Community phytosanitation to manage cassava brown streak disease

Article purchased; Published online: 6 May 2017Cassava viruses are the major biotic constraint to cassava production in Africa. Community-wide action to manage them has not been attempted since a successful cassava mosaic disease control programme in the 1930s/40 s in Uganda. A pilot initiative to investigate the effectiveness of community phytosanitation for managing cassava brown streak disease (CBSD) was implemented from 2013 to 2016 in two communities in coastal (Mkuranga) and north-western (Chato) Tanzania. CBSD incidence in local varieties at the outset was >90%, which was typical of severely affected regions of Tanzania. Following sensitization and monitoring by locally-recruited taskforces, there was effective community-wide compliance with the initial requirement to replace local CBSD-infected material with newly-introduced disease-free planting material of improved varieties. The transition was also supported by the free provision of additional seed sources, including maize, sweet potato, beans and cowpeas. Progress of the initiative was followed in randomly-selected monitoring fields in each of the two locations. Community phytosanitation in both target areas produced an area-wide reduction in CBSD incidence, which was sustained over the duration of the programme. In Chato, maximum CBSD incidence was 39.1% in the third season, in comparison with an incidence of >60% after a single season in a control community where disease-free planting material was introduced in the absence of community phytosanitation. Kriging and geospatial analysis demonstrated that inoculum pressure, which was a function of vector abundance and the number of CBSD-infected plants surrounding monitored fields, was a strong determinant of the pattern of CBSD development in monitored fields. In the first year, farmers achieved yield increases with the new varieties relative to the local variety baseline of 94% in Chato (north-west) and 124% in Mkuranga (coast). Yield benefits of the new material were retained up to the final season in each location. The new variety (Mkombozi) introduced under community phytosanitation conditions in Chato yielded 86% more than the same variety from the same source planted in the no-phytosanitation control location. Although there was an 81% reduction in CBSD incidence in the new variety Kiroba introduced under community phytosanitation compared to control conditions in Mkuranga, there was no concomitant yield increase. Variety Kiroba is known to be tolerant to the effects of CBSD, and tuberous roots of infected plants are frequently asymptomatic. Community phytosanitation has the potential to deliver area-wide and sustained reductions in the incidence of CBSD, which also provide significant productivity gains for growers, particularly where introduced varieties do not have high levels of resistant/tolerance to CBSD. The approach should therefore be considered as a potential component for integrated cassava virus management programmes, particularly where new cassava plantations are being established in areas severely affected by CBSD

Adverse impact of banana Xanthomonas Wilt on farmers’ livelihoods in Eastern and Central Africa

Banana is a key crop in the livelihoods of many people in the Great Lakes region of East and Central Africa. For more than a decade now, the crop has been threatened by Banana Xanthomonas Wilt (BXW) which has spread throughout the region but at different rates. The disease attacks all banana cultivars and can cause up to 100% yield losses at farm level if effective control measures are not put in place. However, limited information on impact of BXW at regional level is available to guide interventions. Thus, this study assessed the impact of BXW on farmers’ livelihoods in Kagera basin of Tanzania, Burundi and Rwanda. A total of 436 households (Tanzania 120, Burundi 208 and Rwanda 108) mostly from major banana-producing and BXW-affected districts were sampled and interviewed in a household survey. Thirty-three to seventy-five of the total banana mats per farm in the three countries were infected with BXW. Banana production losses caused by BXW were valued at US$10.2 million and US$ 2.95 million in Tanzania and Rwanda, respectively, banana sales by farmers dropped by 35% while bunch prices unpredictably doubled. Since banana is a key component of these farming communities, the banana production losses resulted in significant reduction in household food security and incomes. To cope with these challenges, most households are diversifying into other food crops such as maize, cassava and sweet potatoes. This poses a number of socio-economic and biological implications that require further investigation

The dynamics and environmental influence on interactions between cassava brown streak disease and the whitefly, Bemisia tabaci

Cassava brown streak disease (CBSD) is currently the most significant virus disease phenomenon affecting African agriculture. In this study, we report results from the most extensive set of field data so far presented for CBSD in Africa. From assessments of 515 farmers’ plantings of cassava, incidence in the Coastal Zone of Tanzania (46.5% of plants; 87% of fields affected) was higher than in the Lake Zone (22%; 34%), but incidences for both zones were greater than previous published records. The whitefly vector, Bemisia tabaci, was more abundant in the Lake Zone than the Coastal Zone, the reverse of the situation reported previously, and increased B. tabaci abundance is driving CBSD spread in the Lake Zone. The altitudinal “ceiling” previously thought to restrict the occurrence of CBSD to regions <1,000 masl has been broken as a consequence of the greatly increased abundance of B. tabaci in mid-altitude areas. Among environmental variables analyzed, minimum temperature was the strongest determinant of CBSD incidence. B. tabaci in the Coastal and Lake Zones responded differently to environmental variables examined, highlighting the biological differences between B. tabaci genotypes occurring in these regions and the superior adaptation of B. tabaci in the Great Lakes region both to cassava and low temperature conditions. Regression analyses using multi-country data sets could be used to determine the potential environmental limits of CBSD. Approaches such as this offer potential for use in the development of predictive models for CBSD, which could strengthen country- and continent-level CBSD pandemic mitigation strategies

