Streak Disease Research Articles

Pre and Post Training Knowledge of Cassava Viral Disease among Farmer and Extension Officer in Nigeria

This study presents findings of a train-the trainer cassava stakeholders training workshop conducted in 2017. Farmers and agricultural extension officers from 12 states and the Federal Capital Territory (FCT) in Nigeria were invited for a 2-day workshop aimed at raising awareness and educating participants on cassava virus disease management. An open-ended questionnaire was used to measure pre and post workshop knowledge alongside practices and needs. A total of 74 persons, 49 farmers and 25 extension officers were included in the study. Participants were mostly males who had been cultivating cassava for 12±9 years or had been providing extension services for 17±9 years. Pre-workshop knowledge on cassava viral diseases among cassava farmers was poor with farmers scoring an average of 2.04±1.1 out of 5 obtainable points on knowledge. Disease management practices were poor among farmers with 40.8% obtaining planting materials from unverified sources. Post-workshop evaluation showed that farmers’ knowledge of cassava viral diseases had increased as adjudged by an average knowledge score of 7.98±1.80 out of 11 obtainable points. This study highlights the effectiveness of training in improving farmers’ knowledge and equipping them to contribute to the management of cassava viral diseases. Keywords: extension services; Cassava, training; Cassava mosaic disease (CMD); Cassava brown streak disease (CBSD)

Black Sigatoka in bananas: Ecoclimatic suitability and disease pressure assessments.

Black leaf streak disease, or black Sigatoka, is caused by the fungus Pseudocercospora fijiensis, and has been identified as a major constraint to global production of banana and plantain. We fitted a climatic niche model (CLIMEX) for P. fijiensis to gain an understanding of the patterns of climate suitability, and hence hazard from this disease. We then calibrated the climate suitability patterns against the results of an expert elicitation of disease pressure patterns. We found a moderately strong non-linear relationship between modelled climate suitability for P.°fijiensis and the expert ratings for disease pressure. The strength of the relationship provides a cross-validation between the CLIMEX model and the expert elicitation process. The bulk of global banana production experiences high potential threat from P. fijiensis, and the higher yielding areas for banana and plantain production are at greatest threat. By explicitly considering the role of irrigation we have been able to identify how strategic irrigation could be used to support banana production in areas that are at low risk from P. fijiensis.

Utilization of infectious clones to visualize Cassava brown streak virus replication in planta and gain insights into symptom development

Cassava brown streak disease (CBSD) is a leading cause of cassava yield losses across eastern and central Africa and is having a severe impact on food security across the region. Despite its importance, relatively little is known about the mechanisms behind CBSD viral infections. We have recently reported the construction of Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) infectious clones (IC), which can be used to gain insights into the functions of viral proteins and sequences associated with symptom development. In this study, we perform the first reporter gene tagging of a CBSV IC, with the insertion of green fluorescent protein (GFP) sequence at two different genome positions. Nicotiana benthamiana infections with the CBSV_GFP ICs revealed active CBSV replication in inoculated leaves at 2–5 days post inoculation (dpi) and systemic leaves at 10–14 dpi. We also constructed the chimera CBSV_UCP IC, consisting of the CBSV genome with a UCBSV coat protein (CP) sequence replacement. N. benthamiana infections with CBSV_UCP revealed that the CBSV CP may be associated with high levels of viral accumulation and necrosis development during early infection. These initial manipulations pave the way for U/CBSV ICs to be used to understand U/CBSV biology that will inform vital CBSD control strategies.

Assessing the Degeneration of Cassava Under High-Virus Inoculum Conditions in Coastal Tanzania.

Cassava 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%) thanMasika (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.

Engineering Disease-Resistant Cassava.

