Resistance of advanced cassava breeding clones to infection by major viruses in Uganda

Developing broad-spectrum resistance in cassava against viruses causing the cassava mosaic and the cassava brown streak diseases

Growing cassava in Africa requires resistance against the viruses causing cassava mosaic disease (CMD) and the viruses causing cassava brown streak disease (CBSD). A dominant CMD2 resistance gene from a West African cassava landrace provides strong resistance against the cassava mosaic viruses. However, resistance against cassava brown streak viruses is limited to cassava varieties that show tolerance to the disease. A recently identified cassava germplasm that cannot be infected with cassava brown streak viruses provides a new source of the resistance required to protect cassava from CBSD. We present a synopsis of the status of virus resistance in cassava and report on the research to combine resistance against CBSD and CMD. We improve the lengthy and erratic screening for CBSD resistance by proposing a virus infection and screening protocol for the viruses causing CBSD and CMD, which allows a rapid and precise assessment of cassava resistance under controlled conditions. Using this approach, we classified the virus responses of cassava lines from Africa and South America and identified truly virus-resistant clones that cannot be infected with any of the known viruses causing CBSD even under the most stringent virus infections. A modification of this protocol was used to test seedlings from cassava crosses for resistance against both diseases. A broad-spectrum resistance was identified in a workflow that lasted 9 months from seed germination to the identification of virus resistance. The workflow we propose dramatically reduces the evaluation and selection time required in a classical breeding workflow to reach the advanced field trial stage in only 9 months by conducting selections for virus resistance and plant multiplication in parallel. However, it does not bypass field evaluations; cassava resistance assessment prior to the field limits the evaluation to candidates with virus resistance defined as the absence of symptoms and the absence of the virus. The transfer of our virus screening workflow to cassava breeding programs enhances the efficiency by which resistance against viruses can be selected. It provides a precise definition of the plant's resistance response and can be used as a model system to tackle resistance in cassava against other diseases.

Expansion and impact of cassava brown streak and cassava mosaic diseases in Africa: A review

Africa produces over half of global cassava; however, the continent's average yield is below the potential yields achieved under experimental conditions. Many factors contributing to low yield include lack of quality varieties, poor soils, limited access to capital, competition for labor, as well as pests and diseases. Plant diseases are the major biotic constraints to cassava production and have caused considerable food insecurity in Africa. Although there has been some level of disease management which has contributed to the increase in cassava production, the two viral diseases: cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) still claim between 30–40% and upto 70%, respectively of Africa's cassava harvest. Given the importance of the two diseases in Africa, we review the expansion of CBSD and CMD; impacts of the two diseases on food security and how they can be managed. We provide insights in the spread of the two diseases, management efforts, and future directions.

Mutations in DNA polymerase δ subunit 1 co-segregate with CMD2-type resistance to Cassava Mosaic Geminiviruses

Cassava mosaic disease (CMD) suppresses cassava yields across the tropics. The dominant CMD2 locus confers resistance to cassava mosaic geminiviruses. It has been reported that CMD2-type landraces lose resistance after regeneration through de novo morphogenesis. As full genome bisulfite sequencing failed to uncover an epigenetic mechanism for this loss of resistance, whole genome sequencing and genetic variant analysis was performed and the CMD2 locus was fine-mapped to a 190 kilobase interval. Collectively, these data indicate that CMD2-type resistance is caused by a nonsynonymous, single nucleotide polymorphism in DNA polymerase δ subunit 1 (MePOLD1) located within this region. Virus-induced gene silencing of MePOLD1 in a CMD-susceptible cassava variety produced a recovery phenotype typical of CMD2-type resistance. Analysis of other CMD2-type cassava varieties identified additional candidate resistance alleles within MePOLD1. Genetic variation of MePOLD1, therefore, could represent an important genetic resource for resistance breeding and/or genome editing, and elucidating mechanisms of resistance to geminiviruses.

