Cassava Mosaic Disease Research Articles

Epidemiological Analysis of Cassava Mosaic and Brown Streak Diseases, and Bemisia tabaci in the Comoros Islands.

A comprehensive assessment of cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) was carried out in Comoros where cassava yield (5.7 t/ha) is significantly below the African average (8.6 t/ha) largely due to virus diseases. Observations from 66 sites across the Comoros Islands of Mwali, Ngazidja, and Ndzwani revealed that 83.3% of cassava fields had foliar symptoms of CBSD compared with 95.5% for CMD. Molecular diagnostics confirmed the presence of both cassava brown streak ipomoviruses (CBSIs) and cassava mosaic begomoviruses (CMBs). Although real-time RT-PCR only detected the presence of one CBSI species (Cassava brown streak virus, CBSV) the second species (Ugandan cassava brown streak virus, UCBSV) was identified using next-generation high-throughput sequencing. Both PCR and HTS detected the presence of East African cassava mosaic virus (EACMV). African cassava mosaic virus was not detected in any of the samples. Four whitefly species were identified from a sample of 131 specimens: Bemisia tabaci, B. afer, Aleurodicus dispersus, and Paraleyrodes bondari. Cassava B. tabaci comprised two mitotypes: SSA1-SG2 (89%) and SSA1-SG3 (11%). KASP SNP genotyping categorized 82% of cassava B. tabaci as haplogroup SSA-ESA. This knowledge will provide an important base for developing and deploying effective management strategies for cassava viruses and their vectors.

Mitochondrial Genetic Diversity of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Associated with Cassava in Lao PDR.

Simple SummaryThe whitefly species Bemisia tabaci is a known pest of cassava and a vector of cassava geminiviruses in Africa and India, but its role in the recent spread of Cassava Mosaic Disease (CMD) in Southeast Asia is not well known. This is in part due to a lack of data on the occurrence and distribution of B. tabaci in this region. We show here the first results of any country-wide survey and identification of B. tabaci colonizing cassava in Lao PDR.Cassava Mosaic Disease (CMD) caused by Sri Lankan cassava mosaic virus (SLCMV), has rapidly spread in Southeast Asia (SEA) since 2016. Recently it has been documented in Lao PDR. Previous reports have identified whitefly species of B. tabaci as potential vectors of CMD in SEA, but their occurrence and distribution in cassava fields is not well known. We conducted a countrywide survey in Lao PDR for adult whiteflies in cassava fields, and determined the abundance and genetic diversity of the B. tabaci species complex using mitochondrial cytochrome oxidase I (mtCOI) sequencing. In order to expedite the process, PCR amplifications were performed directly on whitefly adults without DNA extraction, and mtCOI sequences obtained using nanopore portable-sequencing technology. Low whitefly abundances and two cryptic species of the B. tabaci complex, Asia II 1 and Asia II 6, were identified. This is the first work on abundance and genetic identification of whiteflies associated with cassava in Lao PDR. This study indicates currently only a secondary role for Asia II in spreading CMD or as a pest. Routine monitoring and transmission studies on Asia II 6 should be carried out to establish its potential role as a vector of SLCMV in this region.

Screening of cultivars against cassava brown streak disease and molecular identification of the phytopathogenic infection-associated viruses

Cassava brown streak disease (CBSD) has been reported as a serious and significant constraint on the cassava crop in Democratic Republic of the Congo (DRC), especially in its eastern part, South Kivu, where cassava has exceedingly high socio-economic value in small-holder farming systems. The recent rapid spread and damage of CBSD are alarming. Compared to cassava mosaic disease (CMD) which causes yield losses of up to 70% in very susceptible cassava cultivars, CBSD destroys whole the field causing 100% crop loss, including immediate and direct losses of tuberous roots, planting material, and quality of leaves. The study gathered information on the circulation of viral species present in the region, and screening of cassava cultivars against CBSD. Here we evaluated the disease incidence (DI) and severity (DS), whitefly vector Bemisia tabaci parameters including number of nymphs and adults. Thereat, the work consisted of quantifying the pressure of the disease to find performing cassava cultivars, which may still be recommendable for farmers. Thirty-one improved and local Congolese cassava cultivars were selected. Disease assessment focused more specifically on morphological, epidemiologic studies, and advanced molecular diagnostic techniques for virus detection. The mean extreme values of disease incidence and severity ranged respectively from 0.0 to 59.44%, and 1 to 2.86, root infection up to 100%, whiteflies up to 2 adults, and 68 nymphs per plant. Accordingly, the viral species identified in circulation are Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) (picornalike (+) ssRNA viruses, Ipomovirus, Potyviridae). Varieties like Cibongoyoka, Dorothea, Kamegere, Korengo, Liyayi, M’Bailo, Nabana, Rav, and Siri showed no symptoms of necrosis on the roots. Some cultivars are even less susceptible or not to the disease and may be recommended in this time of the lack of planting material for farmers.