Manihot esculenta Crantz (cassava) is a food crop originating from South America grown primarily for its starchy storage roots. Today, cassava is grown in the tropics of South America, Africa, and Asia with an estimated 800 million people relying on it as a staple source of calories. In parts of sub-Saharan Africa, cassava is particularly crucial for food security. Cassava root starch also has use in the pharmaceutical, textile, paper, and biofuel industries. Cassava has seen strong demand since 2000 and production has increased consistently year-over-year, but potential yields are hampered by susceptibility to biotic and abiotic stresses. In particular, bacterial and viral diseases can cause severe yield losses. Of note are cassava bacterial blight (CBB), cassava mosaic disease (CMD), and cassava brown streak disease (CBSD), all of which can cause catastrophic losses for growers. In this article, we provide an overview of the major microbial diseases of cassava, discuss current and potential future efforts to engineer new sources of resistance, and conclude with a discussion of the regulatory hurdles that face biotechnology.

Differential expression of the Hsf family in cassava under biotic and abiotic stresses.

Heat shock transcription factors (Hsfs) are important regulators of biotic and abiotic stress responses in plants. Currently, the Hsf gene family is not well understood in cassava, an important tropical crop. In the present study, 32 MeHsf genes were identified from the cassava genome database, which were divided into three types based on functional domain and motif distribution analyses. Analysis of the differential expression of the genes belonging to the Hsf family in cassava was carried out based on published cassava transcriptome data from tissues/organs (leaf blade, leaf midvein, lateral buds, organized embryogenic structures, friable embryogenic callus, fibrous roots, storage roots, stem, petiole, shoot apical meristem, and root apical meristem) under abiotic stress (cold, drought) or biotic stress (mealybugs. cassava brown streak disease, cassava bacterial blight). The results show the expression diversity of cassava Hsfs genes in various tissues/organs. The transcript levels of MeHsfB3a, MeHsfA6a, MeHsfA2a, and MeHsfA9b were upregulated by abiotic and biotic stresses, such as cold, drought, cassava bacterial blight, cassava brown streak disease, and mealybugs, indicating their potential roles in mediating the response of cassava plants to environment stresses. Further interaction network and co-expression analyses suggests that Hsf genes may interact with Hsp70 family members to resist environmental stresses in cassava. These results provide valuable information for future studies of the functional characterization of the MeHsf gene family.

Productivity, Quality, Growth and Pathological Evaluation of Some Promising Sugarcane Genotypes in Egypt

This study was carried out at El-Mattana Agricultural Research Station, Luxor Governorate, Egypt(lat 25° 17' N,long 32° 33' and alt 76 m ASL)during 2015/16, 2016/17 and 2017/18 harvesting seasons to study the productivity, quality,growth characters and natural infection with several diseases. Ten promising sugarcane genotypes (Saccharum spp.)constituted the studied material and check genotype (commercial genotype), namely GT 54-9 were used as control.Sugarcane genotypes were planted in a randomized complete block design with three replicates.The results of this investigation could be summarized as follow: GT 54-9 had the highest stalk length, stalk weight,sucrose%, purity% and all tested productivity characters. The highest infection with pokkah boeng, smut and mosaicdiseases were observed to such genotype while, this genotype uninfected by streak disease. G 84-47 owned the highestbrix%, sucrose% and cane yield and had lowest DS% for pokkah boeng disease and didn’t infect by smut and two testedvirus diseases. However, the highest brix% were recorded to G 2006-6, which gained the lowest infection by pokkahboeng and smut, the highest infected by streak disease and uninfected by mosaic disease. The lowest mean values of stalklength, sugar yield and stalk weight were scrutinized to G 2010-26, G2011-74 and G 2011-79, respectively. Thesegenotypes didn’t infect by smut and mosaic diseases. In regard to streak disease, the highest and least infections wererecorded to G 2010-26 and G 2011-79, respectively. In addition, the lowest stalk diameter and the highest number oftillers/stool were observed in case of G 2011-13 which had the highest infection by smut and uninfected by the two testedvirus diseases. Also, the lowest tested qualitative traits, sugar recovery % and yield were recorded to G2012-50. Also, G2011-13 and G 2012-50 were the highest infected by smut and uninfected by the two tested virus diseases. G 2010-7 andG 2011-82 showed no distinct behavior for all traits. Both genotypes uninfected by smut and the two tested virus diseases.It could be concluded that, GT 54-9 genotype is suitable for regions which were contaminated only by streak disease forhighest stalk weight, cane and sugar yields and G 84-47 for regions which, was contaminated by smut and the two testedvirus for its highest cane yield. The traits of other genotypes were differed from season to other and had differentresponses to tested diseases.