Advances in Genetic Analysis and Breeding of Cassava (Manihot esculenta Crantz): A Review

Cassava (Manihot esculenta Crantz) is the sixth most important food crop and consumed by 800 million people worldwide. In Africa, cassava is the second most important food crop after maize and Africa is the worlds' largest producer. Though cassava is not one of the main commodity crops in South Africa, it is becoming a popular crop among farming communities in frost-free areas, due to its climate-resilient nature. This necessitated the establishment of a multi-disciplinary research program at the Agricultural Research Council of South Africa. The objective of this review is to highlight progress made in cassava breeding and genetic analysis. This review highlights the progress of cassava research worldwide and discusses research findings on yield, quality, and adaptability traits in cassava. It also discusses the limitations and the prospects of the cassava R&D program towards development of the cassava industry in South Africa.

A dataset of cassava whitefly count images

Whiteflies are insect vectors that affect a variety of plants such as tomatoes, cabbages, sweet potatoes, eggplants, and cassava. In Uganda, whiteflies are a major contributor to the spread of Cassava Brown Streak Disease (CBSD). By suckling on infected cassava plants, whiteflies can potentially transfer the Cassava Brown Streak Virus that causes CBSD to uninfected clean plants nearby when they migrate. When they attack the cassava plants in large numbers, whiteflies can also cause significant physical damage through suckling. This eventually can lead to leaf loss or plant death. Whiteflies also excrete “honeydew”, which harbors a fungus known as “sooty mold” that covers the leaves, limiting access to sunlight which in turn affects plant food production. As part of their work, the cassava breeders often conduct studies to assess the population of whiteflies in cassava fields through a manual process of visual inspection which can be arduous and time-consuming. This paper presents a cassava whitefly dataset that has been curated to enable researchers to build solutions for the automation of the count and detection of whiteflies. The dataset contains 3,000 images captured in a whitefly trial site in Uganda. It depicts different variations of whitefly infestation from low to high infestation. This data has already been used to provide a proof-of-concept solution for whitefly counting based on Machine Learning approaches.

Cassava Brown Streak Viruses express second 6-kilodalton (6K2) protein with varied polarity and three dimensional (3D) structures: Basis for trait discrepancy between the virus species

Cassava Brown Streak Virus (CBSV) and Ugandan Cassava Brown Streak Virus (UCBSV) are the two among six virus species speculated to cause the most catastrophic Brown Streak Disease of Cassava (CBSD) in Africa and Asia. Cassava Brown Streak Virus (CBSV) is hard to breed resistance for compared to Ugandan Cassava Brown Streak Virus (UCBSV) species. This is exemplified by incidences of CBSV species rather than UCBSV species in elite breeding line, KBH 2006/0026 at Bagamoyo, Tanzania. It is not yet understood as to why CBSV species could breakdown CBSD-resistance in the KBH 2006/0026 unlike the UCBSV species. This marks the first in silico study conducted to understand molecular basis for the trait discrepancy between CBSV and UCBSV species from structural biology view point. Following ab initio modelling and analysis of physical-chemical properties of second 6-kilodalton (6K2) protein encoded by CBSV and UCBSV species, using ROBETTA server and Protein Parameters tool, respectively we report that; three dimensional (3D) structures and polarity of the protein differs significantly between the two virus species. (95% and 5%) and (85% and 15%) strains of 20 CBSV and 20 UCBSV species respectively, expressed the protein in homo-trimeric and homo-tetrameric forms, correspondingly. 95% and 85% of studied strain population of the two virus species expressed hydrophilic and hydrophobic 6K2, respectively. Based on findings of the curent study, we hypothesize that; (i) The hydrophilic 6K2 expressed by the CBSV species, favour its faster systemic movement via vascular tissues of cassava host and hence result into higher tissue titres than the UCBSV species encoding hydrophobic form of the protein. t and (ii) The hydrophilic 6K2 expressed byCBSV species have additional interaction advantage with Nuclear Inclusion b protease domain (NIb) and Viral genome-linked protein (VPg), components of Virus Replication Complex (VRC) and hence contributing to faster replication of viral genome than the hydrophobic 6K2 expressed by the UCBSV species. Experimental studies are needed to resolve the 3D structures of the 6K2, VPg and NIb and comprehend complex molecular interactions between them. We suggest that, the 6K2 gene should be targeted for improvement of RNA interference (RNAi)-directed transgenesis of virus-resistant cassava as a more effective way to control the CBSD besides breeding.

