Assessing the degeneration of cassava under high-virus inoculum conditions in coastal Tanzania

Open Access Article; Published online: 19 Jul 2019Cassava brown streak disease (CBSD), caused by cassava brown streak ipomoviruses (CBSIs), has become the most debilitating biotic stress to cassava production in East and Central Africa. Lack of CBSD-resistant varieties has necessitated the search for alternative control measures. Most smallholder farmers reuse stems from previous crops for planting in the new season. Recycling planting material in this way can lead to “degeneration” owing to the compounding effects of disease. In this study, degeneration was defined as the increase in CBSD incidence and reduction in marketable root yield over time. An experiment was established to study the rates of degeneration in selected cassava varieties Chereko, KBH2002_135, Kipusa, Kizimbani, and Mkuranga1 and cultivars Kiroba and Kikombe under high-CBSD inoculum conditions in Bagamoyo, Tanzania from 2013 to 2017. The experiment was replicated across two seasons: the first planted during the long rains (Masika) between March and June and the second planted during the short rains (Vuli) between October and December. Mean abundance of the whitefly vector (Bemisia tabaci) was much greater during the Vuli season (>19 insects per plant) than the Masika season (<2 insects per plant). CBSD shoot symptoms occurred naturally and were observed only on Kikombe, Kiroba, and Kipusa. New materials had overall lower CBSD shoot incidences (1.5%) compared with recycled materials (6.9%) in Masika, although no significant differences were obvious in Vuli. However, Masika (8.7%) had an overall lower CBSD shoot incidence than Vuli (16.5%) in the varieties that had shoot symptoms. CBSD root incidences were higher in Vuli (10.3%) than Masika (4.4%), and root yields in Masika (29.4 t/ha) were significantly greater than those in Vuli (22.5 t/ha). The highest percentage of roots rendered unusable owing to CBSD was observed in Vuli. There was significantly higher unusable root incidence in recycled materials (3.7%) than in new materials (1.4%) in Masika but not in Vuli. Overall root yield was similar between recycled and new materials in either season. Significant reductions in root yield over the course of the experiment were observed both in Masika and Vuli, whereas changes in marketable yield were significant only in Masika. Differences in the response of varieties to degeneration led to the identification of four degeneration patterns, namely “strong,” “moderate,” “mild,” and “delayed” degeneration. The strongest effects of degeneration were most obvious in the susceptible cultivar (Kikombe), which also had the lowest marketable yield in either season. Seasonal differences were a key driver of degeneration, because its effects were much greater in Vuli than Masika. To the best of our knowledge, this work reports the first study of degeneration caused by cassava viruses

Genotype by environment cultivar evaluation for cassava brown streak disease resistance in Tanzania

Open Access Article; Published online: 24 May 2020Cassava brown steak disease (CBSD), caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), is the most important biotic constraint to cassava production in East and Central Africa. Concerted efforts are required to prevent further spread into West Africa as well as to reduce losses in areas already affected. The study reported here was part of a five-country (Kenya, Malawi, Mozambique, Tanzania and Uganda) programme that aimed to identify superior cassava cultivars resistant to CBSD and to disseminate them widely in the region. Seventeen tissue-cultured and virus-tested cultivars were evaluated in Tanzania across nine sites with diverse CBSD inoculum conditions. Experiments were planted using an alpha-lattice design and assessments were made of surrounding inoculum pressure, CBSD foliar and root incidence and root yield at harvest. There were large differences in CBSD infection between sites, with greatest spread recorded from the north-western Lake (Victoria) zone. Differences were driven by Bemisia tabaci whitefly vector abundance and CBSD inoculum pressure. Both CBSV and UCBSV were almost equally represented in cassava fields surrounding experimental plots, although CBSV predominated in the north-west whilst UCBSV was more frequent in coastal and southern sites. However, the incidence of CBSV was much greater than that of UCBSV in initially virus-free experimental plots, suggesting that CBSV is more virulent. Cultivars could be categorised into three groups based on the degree of CBSD symptom expression in shoots and roots. The seven cultivars (F10_30R2, Eyope, Mkumba, Mkuranga1, Narocass1, Nase3 and Orera) in the most resistant category each had shoot and root incidences of less than 20%. Fresh root yield differed between sites and cultivars, but there was no genotype by environment interaction for this trait, probably attributable to the large fertility and soil moisture differences between sites. Susceptible cultivars and the local check performed well in the absence of CBSD pressure, highlighting the importance of exploiting quality and yield traits of local landraces in breeding programmes. Overall, our results emphasized the importance of applying a balanced strategy for CBSD management. This should use both improved and local germplasm resources to generate high yielding cultivars for specific end-user traits, and combine the deployment of improved cultivars with phytosanitary control measures including the use of healthy planting material and planting during periods of reduced CBSD infection

Collection, genotyping and virus elimination of cassava landraces from Tanzania and documentation of farmer knowledge

