Unsuccessful Cassava Brown Streak Disease (CBSD) evaluation attempts in western Democratic Republic of Congo and implications with cassava root necrosis disease (CRND) etiology

Open Access ArticleCassava brown streak disease (CBSD) is the second most important virus disease after Cassava mosaic disease (CMD), infecting cassava (ManihotesculetaCrantz) in Africa. The disease is caused by two distinct viruses, Cassava brown streak virus [2, 3] and Ugandan Cassava brown streak virus (family, Potyviridae: genus, Ipomovirus). Transmission of CBSV from one plant to another is reported to occur through grafting CBSV-free with infected cuttings and subsequent dissemination by infected cuttings. The basic approach to control of CBSD is selecting planting material from symptomless mother plants. Graft inoculation is the most efficient and effective of the techniques for CBSD virus transmission and consequently cuttings are the most effective way of the disease spreading. In early 2000s, cassava root necrosis similar to those of CBSD were reported in western provinces of Democratic Republic of Congo (RDC) (Kinshasa and Kongo Central) and up to date PCR diagnoses did not detect any causal agent related to the observed symptoms and the disease which was still referred as ‘CBSD-like disease’. Due to lack of molecular data and the similarity of root symptoms with CBSD, the existence of a virus has always been suspected to be the cause of CBSD-like propagation. Thus, 2 field experiments were proposed in order to verify the existence of a systematic transmission of a possible CBSD related virus, knowing that CBSD viruses are transmitted efficiently by cuttings. The first trial focused on the field evaluation of CBSD – like infected and apparently uninfected planting materials, while the second trial involved the importation of tanzanian CBSD resistant genotypes for evaluation in INERA Mvuazi research center under CBSD-like infection conditions. Results of the first trial did not show a systemic transmission of any CBSD-like pathogen while CBSD-resistant parents involved in the second trial all succumbed to CBSD-like disease

A time series transcriptome analysis of cassava (Manihot esculenta Crantz) varieties challenged with Ugandan cassava brown streak virus

Open Access Journal; Published online: 29 August 2017A time-course transcriptome analysis of two cassava varieties that are either resistant or susceptible to cassava brown streak disease (CBSD) was conducted using RNASeq, after graft inoculation with Ugandan cassava brown streak virus (UCBSV). From approximately 1.92 billion short reads, the largest number of differentially expressed genes (DEGs) was obtained in the resistant (Namikonga) variety at 2 days after grafting (dag) (3887 DEGs) and 5 dag (4911 DEGs). At the same time points, several defense response genes (encoding LRR-containing, NBARC-containing, pathogenesis-related, late embryogenesis abundant, selected transcription factors, chaperones, and heat shock proteins) were highly expressed in Namikonga. Also, defense-related GO terms of ‘translational elongation’, ‘translation factor activity’, ‘ribosomal subunit’ and ‘phosphorelay signal transduction’, were overrepresented in Namikonga at these time points. More reads corresponding to UCBSV sequences were recovered from the susceptible variety (Albert) (733 and 1660 read counts per million (cpm)) at 45 dag and 54 dag compared to Namikonga (10 and 117 cpm respectively). These findings suggest that Namikonga’s resistance involves restriction of multiplication of UCBSV within the host. These findings can be used with other sources of evidence to identify candidate genes and biomarkers that would contribute substantially to knowledge-based resistance breeding

Field evaluation of selected cassava genotypes for cassava brown streak disease based on symptom expression and virus load

Background Production of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties. Methods This study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene. Results A complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species. Conclusions A substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread.Background Production of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties. Methods This study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene. Results A complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species. Conclusions A substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread.Background Production of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties. Methods This study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene. Results A complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species. Conclusions A substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread.Background Production of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties. Methods This study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene. Results A complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species. Conclusions A substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread

Identification of F1 cassava (Manihot esculenta Crantz) progeny using microsatellite markers and capillary electrophoresis