Gene action and heterosis in F1 clonal progenies of cassava for β-Carotene and farmers’ preferred traits

Gene action and heterosis provides information to assist breeder for selecting and generating improved plant recombinants. This study aimed at determining the gene action of selected cassava traits. The F1 clones exhibited considerable phenotypic variability between families and offsprings. The best F1 progenies had a higher amount of β-carotene (β-C) of 6.12 mg 100 g−1 against 1.32 mg 100 g−1 of the best parent. This superiority could be attributed to the over-dominance from the recombination of additive gene action and epistasis. The general combining ability (GCA) of parents and specific combining ability (SCA) of combinations were significant for different traits, and indicating the role of additive and non-additive gene action in controlling such traits. The significant GCA for β-C and postharvest physiological deterioration (PPD) indicates the role of additive gene action. The significant SCA for cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) showed a predominance of non-additive gene action. The F1 progenies from the family Mavoka x Garukunsubire expressed the highest positive heterosis for CMD, dry matter and β-C. The high positive heterosis for β-C and DMC could be linked to transgressive segregation, because one of the parents was poor combiner.

Cassava brown streak virus Ham1 protein hydrolyses mutagenic nucleotides and is a necrosis determinant.

SummaryCassava brown streak disease (CBSD) is a leading cause of cassava losses in East and Central Africa, and is currently having a severe impact on food security. The disease is caused by two viruses within the Potyviridae family: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), which both encode atypical Ham1 proteins with highly conserved inosine triphosphate (ITP) pyrophosphohydrolase (ITPase) domains. ITPase proteins are widely encoded by plant, animal, and archaea. They selectively hydrolyse mutagenic nucleotide triphosphates to prevent their incorporation into nucleic acid and thereby function to reduce mutation rates. It has previously been hypothesized that U/CBSVs encode Ham1 proteins with ITPase activity to reduce viral mutation rates during infection. In this study, we investigate the potential roles of U/CBSV Ham1 proteins. We show that both CBSV and UCBSV Ham1 proteins have ITPase activities through in vitro enzyme assays. Deep‐sequencing experiments found no evidence of the U/CBSV Ham1 proteins providing mutagenic protection during infections of Nicotiana hosts. Manipulations of the CBSV_Tanza infectious clone were performed, including a Ham1 deletion, ITPase point mutations, and UCBSV Ham1 chimera. Unlike severely necrotic wild‐type CBSV_Tanza infections, infections of Nicotiana benthamiana with the manipulated CBSV infectious clones do not develop necrosis, indicating that that the CBSV Ham1 is a necrosis determinant. We propose that the presence of U/CBSV Ham1 proteins with highly conserved ITPase motifs indicates that they serve highly selectable functions during infections of cassava and may represent a euphorbia host adaptation that could be targeted in antiviral strategies.

Control of the onion thrips, &lt;i&gt;Thrips tabaci&lt;/i&gt; Lindeman, and necrotic streak disease, by Pyrifluquinazon application

Control of the onion thrips, &lt;i&gt;Thrips tabaci&lt;/i&gt; Lindeman, and necrotic streak disease, by Pyrifluquinazon application

High-throughput sequencing of complete genomes of ipomoviruses associated with an epidemic of cassava brown streak disease in the Comoros Archipelago.

Using high-throughput sequencing of small interfering RNAs (siRNAs), virion-associated nucleic acid (VANA), and double stranded RNAs (dsRNAs), we have determined the complete genome sequences of Comorian isolates of two ipomoviruses, cassava brown streak virus (CBSV) and a divergent isolate of Ugandan cassava brown streak virus (UCBSV-KM) representing a new strain of this virus. While the large ORF of CBSV shares the highest nucleotide sequence identity (95.9%) with a Tanzanian isolate of CBSV, the large UCBSV-KM ORF shares the highest nucleotide sequence identity (77.5%) with a Malawian isolate of UCBSV. This low value is near the species demarcation threshold for the family Potyviridae (<76%). Phylogenetic analysis confirms that UCBSV-KM represents a new lineage that is genetically distinct from the currently described UCBSV strains.