Selection for resistance to cassava mosaic disease in African cassava germplasm using single nucleotide polymorphism markers

Cassava mosaic disease (CMD) is one of the main constraints that hamper cassava production. Breeding for varieties that are CMD resistant is a major aim in cassava breeding programmes. However, the use of the conventional approach has its limitations, including a lengthy growth cycle and a low multiplication rate of planting materials. To increase breeding efficiency as well as genetic gain of traits, SNP markers can be used to screen and identify resistant genotypes. The objective of this study was to predict the performance of 145 cassava genotypes from open-pollinated crosses for CMD resistance using molecular markers. Two SNP markers (S12_7926132 and S14_4626854), previously converted into Kompetitive allele-specific PCR (KASP) assays, as well as CMD incidence and severity scores, were used for selection. About 76% of the genotypes were revealed to be resistant to CMD based on phenotypic scores, while over 24% of the total population were found to be susceptible. Significant effects were observed for alleles associated with marker S12_7926132 while the other marker had non-significant effects. The predictive accuracy (true positives and true negatives) of the major CMD2 locus on chromosome 12 was 77% in the population used in this study. Our study provides insight into the potential use of marker-assisted selection for CMD resistance in cassava breeding programmes.

A population based expression atlas provides insights into disease resistance and other physiological traits in cassava (Manihot esculenta Crantz)

Cassava, a food security crop in Africa, is grown throughout the tropics and subtropics. Although cassava can provide high productivity in suboptimal conditions, the yield in Africa is substantially lower than in other geographies. The yield gap is attributable to many challenges faced by cassava in Africa, including susceptibility to diseases and poor soil conditions. In this study, we carried out 3’RNA sequencing on 150 accessions from the National Crops Resources Research Institute, Uganda for 5 tissue types, providing population-based transcriptomics resources to the research community in a web-based queryable cassava expression atlas. Differential expression and weighted gene co-expression network analysis were performed to detect 8820 significantly differentially expressed genes (DEGs), revealing similarity in expression patterns between tissue types and the clustering of detected DEGs into 18 gene modules. As a confirmation of data quality, differential expression and pathway analysis targeting cassava mosaic disease (CMD) identified 27 genes observed in the plant–pathogen interaction pathway, several previously identified CMD resistance genes, and two peroxidase family proteins different from the CMD2 gene. Present research work represents a novel resource towards understanding complex traits at expression and molecular levels for the development of resistant and high-yielding cassava varieties, as exemplified with CMD.