Comparing the regional epidemiology of the cassava mosaic and cassava brown streak virus pandemics in Africa

The rapid geographical expansion of the cassava mosaic disease (CMD) pandemic, caused by cassava mosaic geminiviruses, has devastated cassava crops in twelve countries of East and Central Africa and is well documented. Region-level epidemiological studies continue to reveal a more-or-less regular pattern of annual spread along a contiguous ‘front’. More recently, outbreaks of cassava brown streak disease (CBSD) were reported from Uganda and other parts of East Africa that had been hitherto unaffected. Recent survey data reveal several significant contrasts between the regional epidemiology of the two pandemics: i) severe CMD spreads in an ‘expanding concentric rings’-like manner, whilst CBSD seems to be spreading from independent hot-spots; ii) the severe CMD pandemic has arisen from virus recombination and inter-species synergy, whilst current knowledge suggests that the CBSD pandemic is a ‘new encounter’ situation; iii) CMD pandemic spread has been tightly linked with the appearance of super-abundant B. tabaci whitefly populations, in contrast to CBSD, where outbreaks have occurred 3-12 years after whitefly population increases; iv) the cassava mosaic geminiviruses (CMGs) causing CMD are transmitted in a persistent manner, whilst cassava brown streak viruses (CBSVs) appear to be semi-persistently transmitted; and v) different patterns of symptom expression mean that phytosanitary measures are easy to implement for CMD but have limited effectiveness whilst the same measures are hard to do for CBSD but potentially very effective. One important similarity between these two cassava virus pandemics is that the viruses occurring in pandemic-affected areas are also found elsewhere, indicating that contrary to earlier published information, the viruses per se are unlikely to be the key factors driving the two pandemics. A model is proposed for the temporal relationship between B. tabaci abundance and changing incidences of both CMD and CBSD in the Great Lakes region. This emphasizes the pivotal role played by the vector in both pandemics and highlights the urgent need to identify effective and sustainable strategies for controlling whiteflies on cassava

Spatiotemporal patterns of genetic change amongst populations of cassava Bemisia tabaci whiteflies driving virus pandemics in East and Central Africa

The greatest current threat to cassava in sub-Saharan Africa, is the continued expansion of plant virus pandemics being driven by super-abundant populations of the whitefly vector, Bemisia tabaci. To track the association of putatively genetically distinct populations of B. tabaci with pandemics of cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), a comprehensive region-wide analysis examined the phylogenetic relationships and population genetics of 642 B. tabaci adults sampled from cassava in six countries of East and Central Africa, between 1997 and 2010, using a mitochondrial DNA cytochrome oxidase I marker (780 bases). Eight phylogenetically distinct groups were identified, including one, designated herein as ‘East Africa 1’ (EA1), not previously described. The three most frequently occurring groups comprised >95% of all samples. Among these, the Sub-Saharan Africa 2 (SSA2) group diverged by c. 8% from two SSA1 sub-groups (SSA1-SG1 and SSA1-SG2), which themselves were 1.9% divergent. During the 14-year study period, the group associated with the CMD pandemic expansion shifted from SSA2 to SSA1-SG1. Population genetics analyses of SSA1, using Tajima's D, Fu's Fs and Rojas’ R2 statistics confirmed a temporal transition in SSA1 populations from neutrally evolving at the outset, to rapidly expanding from 2000 to 2003, then back to populations more at equilibrium after 2004. Based on available evidence, hybrid introgression appears to be the most parsimonious explanation for the switch from SSA2 to SSA1-SG1 in whitefly populations driving cassava virus pandemics in East and Central Africa

Successful innovations and lessons learnt in cassava improvement and deployment by IITA in Eastern African Region

The International Institute of Tropical of Agriculture (IITA) established its East and Southern Africa Regional Research Centre (ESARC) in Uganda at the former Namulonge Agricultural and Animal Research Institute (NAARI), presently Namulonge Animal and Crops Research Institute (NaCRRI), to address issues of cassava, banana, and plantain development; coordinate all related activities, and work closely with the national agricultural research institutes (NARS). IITA-ESARC began extensive cassava germplasm development to counter the pandemic of African cassava mosaic disease (ACMD) in the region in 1995 through the Eastern Africa Root Crops Research Network (EARRNET). More than 100,000 seeds were evaluated through the conventional plant breeding scheme. Selected genotypes were kept in in-situ conservation from where the regional cassava national programs selected clones for further evaluation in their own countries. Burundi, Democratic Republic of Congo (DRC), Kenya, Rwanda, Tanzania, and Uganda benefited immensely. Through EARRNET, the region gained significantly from the large germplasm base to mitigate the scourge of ACMD and the production of cassava was restored. A new joint effort that was established between Catholic Relief Services and IITA in collaboration with the NARS and other stakeholders promoted the adoption of improved germplasm through participatory variety selection. The breeding approach used enabled to reduce selection period for NARS to release new varieties to farmers as they received elite materials for evaluation. However, the spread of cassava brown streak disease (CBSD) in mid altitude threatened the achievements already made as the new disease attacked most of the ACMD-resistant and high yielding varieties. Its spread in the region calls for more effective collaborative action than before from IITA and its partners to develop new resistant materials to mitigate the effects of both ACMD and CBSD. The present paper attempts to summarize the breeding work efforts made and demonstrate how the germplasm development at this regional center has been useful to the region through effective partnership