time frame of 20 to 30 seconds did not completely sterilize the blade.
The other methods tested, like inserting tools into hot and cold wood ash, forcefully and repeatedly inserting the tools into the soil, exposing tools to direct sun, washing tools with cold water only, were only marginally better than the “do nothing treatment”.
The worst option was leaving the tools on a dry surface for 7 days. The bacterial population was as high as in the “do nothing treatment”, meaning that if a farmer does nothing at all, contaminated cutting tools can spread the disease for at least 7 days.
We hope that the incorporation and promotion of the use of soap or detergents and of boiling water for sterilizing contaminated cutting tools will increase farmers’ flexibility and reduce the cost of the practice, ultimately improving the control of BXW on farms.
For more on how soap and water or boiling water can clean/sterilize farming tools read here: https://www.frontiersin.org/articles/10.3389/fagro.2021.655824/full
This blog was written in collaboration with Guy Blomme and Anthony F. Tazuba
Suggested reading
Blomme, G. and Ocimati, W. (2018) Xanthomonas bacterial wilt. In: Chapter 5: Diseases caused by bacteria and phytoplasmas. Handbook of diseases of banana, abacá and enset. (Jones, D.R. (ed.)). Wallingford (UK) CAB International. pp. 296-313. ISBN: 9781780647197
Blomme, G., Dita, M., Jacobsen, K.S., Pérez Vicente, L., Molina, A., Ocimati, W., … and Prior, P. (2017). Bacterial diseases of bananas and enset: current state of knowledge and integrated approaches toward sustainable management. Front. Plant Sci., 8, 1290. https://doi.org/10.3389/fpls.2017.01290
Blomme, G., Dusingizimana, P., Ntamwira, J., Kearsley, E., Gaidashova, S., Rietveld, A., … and Ocimati, W. (2021). Comparing effectiveness, cost-and time-efficiency of control options for Xanthomonas wilt of banana under Rwandan agro-ecological conditions. European Journal of Plant Pathology, 1-15. https://doi.org/10.1007/s10658-021-02258-z
Blomme, G., Ocimati, W., Sivirihauma, C., Lusenge, V., Bumba, M., and Ntamwira, J. (2019). Controlling Xanthomonas wilt of banana: Influence of collective application, frequency of application, and social factors on the effectiveness of the Single Diseased Stem Removal technique in eastern Democratic Republic of Congo. Crop Prot. 118, 79–88. doi: 10.1016/j.cropro.2018.12.015
Kikulwe, E. M., Kyanjo, J. L., Kato, E., Ssali, R. T., Erima, R., Mpiira, S., … and Karamura, E. (2019). Management of Banana Xanthomonas Wilt: Evidence from Impact of Adoption of Cultural Control Practices in Uganda. Sustainability, 11(9), 2610. DOI: 10.3390/su11092610
Ocimati, W., van Schagen, B., Kozicka, M., Kikulwe, E., Gotor, E., & Blomme, G. (2019). Banishing banana wilt: Can it get any easier?. https://hdl.handle.net/10568/101557