Farmers' Sources and Varieties of Cassava Planting Materials in Coastal Kenya

Cassava (Manihot esculenta Crantz) grows in diverse agro-ecological zones. In Kenya, it is widely cultivated in Western and Coastal regions. It is mainly grown for food and nutrition security and excess roots are sold to generate income for the farm households. Its productivity per unit of the land area is high compared to maize and wheat which are staple crops in the country. However, scarcity of cassava planting materials and pests and diseases limit production in these regions. This study aimed at revealing the sources and varieties of cassava planting materials used by farmers and other farmers' practices in coastal Kenya. Four focus group discussions (FGD) and a survey were conducted in 2018, using a semi-structured questionnaire targeting 250 farmers. The data collected on the sources of planting materials, preferred varieties and the practices employed by the farmers in cassava production, was analyzed using descriptive statistics. Majority of farmers (83%) interviewed indicated that they recycled planting materials from the previous crop while some 67% respondents obtained the planting material from their neighbors. Kenya Agricultural and Livestock Research Organization's (KALRO) and the local markets were reported as sources of planting materials by 11 and 5% farmer respondents, respectively. The only formal seed source reported was KALRO. The rest, own seed, neighbors, and the local markets, were informal seed sources. Piecemeal harvesting practiced by 98% of the farmers favored planting material recycling. Farmers dedicated a small proportion of their land (mean 0.2 ha) to cassava production as reported by 60 and 72.8% of farmers from Taita Taveta and Kilifi, respectively. Slightly above half (56%) of the farmers in Kilifi dedicated slightly more land to cassava, planting between 100 and 4,000 cuttings (2.5% of an acre up to a full acre) compared to 87% farmers from Taita Taveta who planted 100 cuttings or less (which is about 2.5% of an acre or less). A majority (81.1%) of farmers in Taita Taveta planted local cassava varieties compared to Kilifi's 57.8%. Slightly above half of the farmer respondents reported Kibandameno as the preferred variety followed by Tajirika as the second preferred variety as reported by 18% farmers. Kibandameno was preferred for its sweet taste by 75.6% farmers while Tajirika was preferred by 52.4% farmers because of the high yielding capacity. Nearly all farmer respondents, in Taita Taveta County obtained the planting material from informal seed sources, except a negligible number, who reported buying their planting material from KALRO, a formal seed source, far from their locality. Farmers sourcing cuttings from a formal seed source such as those from Kilifi County were more likely to use a tractor for land preparation compared to those who sourced planting materials informally who more likely had scarce knowledge on cassava production and the value of cassava. Therefore, interventions to establish a sustainable healthy cassava planting materials seed system are needed to address the systemic constraint and help develop a viable cassava value chain.

Smallholder Cassava Planting Material Movement and Grower Behavior in Zambia: Implications for the Management of Cassava Virus Diseases

Cassava (Manihot esculenta) is an important food crop across sub-Saharan Africa, where production is severely inhibited by two viral diseases, cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), both propagated by a whitefly vector and via human-mediated movement of infected cassava stems. There is limited information on growers' behavior related to movement of planting material, as well as growers' perception and awareness of cassava diseases, despite the importance of these factors for disease control. This study surveyed a total of 96 cassava subsistence growers and their fields across five provinces in Zambia between 2015 and 2017 to address these knowledge gaps. CMD symptoms were observed in 81.6% of the fields, with an average incidence of 52% across the infected fields. No CBSD symptoms were observed. Most growers used planting materials from their own (94%) or nearby (<10 km) fields of family and friends, although several large transactions over longer distances (10 to 350 km) occurred with friends (15 transactions), markets (1), middlemen (5), and nongovernmental organizations (6). Information related to cassava diseases and certified clean (disease-free) seed reached only 48% of growers. The most frequent sources of information related to cassava diseases included nearby friends, family, and neighbors, while extension workers were the most highly preferred source of information. These data provide a benchmark on which to plan management approaches to controlling CMD and CBSD, which should include clean propagation material, increasing growers' awareness of the diseases, and increasing information provided to farmers (specifically disease symptom recognition and disease management options).[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Mesoporous activated carbon yielded from pre-leached cassava peels