Generation of genetic diversity is necessary in improving on the potential of cassava when faced with various biotic and abiotic challenges. Presently, cassava breeders are breeding for a number of traits, such as drought tolerance, early root bulking, yield, starch, beta-carotene, protein, dry matter, pest and disease resistance, by relying on genetic diversity that exists in manihot esculenta germplasm. Controlled pollination is one of the main methods used to generate genetic diversity in cassava. However, the process of controlled pollination especially in an open field is prone to contamination by illegitimate pollen right from the time of pollination, seed collection, nursery bed establishment to planting of the trials. Therefore, authentication of the progeny obtained from cas-sava crosses is very important for genetic studies. Twelve informative microsatellite markers were used to verify the authenticity of 364 F1 progeny thought to come from four controlled parental crosses. The transmission of each allele at nine microsatellite loci was tracked from parents to progeny in each of the four Namikonga-derived F1 cassava families. Out of the 364 F1 progeny, 317 (87.1%) were true-to-type, 44 (12.1%) were a product of self-pollination and 3 (0.8%) were a product of open pollination. The consistency of the results obtained using microsatellite markers makes this technique a reliable tool for assessing the purity of progeny generated from cassava crosses

Effect of time of harvesting and disease resistance in reducing Cassava (Manihot esculenta Crantz) yield losses by two viral diseases

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are two important biotic constraints for cassava (Manihot esculenta Crantz) production in Eastern and Southern Africa. CMD causes a general decline in yield in affected plants of susceptible cassava varieties but CBSD causes rotting of edible roots. Delayed harvesting can increase rotting of roots and making them unfit for consumption or marketing, and thus affecting the livelihoods of poor farmers. This study investigated the effect of interaction between time of harvesting and levels of disease resistance to identify ideal harvesting times for reducing yield losses. The resistant cassava variety Namikonga remained in the field for the duration of the study, up to 24 months after planting without incurring significant yield losses, while the tolerant varieties Kiroba and Kizimbani could only be maintained up to 21 months. Susceptible varieties Mreteta and Albert suffered significant yield losses beyond 15 months. Among the varieties, Kizimbani had the least CBSD and CMD foliar symptoms as well as farmer desirable traits including high root weight, quantity of marketable roots and dry matter content. Harvesting of cassava can depend on the resistance or susceptibility of the varieties grown. Therefore, the above harvesting times for different varieties were recommended for minimizing yield losses due to the diseases and thus maximizing yields to the farmers

QTL associated with resistance to cassava brown streak and cassava mosaic diseases in a bi-parental cross of two Tanzanian farmer varieties, Namikonga and Albert

Article purchasedCassava production in Africa is compromised by cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). To reduce costs and increase the precision of resistance breeding, a QTL study was conducted to identify molecular markers linked to resistance against these diseases. A bi-parental F1 mapping population was developed from a cross between the Tanzanian farmer varieties, Namikonga and Albert. A one-step genetic linkage map comprising 943 SNP markers and 18 linkage groups spanning 1776.2 cM was generated. Phenotypic data from 240 F1 progeny were obtained from two disease hotspots in Tanzania, over two successive seasons, 2013 and 2014. Two consistent QTLs linked to resistance to CBSD-induced root necrosis were identified in Namikonga on chromosomes II (qCBSDRNFc2Nm) and XI (qCBSDRNc11Nm) and a putative QTL on chromosome XVIII (qCBSDRNc18Nm). qCBSDRNFc2Nm was identified at Naliendele in both seasons. The same QTL was also associated with CBSD foliar resistance. qCBSDRNc11Nm was identified at Chambezi in both seasons, and was characterized by three peaks, spanning a distance of 253 kb. Twenty-seven genes were identified within this region including two LRR proteins and a signal recognition particle. In addition, two highly significant CMD resistance QTL (qCMDc12.1A and qCMDc12.2A) were detected in Albert, on chromosome 12. Both qCMDc12.1A and qCMDc12.2A lay within the range of markers reported earlier, defining the CMD2 locus. This is the first time that two loci have been identified within the CMD2 QTL, and in germplasm of apparent East African origin. Additional QTLs with minor effects on CBSD and CMD resistance were also identified