Whitefly resistance in African cassava genotypes

Whitefly ( Bemisia tabaci ), a major pest and vector of viruses in cassava, is the greatest current threat to cassava production in sub-Saharan Africa (SSA). Research efforts have focused on management of the two viral diseases: cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), and have ignored the whitefly vector that is driving the spread of the viruses, causing CMD and CBSD in SSA. The objective of this study was to evaluate cassava genotypes for resistance to B. tabaci based on field infestation and damage in Uganda. The study was carried out in four sites with diverse agro-ecologies including: Namulonge, Kasese, Ngetta and Serere during 2015 and 2016.Whitefly nymph abundance and feeding damage were assessed on each test genotype from 3 to 6 months after planting (MAP). In 2015, the highest broad sense heritability estimates were 39% (4 MAP) and 53% (5 MAP) for whitefly nymph abundance and feeding damage, respectively. In 2016, broad sense heritability estimates were 23% (3 MAP) and 41% (4 MAP) for whitefly nymph abundance and feeding damage, respectively.Analysis of variance of whitefly nymph abundance showed a significant (P< 0.05) location × genotype × season interactions at 3, 4, 5 and 6 MAP. There were also significant (P< 0.05) location × genotype × season interactions at 3 and 4 MAP for whitefly feeding damage. Ten genotypes showed good levels of resistance to whitefly infestation and feeding damage including: UG120202, UG120174, NASE13, UG120160, UG120286, UG120293, UG130075, CSI-142, CS1-144 and UG130085. These genotypes may serve as parental materials for breeding programmes for whitefly and viral disease control. Key words: Bemisia tabac i, cassava brown streak disease, cassava mosaic disease

Genome of the African cassava whitefly Bemisia tabaci and distribution and genetic diversity of cassava-colonizing whiteflies in Africa

The whitefy Bemisia tabaci, a species complex consisting of many morphologically indistinguishable species divided into distinct clades, is one of the most globally important agricultural pests and plant virus vectors. Cassava-colonizing B. tabaci transmits viruses that cause cassava mosaic disease (CMD) and cassava brown streak disease (CBSD). Half of all cassava plants in Africa are affected by these viral diseases, resulting in annual production losses of more than US$ 1 billion. Here we report the draft genome of the cassava whitefly B. tabaci Sub-Saharan Africa - East and Central Africa (SSA-ECA), the super-abundant population that has been associated with the rapid spread of viruses causing the pandemics of CMD and CBSD. The SSA-ECA genome assembled from Illumina short reads has a total size of 513.7 Mb and a scaffold N50 length of 497 kb, and contains 15,084 predicted protein-coding genes. Phylogenetic analysis suggests that SSA-ECA diverged from MEAM1 around 5.26 million years ago. A comprehensive genetic analysis of cassava-colonizing B. tabaci in Africa was also conducted, in which a total of 243 whitefly specimens were collected from 18 countries representing all major cassava-growing regions in the continent and genotyped using NextRAD sequencing. Population genomic analyses confirmed the existence of six major populations linked by gene flow and inferred the distribution patterns of these populations across the African continent. The genome of SSA-ECA and the genetic findings provide valuable resources and guidance to facilitate whitefly research and the development of strategies to control cassava viral diseases spread by whiteflies.

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.