The search for alternatives to fossil-based commercial activated carbon (AC) continues to reveal new eco-friendly potential precursors, among which is agricultural waste. The key research aspect in all these endeavors is empirical ascertainment of the core properties of the resultant AC to suit a particular purpose. These properties include: yield, surface area, pore volume, and the active surface groups. It is therefore pertinent to have process conditions controlled and tailored towards these properties for the required resultant AC. Pre-leaching cassava peels with NaOH followed by KOH activation and carbonization at holding temperatures (780 °C) above the melting point of K (760 °C) yielded mesoporous activated carbon with the highest surface area ever reported for cassava peel-based AC. The carbonization temperatures were between 480 and 780 °C in an activation-carbonization stepwise process using KOH as the activator at a KOH:peel ratio of 5:2 (mass basis). A 42% maximum yield of AC was realized along with a total pore volume of 0.756 cm3g-1 and BET surface area of 1684 m2g-1. The AC was dominantly microporous for carbonization temperatures below 780 °C, but a remarkable increase in mesopore volume (0.471 cm3g-1) relative to the micropore volume (0.281 cm3g-1) was observed at 780 °C. The Fourier transform infrared (FTIR) spectroscopy for the pre-treated cassava peels showed distortion in the C-H bonding depicting possible elaboration of more lignin from cellulose disruption by NaOH. A carboxylate stretch was also observed owing to the reaction of Na+ ions with the carboxyl group in the raw peels. FTIR showed possible absorption bands for the AC between 1425 and 1712 cm-1 wave numbers. Besides the botanical qualities of the cassava peel genotype used, pre-leaching the peels and also increasing holding activation temperature above the boiling point of potassium enabled the modified process of producing highly porous AC from cassava peel. The scanning electron microscope (SEM) and transmission electron microscope (TEM) imaging showed well-developed hexagonal pores in the resultant AC and intercalated K profile in the carbon matrices, respectively.

Genetic Gains for Yield and Virus Disease Resistance of Cassava Varieties Developed Over the Last Eight Decades in Uganda

Achieving food security for an ever-increasing human population requires faster development of improved varieties. To this end, assessment of genetic gain for key traits is important to inform breeding processes. Despite the improvements made to increase production and productivity of cassava in Uganda at research level, there has been limited effort to quantify associated genetic gains. Accordingly, a study was conducted in Uganda to assess whether or not genetic improvement was evident in selected cassava traits using cassava varieties that were released from 1940 to 2019. Thirty-two varieties developed during this period, were evaluated simultaneously in three major cassava production zones; central (Namulonge), eastern (Serere), and northern (Loro). Best linear unbiased predictors (BLUPs) of the genotypic value for each clone were obtained across environments and regressed on order of release year to estimate annual genetic gains. We observed that genetic trends were mostly quadratic. On average, cassava mosaic disease (CMD) resistance increased by 1.9% per year, while annual genetic improvements in harvest index (0.0%) and fresh root yield (-5 kg per ha or -0.03% per ha) were non-substantial. For cassava brown streak disease (CBSD) resistance breeding which was only initiated in 2003, average annual genetic gains for CBSD foliar and CBSD root necrosis resistances were 2.3% and 1.5%, respectively. It's evident that cassava breeding has largely focused on protecting yield against diseases. This underpins the need for simultaneous improvement of cassava for disease resistance and high yield for the crop to meet its current and futuristic demands for food and industry.

Differential Tropism in Roots and Shoots of Resistant and Susceptible Cassava (Manihot esculenta Crantz) Infected by Cassava Brown Streak Viruses

Cassava brown streak disease (CBSD) is a destructive disease of cassava in Eastern and Central Africa. Because there was no source of resistance in African varieties to provide complete protection against the viruses causing the disease, we searched in South American germplasm and identified cassava lines that did not become infected with the cassava brown streak viruses. These findings motivated further investigations into the mechanism of virus resistance. We used RNAscope^® in situ hybridization to localize cassava brown streak virus in cassava germplasm lines that were highly resistant (DSC 167, immune) or that restricted virus infections to stems and roots only (DSC 260). We show that the resistance in those lines is not a restriction of long-distance movement but due to preventing virus unloading from the phloem into parenchyma cells for replication, thus restricting the virus to the phloem cells only. When DSC 167 and DSC 260 were compared for virus invasion, only a low CBSV signal was found in phloem tissue of DSC 167, indicating that there is no replication in this host, while the presence of intense hybridization signals in the phloem of DSC 260 provided evidence for virus replication in companion cells. In neither of the two lines studied was there evidence of virus replication outside the phloem tissues. Thus, we conclude that in resistant cassava lines, CBSV is confined to the phloem tissues only, in which virus replication can still take place or is arrested.