Open Access Journal; Published online: 17 Aug 2021Cassava (Manihot esculenta Crantz.) has been a vital staple and food security crop in Tanzania for several centuries, and it is likely that its resilience will play a key role in mitigating livelihood insecurities arising from climate change. The sector is dominated by smallholder farmers growing traditional landrace varieties. A recent surge in virus diseases and awareness in the commercial potential of cassava has prompted a drive to disseminate improved varieties in the country. These factors however also threaten the existence of landraces and associated farmer knowledge. It is important that the landraces are conserved and utilized as the adaptive gene complexes they harbor can drive breeding for improved varieties that meet agro-ecological adaptation as well as farmer and consumer needs, thereby improving adoption rates. Here we report on cassava germplasm collection missions and documentation of farmer knowledge in seven zones of Tanzania. A total of 277 unique landraces are identified through high-density genotyping. The large number of landraces is attributable to a mixed clonal/sexual reproductive system in which the soil seed bank and incorporation of seedlings plays an important role. A striking divergence in genetic relationships between the coastal regions and western regions is evident and explained by (i) independent introductions of cassava into the country, (ii) adaptation to prevailing agro-ecological conditions and (iii) farmer selections according to the intended use or market demands. The main uses of cassava with different product profiles are evident, including fresh consumption, flour production, dual purpose incorporating both these uses and longer-term food security. Each of these products have different trait requirements. Individual landraces were not widely distributed across the country with limited farmer-to-farmer diffusion with implications for seed systems

Distribution of single nucleotide polymorphism markers towards tagging sources of resistance to cassava brown streak disease in cassava

Cassava roots represent the future of food and income generation for over 800 million people in the world however, its production is threatened by virus disease; Cassava brown streak disease (CBSD). Biotechnology approaches are fast and powerful methodologies in cassava improvement and breeding. Construction of high-density and high quality genetic map in cassava would be of great benefit in combating CBSD. Conventional study was conducted in Naliendele, Kibaha and Dodoma for Tanzania and Molecular work performed at the International Livestock Research Institute, Kenya and University of Berkeley in United States of America. This study involved assessing the integrity of F1 population from a cross between AR40-6 x Albert cassava cultivars using simple sequence repeat (SSR) polymorphisms. An F1 population of 156 individuals were developed. Population evaluation resulted into 72% individuals as true F1 hybrids, 18.7% were non hybrids and 8.2% were selfed individual plants. Evaluation of F1 population validated SSR markers to be useful and efficient tools in identification of true F1 hybrids in controlled crosses. On the other hand, the true F1hybrids obtained were used to construct high dense SNP based linkage map using high throughput genotyping by sequencing (GBS) approach. The GBS is simple, low cost and de novo sequencing that makes an attractive option for large number of markers and individuals. Linkage analysis resulted into comprehensive genetic map with 19 linkage groups with a total of 4784 SNP markers: 2159 of these were mapped to the female genetic map, 2169 to the male map, and 3449 SNP markers to the integrated genetic map. Comprehensive genetic map encompassed 4250cM with mean distance of 1.26cM between the markers. This high density SNP based genetic linkage map of cassava can be used as base in locating genes controlling resistance to cassava brown streak disease and other genomic studies such as QTL detection, sequence assembly and genome comparison of the crop.Bill and Melinda Gates Foundatio

Distribution of single nucleotide polymorphism markers towards tagging sources of resistance to cassava brown streak disease in cassava

Cassava roots represent the future of food and income generation for over 800 million people in the world however, its production is threatened by virus disease; Cassava brown streak disease (CBSD). Biotechnology approaches are fast and powerful methodologies in cassava improvement and breeding. Construction of high-density and high quality genetic map in cassava would be of great benefit in combating CBSD. Conventional study was conducted in Naliendele, Kibaha and Dodoma for Tanzania and Molecular work performed at the International Livestock Research Institute, Kenya and University of Berkeley in United States of America. This study involved assessing the integrity of F1 population from a cross between AR40-6 x Albert cassava cultivars using simple sequence repeat (SSR) polymorphisms. An F1 population of 156 individuals were developed. Population evaluation resulted into 72% individuals as true F1 hybrids, 18.7% were non hybrids and 8.2% were selfed individual plants. Evaluation of F1 population validated SSR markers to be useful and efficient tools in identification of true F1 hybrids in controlled crosses. On the other hand, the true F1hybrids obtained were used to construct high dense SNP based linkage map using high throughput genotyping by sequencing (GBS) approach. The GBS is simple, low cost and de novo sequencing that makes an attractive option for large number of markers and individuals. Linkage analysis resulted into comprehensive genetic map with 19 linkage groups with a total of 4784 SNP markers: 2159 of these were mapped to the female genetic map, 2169 to the male map, and 3449 SNP markers to the integrated genetic map. Comprehensive genetic map encompassed 4250cM with mean distance of 1.26cM between the markers. This high density SNP based genetic linkage map of cassava can be used as base in locating genes controlling resistance to cassava brown streak disease and other genomic studies such as QTL detection, sequence assembly and genome comparison of the crop.Bill and Melinda Gates Foundatio