Promising High-Yielding Tetraploid Plantain-Bred Hybrids in West Africa

The devastating threat of black leaf streak disease caused by Pseudocercospora fijiensis on plantain production in West Africa spurred the development of resistant hybrids. The goal of this research and development (R&amp;D) undertaken was assessing the development and dissemination of two plantain hybrids PITA 3 and FHIA 21 bred in the 1980s by the International Institute of Tropical Agriculture (IITA, Nigeria) and the Fundación Hondureña de Investigación Agrícola (FHIA, Honduras), respectively. In Côte d’Ivoire, plantain growers selected PITA 3 and FHIA 21 based on their improved agronomic characteristics and, between 2012 and 2016, they were massively propagated and distributed to farmers in Benin, Burkina Faso, Côte d’Ivoire, and Togo under the West Africa Agricultural Productivity Program (WAAAP) coordinated by the West and Central Africa Council for Agricultural Research and Development (CORAF). In 2016, the Centre National de Recherche Agronomique in Côte d’Ivoire included the hybrids in the improved cultivar directory. This R&amp;D activity illustrates how three decades of crossbreeding, selection, and distribution led to local acceptance. It also highlights how a CORAF-led partnership harnessed CGIAR research for development. The dissemination and acceptance of these plantain hybrids will enhance the sustainable intensification in plantain-based farming systems across the humid lowlands of West and Central Africa.

Shelf-life estimation and quality of resistant bananas to black leaf streak disease during ripening

Black Leaf Streak Disease (BLSD), also known as Black Sigatoka, is considered the most important banana leaf disease worldwide, because it reduces, in average (↓), both yield (↓ 38%) and postharvest banana green-life (↓ 40%). The control of this disease requires weekly applications of fungicides, which increase the economic and environmental costs for banana production, especially in a few countries in Latin American, which account for over 80% of total banana traded internationally. However, frequent fungicide spraying leads to rapid evolution of fungicide-resistance fungal strains that cause disease-control failures. This work aimed to evaluate the shelf-life and postharvest quality of three genotypes of banana (Musa spp.) resistant to BLSD. Survival analysis methodology was used to estimate shelf-life using the Log-Rank test. Postharvest quality included variables such as peel weight, pulp weight, soluble solids, total sugar, starch, total chlorophyll and total carotenoids. The best results were highlighted by BRS Tropical including a high shelf-life for green life (GL) phase of 22.28 days, followed by BRS Vitória (13.45 days). BRS Platina presented the lowest GL phase (11.23 days). BRS Tropical presented a highest yellow life (YL), including good postharvest quality at end of ripening as soluble solids (25.53°Brix), total sugars (13.73 g 100 g−1) and starch (1.32 g 100 g−1). Results suggest that BRS Tropical, which presented a longer shelf-life (27.85 days), may be indicated for exports purposes as well as in natura consumption. Due to the short shelf-life, BRS Vitória and BRS Platina were more suitable for processing industry.

Insight into Re-emergence of Cassava Brown Streak Disease: the Need to explore Diverse Approaches for Effective Control

The consumption of cassava crop per capita calories in many Africa countries ranked it as the second most important staple food crop. Despite its diverse utilities, the yield potential of cassava is greatly hampered by viral diseases and these differ from country to country, depending on the type of viruses present. Unfortunately, the recent sudden re-emergence of cassava brown streak disease (CBSD) in East and Central Africa is currently gaining more aggressive international attention in Africa and other concerned countries, due to the significant economic yield losses imparting to most poor resource farmers fields. However, selfless efforts from scientific researches to manage CBSD, including recommendations, still seems not to be complimented as expected with pro-active measures from relevant stakeholders, nevertheless the concerned farmers. Overall, the present review revisited a brief glance of the historical development of CBSD, progresses made so far with respect to management strategies. Even more, the paper suggests opinions which could be explore by the ones interested to help within research findings, as well as to bring the awareness and to prevent further spread in countries and/or regions where there is currently the outbreak of CBSD and which had not been yet affected.