Kawuki R. Cassava Brown Streak Disease Response and Association With Agronomic Traits in Elite Nigerian Cassava Cultivars

Cassava mosaic geminiviruses (CMGs) and cassava brown streak viruses (CBSVs) cause the highest yield losses in cassava production in Africa. In particular, cassava brown streak disease (CBSD) is and continues to be a significant constraint to optimal cassava production in Eastern and Southern Africa. While CBSD has not been reported in West Africa, its recent rapid spread and damage to cassava productivity in Eastern, and Southern Africa is alarming. The aim of this study was to evaluate Nigerian cassava genotypes in order to determine their responses to CBSD, in the event that it invades Nigeria, the world's largest cassava producer. The study gathered information on whether useful CBSD resistance alleles are present in the elite Nigerian cassava accessions. A total of 1,980 full-sib cassava seedlings from 106 families were assessed in the field at the seedling stage for a year. A subset of 569 clones were selected and assessed for another year at the clonal stage in Namulonge, central Uganda, a known hotspot for CBSD screening. Results indicated that foliar and root incidences and severities varied significantly (p ≤ 0.01, p ≤ 0.001) except for CBSD foliar incidence at 6 months (CBSD_6i ). Highest and lowest plot-based heritability estimates for CBSD were registered for CBSD root severity (CBSD _rs ) (0.71) and CBSD_6i (0.5). Positive and highly significant correlations were noted between CBSD root incidence (CBSD _ri ) and CBSD _rs (r = 0.90^***). Significant positive correlations were also noted between CBSD foliar severity at 3 months (CBSD_3s ) and CBSD foliar incidence at 6 months (CBSD_6i ) (r = 0.77^***), CBSD_3s and CBSD _rs (r = 0.35^***). Fresh root weight (Fresh _RW ) negatively correlated with CBSD _ri and CBSD _rs , respectively (r = -0.21^*** and r = -0.22^***). Similarly, CBSD_3s correlated negatively with cassava mosaic disease severity at 3 (CMD_3s ) and 6 months (CMD_6s ), respectively (r = -0.25^*** and r = -0.21^***). Fifteen clones were selected using a non-weighted summation selection index for further screening. In conclusion, results revealed that the elite Nigerian accessions exhibited significant susceptibility to CBSD within 2 years of evaluation period. It is expected that this information will aid future breeding decisions for the improvement of CBSD resistance among the Nigerian cassava varieties.

The metabotyping of an East African cassava diversity panel: A core collection for developing biotic stress tolerance in cassava

Cassava will have a vital role to play, if food security is to be achieved in Sub-Saharan Africa, especially Central and East Africa. The whitefly Bemisia tabaci poses a major threat to cassava production by small holder farmers in part due to their role as a vector of cassava mosaic begomoviruses (CMBs) and cassava brown streak ipomoviruses (CBSIs). In the present study untargeted metabolomics has been used as a tool to assess natural variation, similarities and attempts to identify trait differentiators among an East African cassava diversity panel that displayed tolerance/resistance to the effects of Bemisia tabaci infestation. The metabolome captured, was represented by 1529 unique chemical features per accession. Principal component analysis (PCA) identified a 23% variation across the panel, with geographical origin/adaption the most influential classification factors. Separation based on resistance and susceptible traits to Bemisia tabaci could also be observed within the data and was corroborated by genotyping data. Thus the metabolomics pipeline represented an effective metabotyping approach. Agglomerative Hierarchical Clustering Analysis (HCA) of both the metabolomics and genotyping data was performed and revealed a high level of similarity between accessions. Specific differentiating features/metabolites were identified, including those potentially conferring vigour to whitefly tolerance on a constitutive manner. The implications of using these cassava varieties as parental breeding material and the future potential of incorporating more exotic donor material is discussed.

