Sustainability of banana-based agroecosystems affected by Xanthomonas wilt disease of banana

Walter Ocimati

Research output: Thesisinternal PhD, WU

Abstract

Banana (plantains inclusive) in addition to being an important food and income security for over 70 million people across the African Great Lakes region (AGLR),  is an important source of several regulatory and supporting services that to date have receive little attention. The recent outbreak of Xanthomonas wilt disease (XW) of banana (first reported in 2001) has compromised food and income security of the households and communities in the banana-based agroecosystems. XW effects on other ecosystem services are not well known and could affect the sustainability of these agroecosystems. XW management has been mainly reactive and mitigative and the disease has as such persisted and continues to spread. Understanding of the risk factors at landscape and field/ farm level are thus crucial. This study explored strategies for preventing and minimising shocks caused by XW disease outbreaks and improving the adaptive and/or buffering capacity of banana-based agroecosystems. To achieve this, the study i) determined retrospectively the XW-driven land-use changes and trajectories across landscapes in eastern DR Congo, ii) potential effects XW and land-use changes on the supply of key ecosystem services, and iii) developed XW risk maps for the AGLR and banana producing zones across Africa. At farm/ field level, iv) field level risk factors including the role of intercrops and weeds in harbouring and perpetuating XW, and v) the effect of banana leaf pruning to integrate intercrops on the efficiency of the system as a basis for discouraging and/or improving the intercrop management were determined. Finally, vi) the study explored the opportunity of integrating shade and drought tolerant species within heavily shaded banana fields so as to increase biomass production without profoundly affecting performance of the banana crop.

To retrospectively characterise land-use changes/ trajectories due to XW and to determine XW effects on ecosystem services, focus group discussions, a four-cell analysis, diagnosis of farmer fields and a review of available literature were conducted. The dominance of the banana declined across all XW affected landscapes with increases in importance of other crops, mainly annuals. Crop diversity increased at household level but not at field level. XW was observed to also reduce the supply of supporting and regulatory ecosystem services. These findings suggest a need for an ecosystem services broad framework for addressing XW and other diseases of banana and other crops. 

At landscape level, XW incidence and a range of covariates were used to develop spatial spread maps of XW disease. XW increased with increasing precipitation and declining investment in disease and crop management. The spatial XW spread map highlights XW hotspots, front lines (e.g. eastern DR Congo) and the vulnerable landscapes with low (e.g. north-western Tanzania) or no XW (northern Mozambique). The eastern DR Congo, a zone where the plantains (Musa AAB) and the East African highland bananas (Musa AAA) meet was a major hotspot and is thus a potential gateway for XW to spread into the plantain belt of Central and West Africa. These maps are a good starting point for guiding proactive strategies for XW prevention, eradication and control.

To elucidate the field/ farm level risk factors of XW a combination of laboratory and screenhouse experiments and a farm diagnostic study were used. Xanthomonas campestris pv. musacearum the causal organism of XW was observed to survive within some banana intercrops (e.g. maize, millet, sorghum, sugarcane) and weeds (Canna spp. and wild sorghum), with XW characteristic symptoms on millet, sorghum and Canna spp. Insect spread while collecting nectar, sap and pollen, plant to plant spread through the rhizome are important for XW perpetuation whereas the pathogen does not survive for long in absence of a living host. Thus the risk from the annual crops was rated zero to low, from sugarcane (perennial and propagates through the stem and rhizome) as low-moderate and from Canna spp. (propagates through the rhizome, highly susceptible and prevalent on farms) as moderate to high. Field diagnostic studies only showed a 0.02% disease incidence in Canna spp. XW risk on farm was instead lowered by growing banana cultivar mixtures and increasing access to information on disease epidemiology and management; and increased by the presence of the highly susceptible ABB banana types. Though Canna spp. has no association with the observed XW risk on farm it warrants close monitoring. The impact of cultivar mixtures on XW is novel and needs to be further investigated and integrated into the current XW management package.

At field level, farmers prune banana leaves to increase light intercepted by the shorter crops. This practice currently enhances XW spread. Field experiments and a bio-economic optimisation model (FarmDESIGN) were used to explore the impact of these practices on the performance of the system. The model was also used to explore different external input (hedges, inorganic fertilizers and organic manure) scenarios for the system. Severe leaf pruning (to 4 leaves) was inefficient both agronomically and economically. Mild pruning (7 leaves) however improved the agronomic efficiency whereas unpruned banana (as intercrop or sole crop) was more efficient economically. Moderate leaf pruning to integrate legumes and not pruning at all could thus be considered as a trade-off between agronomic and economic efficiency for the banana-legume intercrop system. Trade-offs occurred between nitrogen input (minimized) and operating profit, soil organic matter balance and protein yield that were maximised. Profound improvements for all the above objectives only occurred with addition of external inputs, especially inorganic fertilizers and manure. For example, for the no or low input scenario(s), the model predominantly allocated land to severely pruned banana-legume intercrop and bush bean monocrop. In contrast, with addition of external inputs, the model allocated land to the more profitable options i.e. unpruned banana either as a sole crop or intercrop with legumes. Discouraging severe leaf pruning and improving the performance of the system for all objectives will necessitate investments in external inputs. These findings are a good basis for co-innovating the system for better performance. The FarmDESIGN model is thus useful for supporting field level explorations, decision making and overall co-designing of the banana agroecosystems.

Available literature was also reviewed to identify potential shade and drought tolerant species for integration into heavily shaded banana fields. Integration of such species within the banana fields will increase overall biomass yield, minimise XW spread through leaf pruning and improve the overall performance of the system.

This study thus shows XW to still be an important threat across landscapes in the AGLR and to both indirectly and directly affect the supply of a broad range of ecosystem services. Adopting proactive and holistic measures taking to account the broader range of ecosystems services is crucial at farm and landscape level for managing the disease. At field level, understanding the risk factors including crop management practices, and the role of other plant species in a system is important. This study also proposes agroecological practices (e.g. such as cover crops, hedges, crop cultivar mixtures) for improving the buffering and adaptive capacity of the banana-based agroecosystems.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Tittonell, P., Promotor
  • Groot, Jeroen, Co-promotor
Award date11 Dec 2019
Place of PublicationWageningen
Publisher
Print ISBNs9789463952026
DOIs
Publication statusPublished - 2019

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