Sequencing wild and cultivated cassava and related species reveals extensive interspecific hybridization and genetic diversity

Cassava (Manihot esculenta) provides calories and nutrition for more than half a billion people. It was domesticated by native Amazonian peoples through cultivation of the wild progenitor M. esculenta ssp. flabellifolia and is now grown in tropical regions worldwide. Here we provide a high-quality genome assembly for cassava with improved contiguity, linkage, and completeness; almost 97% of genes are anchored to chromosomes. We find that paleotetraploidy in cassava is shared with the related rubber tree Hevea, providing a resource for comparative studies. We also sequence a global collection of 58 Manihot accessions, including cultivated and wild cassava accessions and related species such as Ceará or India rubber (M. glaziovii), and genotype 268 African cassava varieties. We find widespread interspecific admixture, and detect the genetic signature of past cassava breeding programs. As a clonally propagated crop, cassava is especially vulnerable to pathogens and abiotic stresses. This genomic resource will inform future genome-enabled breeding efforts to improve this staple crop

Cassava brown streak disease: a threat to food security in Africa

Published Online: 01/05/2015Cassava brown streak disease (CBSD) has emerged as the most important viral disease of cassava (Manihot esculenta) in Africa and is a major threat to food security. CBSD is caused by two distinct species of ipomoviruses, Cassava brown streak virus and Ugandan cassava brown streak virus, belonging to the family Potyviridae. Previously, CBSD was reported only from the coastal lowlands of East Africa, but recently it has begun to spread as an epidemic throughout the Great Lakes region of East and Central Africa. This new spread represents a major threat to the cassava-growing regions of West Africa. CBSD-resistant cassava cultivars are being developed through breeding, and transgenic RNA interference-derived field resistance to CBSD has also been demonstrated. This review aims to provide a summary of the most important studies on the aetiology, epidemiology and control of CBSD and to highlight key research areas that need prioritization

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

QTL associated with resistance to cassava brown streak and cassava mosaic diseases in a bi-parental cross of two Tanzanian farmer varieties, Namikonga and Albert

Cassava production in Africa is compromised by cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). To reduce costs and increase the precision of resistance breeding, a QTL study was conducted to identify molecular markers linked to resistance against these diseases. A bi-parental F1 mapping population was developed from a cross between the Tanzanian farmer varieties, Namikonga and Albert. A one-step genetic linkage map comprising 943 SNP markers and 18 linkage groups spanning 1776.2 cM was generated. Phenotypic data from 240 F1 progeny were obtained from two disease hotspots in Tanzania, over two successive seasons, 2013 and 2014. Two consistent QTLs linked to resistance to CBSD-induced root necrosis were identified in Namikonga on chromosomes II (qCBSDRNFc2Nm) and XI (qCBSDRNc11Nm) and a putative QTL on chromosome XVIII (qCBSDRNc18Nm). qCBSDRNFc2Nm was identified at Naliendele in both seasons. The same QTL was also associated with CBSD foliar resistance. qCBSDRNc11Nm was identified at Chambezi in both seasons, and was characterized by three peaks, spanning a distance of 253 kb. Twenty-seven genes were identified within this region including two LRR proteins and a signal recognition particle. In addition, two highly significant CMD resistance QTL (qCMDc12.1A and qCMDc12.2A) were detected in Albert, on chromosome 12. Both qCMDc12.1A and qCMDc12.2A lay within the range of markers reported earlier, defining the CMD2 locus. This is the first time that two loci have been identified within the CMD2 QTL, and in germplasm of apparent East African origin. Additional QTLs with minor effects on CBSD and CMD resistance were also identified.The Bill and Melinda Gates Foundation for funding under Contract ID OPPGD1016.http://www.sherpa.ac.uk/romeo/issn/0040-5752/am2017Forestry and Agricultural Biotechnology Institute (FABI)Genetic