Alternative Approaches for Assessing Cassava Brown Streak Root Necrosis to Guide Resistance Breeding and Selection

Cassava brown streak disease (CBSD) caused by the rapidly evolving cassava brown streak viruses (CBSVs), causes immense yield losses to the cassava value chain in eastern and southern Africa. Western Africa, another region that heavily depends on cassava is under eminent threat from CBSD. Resistance breeding is the best practical solution. However, complexities associated with CBSD resistance screening i.e., variable root sampling units, limit systematic attainment of genetic progress. Accordingly, we compared efficiency of five CBSD root necrosis assessment methods to guide selection: cassava brown streak disease root incidence (CBSDRi), cassava brown streak disease root severity (CBSDRs), cassava brown streak disease root severity computed as harmonic mean (CBSD-Harmonic), proportion-based root necrosis index (CBSD-proportion), and standardized root necrosis index (CBSD-standardized). The indexes (CBSD-proportion and CBSD-standardized) correct for variable sample size. We analyzed CBSD root necrosis data of 256 clones evaluated across 12 environments. Higher and variable standard errors were associated with root severity score 1 (no CBSD root necrosis). Lowest and highest plot-based heritability were respectively registered for CBSD-standardized (0.22) and CBSD-proportion (0.71). CBSDRs was only positively correlated with CBSDRi (r = 0.92) and CBSD-Harmonic (r = 0.97). Using best linear unbiased predictions (BLUPs), we ranked the top 15 CBSD resistant clones; only one clone (UG130014) featured in all the five assessment methods; two clones (UG130006 and UG120156) featured in four (CBSD-Harmonic, CBSDRi, CBSDRs, and CBSD-standardized); and five clones (UG120180, UG120063, UG130002, UG130033, and UG120183) featured in three methods (CBSD-Harmonic, CBSDRi, and CBSDRs). Influence of sample size was also quantified by sub-setting and analyzing CBSDRs data to have plots with at least 40 or 30 roots. Data stabilization was evident in plots with 30 roots. The significant influence of root sample sizes on overall ranking of clones, justifies the use of CBSD root necrosis indexes in early selection stages i.e., seedling and/or clonal trials, that are often characterized by high variations in roots assessed per plot. It is expected that this information will provide a foundation for harmonizing and/or optimizing on-going and future CBSD resistance breeding efforts.

Resistance Against Cassava Brown Streak Viruses From Africa in Cassava Germplasm From South America

Cassava brown streak disease (CBSD) is a severe virus disease of cassava and prevalent in the eastern regions of Africa. The disease is characterized by distinct vein chlorosis and streak symptoms on leaves and stems and necrosis of storage roots. This necrosis can encompass large areas of the root, rendering it inedible so that the entire cassava harvest can be lost. African cassava varieties are susceptible to either of the two viruses causing the disease, cassava brown streak virus (CBSV) and Uganda cassava brown streak virus, and while there are less sensitive varieties, all cassava eventually succumb to the disease. The lack of CBSD resistance in African cassava varieties prompted this search for new sources of virus resistance in the diversity of South American cassava germplasm held in the collection at International Center for Tropical Agriculture, Columbia. Our search for CBSD resistance in South American cassava germplasm accessions revealed that most of the 238 South American cassava lines infected with CBSV established systemic virus infections with moderate to severe disease symptoms on leaves and stems. Fifteen cassava accessions did not become virus infected, remained free of symptoms, and CBSV was undetected by qRT-PCR. When tuberous roots of those lines were examined, necrotic tissue was found in eight lines and CBSV was detected. The remaining seven cassava accessions remained clear of symptoms on all tissues and organs and were virus free. A broad spectrum of virus resistance also including other virus isolates was confirmed for the breeding lines DSC167 and DSC118. While detailed infection experiments with other cassava lines selected for resistance are still ongoing, this indicates that the resistance identified may also hold against a broader diversity of CBSVs. Taken together, we present the results of a comprehensive study on CBSV resistance and susceptibility in cassava germplasm accessions from South America. The virus resistance in cassava germplasm identified provides compelling evidence for the invaluable contribution of germplasm collections to supply the genetic resources for the improvement of our crops.