Distribution of Common Maize Diseases and Molecular Characterization of Maize Streak Virus in Kenya

Maize is an important food crop in Kenya and is susceptible to a wide range of diseases. A survey was conducted in 2012 in different agro-ecological zones (AEZ) of Kiambu, Embu and Nakuru counties to determine the distribution of northern leaf blight (NLB), common rust (CR), maize streak disease (MSD), gray leaf spot (GLS), head smut (HS) and common smut (CS). Data collected included prevalence, incidence and severity of each of the diseases. Maize leaf samples infected with MSD were also collected for molecular characterization of Maize streak virus (MSV). Northern leaf blight was reported in all counties surveyed with 100% disease prevalence. Kiambu had the highest incidence (100%) of CR whereas Embu had the highest prevalence (45%) of MSD. The incidences of GLS and HS were very low with averages of below 2.5%. The highest incidence of GLS was in Kiambu (5%). High altitude areas had higher incidences of NLB and GLS while CS and MSD were widespread in the three counties. Comparison of 797 nucleotides from the open reading frame (ORF) C2/C1 of MSV with other sequences from the GenBank showed sequence similarities of 99 to 100% with MSV-A strain. The study revealed that the major foliar diseases of maize are widespread in Kenya and therefore there is need to institute measures to manage these diseases and reduce associated losses. Also, the high percent sequence similarities of MSV indicate low variability which is good for breeders since developed resistant varieties can be adopted over a wider region.

Analyses of seven new whole genome sequences of cassava brown streak viruses in Mozambique reveals two distinct clades: evidence for new species.

Cassava brown streak disease (CBSD) caused by Cassava brown streak virus (CBSV) and Uganda cassava brown streak virus (UCBSV) is a major constraint to cassava production in Mozambique. Full genome sequences of CBSD‐associated virus isolates contribute to the understanding of genetic diversity and the development of new diagnostic primers that can be used for early detection of the viruses for sustainable disease management. This study determined seven new whole CBSV genomes from total RNA isolated from cassava leaves with CBSD symptoms collected from Nampula and Zambezia in Mozambique. Phylogenetic analyses of the new genomes with published CBSV and UCBSV sequences in GenBank grouped the CBSV isolates from Mozambique into two distinct clades together with CBSV isolates from Tanzania. Clade 1 and 2 isolates shared low nucleotide (79.1–80.4%) and amino acid (86.5–88.2%) sequence identity. Further, comparisons within the seven new CBSV isolates, and between them and the single published complete CBSV sequence (CBSV_MO_83_FN434436) from Mozambique, revealed nucleotide sequence identities of 79.3–100% and 79.3–98%, respectively, and amino acid identities of 86.7–100% and 86.7–98.8%. In addition, using RDP4, a recombination analysis comprising all CBSV and UCBSV genome sequences from GenBank detect 11 recombination events. Using several comprehensive evolutionary models and statistical programs, it was confirmed that CBSV and UCBSV are distinct virus species, with an additional probable new species (clade 2).

Genetic Variation and Trait Correlations in an East African Cassava Breeding Population for Genomic Selection.

Cassava (Manihot esculenta Crantz) is a major source of dietary carbohydrates for >700 million people globally. However, its long breeding cycle has slowed the rate of genetic gain for target traits. This study aimed to asses genetic variation, the level of inbreeding, and trait correlations in genomic selection breeding cycles. We used phenotypic and genotypic data from the National Crops Resources Research Institute (NaCRRI) foundation population (Cycle 0, C0) and the progeny (Cycle 1, C1) derived from crosses of 100 selected C0 clones as progenitors, both to evaluate and optimize genomic selection. The highest broad-sense heritability (H2 = 0.95) and narrow-sense heritability (h2 = 0.81) were recorded for cassava mosaic disease severity and the lowest for root weight per plot (H2 = 0.06 and h2 = 0.00). We observed the highest genetic correlation (rg= 0.80) between cassava brown streak disease root incidence measured at seedling and clonal stages of evaluation, suggesting the usefulness of seedling data in predicting clonal performance for cassava brown streak root necrosis. Similarly, high genetic correlations were observed between cassava brown streak disease severity (rg= 0.83) scored at 3 and 6 mo after planting (MAP) and cassava mosaic disease, scored at 3 and 6 MAP (rg= 0.95), indicating that data obtained on these two diseases at 6 MAP would suffice. Population differentiation between C0 and C1 was not well defined, implying that the 100 selected progenitors of C1 captured the diversity in the C0. Overall, genetic gain for most traits were observed from C0 to C1.