Study of the Nutritional Quality and Therapeutic Virtues of Cassava Leaves Infected by the Mosaic Virus and Healthy in Central African Republic

Research Problem: Cassava leaves play an important role in the diet of the Central African population. Several studies have investigate the composition in nutrients of cassava leaves but only few study focused on the impact of the cassava mosaic virus disease, the main threat of the cassava production in sub-Saharan Africa, on the nutritional value of infected cassava leaves. The objective of this work is to determine the content of nutritional and anti-nutritional compounds in cassava leaves infected by the mosaic virus and healthy leaves.
 Material and Methods: The study was performed on two cassava varieties that are M66033 (susceptible to be infected by the mosaic virus) and Togo or TMS66 (resistant to the virus) planted in the locality of Bangui, the Capital of the Central African Republic. Cassava leaves were collected 3 months after planting, separated according to the mosaic severity symptoms (0 to 5) and kept at 4°C in a fridge. Biochemical analyses were performed on leaves for the determination of fiber, protein, lipid and carbohydrate contents as well as antioxidant contents.
 Result: The results showed that, the dietary fiber content is 44% in healthy leaves, it decreases from 42% to 26% in cassava leaves infected with mosaic virus. Protein content was 4% in the infected cassava leaf samples and 3% in the healthy cassava leaves. Lipid content decreases from 7% to 2% in mosaic-infected cassava leaves, it is 5% in healthy leaves. The carbohydrate content is 59% in mosaic-infected leaves and 43% in healthy leaves. The polyphenol content is 30 mgeq GA/ g in cassava leaves with mosaic symptoms and 28 mgeq GA/ g in healthy leaves. Flavonoid content decreases from 6 10-2 mgeq Q/ g to 0.510-2 mgeq Q/ g in cassava leaves with the mosaic symptom. Tannin content decreases from 6 10-2 to 0 mgeq cat/ g in mosaic infected cassava leaves. Tannin content is 210-2 mgeq cat/ g in healthy leaves. The defense mechanism developed by cassava plants infected with mosaic virus has an effect on the synthesis of biochemical compounds, fibers, proteins, lipids, carbohydrates, polyphenols, Flavonoids and tannins. These results suggest that cassava leaves infected by mosaic virus are an excellent food for consumer health.

Susceptibility of Cassava Varieties to Disease Caused by Sri Lankan Cassava Mosaic Virus and Impacts on Yield by Use of Asymptomatic and Virus-Free Planting Material

Cassava (Manihot esculenta Crantz) is a rainfed, smallholder-produced crop in mainland Southeast Asia, and is currently facing a serious challenge posed by the introduction of cassava mosaic disease (CMD). This study assessed the susceptibility of popular Asian varieties to CMD, yield penalties associated with the disease, and the efficacy of selecting clean or asymptomatic plants as seed for the following season. Field experiments evaluated agronomic management practices (i.e., fertilizer application, use of symptomatic and asymptomatic seed stakes) in Cambodia with six to nine popular varieties over three seasons under natural disease pressure. Popular cassava varieties KU50 and Huaybong60 showed superior CMD tolerance, with consistently fewer symptomatic plants, lower disease progress measures, and higher yields. Plants demonstrating symptoms at early stages of development, i.e., 60 days after planting, yielded significantly less than those developing symptoms later (i.e., 270 DAP) or not at all. Plants grown from clean stems yielded on average 20% to 2.7-fold higher than those grown from symptomatic planting material. A yield decline of ~50% was recorded with symptomatic planting materials of susceptible varieties (e.g., SC8, ~25 t ha−1) over successive years. The findings emphasize that farmers could use positive selection by choosing asymptomatic plants to significantly reduce yield losses.

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.

Molecular identification of Sri Lankan cassava mosaic virus causing mosaic disease of cassava in Nghe An province, Vietnam

Cassava mosaic disease (CMD), caused by Sri Lankan cassava mosaic virus (SLCMV), was first detected in Tay Ninh province in the southeast region of Vietnam in 2017 and is an emerging threat for cassava production in Vietnam. This study was conducted to characterize the causal agent of CMD in Nghe An province in the north-central region of Vietnam, which is more than 1,200 km away from Tay Ninh province. Genetic relationships between SLCMV isolates indicated that both DNA-A and DNA-B sequences from Nghe An province were identical with the isolates from Tay Ninh province and were highly homologous (>99.7%) with isolates from Cambodia, Thailand and China. This study confirms the gradual spread of SLCMV in Vietnam as an emerging threat to cassava production. In addition, the outcome of this study could help in the timely enforcement of effective management strategies for the cassava mosaic disease in Vietnam.

Cassava mosaic disease incidence and severity and whitefly vector distribution in Gabon

Cassava mosaic disease (CMD) is a major constraint to cassava (Manihot esculenta Crantz) production in sub-Saharan Africa. The objective of this study was to gain insights into the epidemiology of CMD in cassava production systems in Gabon. An epidemiological survey was conducted throughout Gabon from October 2020 to May 2021 to evaluate the epidemiology of cassava mosaic disease (CMD) in cassava farms. A phytosanitary diagnosis was conducted at each farm, using the harmonised and unified WAVE protocol to assess the incidence and severity of CMD and the abundance of its whitefly vector. All data collected were recorded using WAVE’s mobile application and uploaded into the WAVE Cube data system. The results showed that CMD was present in all farms surveyed (n = 227; 100%), but with a variable incidence rate. At national level, the incidence of CMD was high, with an average of 64.29%. However, incidence differed among regions with 82.22 and 79.20% for Ogooue-Maritime and Nyanga; respectively, but only 40.42% for Estuaire. Woleu-Ntem had the highest mean severity (3.47) and Ogooue-Maritime the lowest (2.64). Overall, incidence and severity differed significantly among regions (P< 0.05). The average abundance was 2.58 whiteflies per plant (w/p), varying between 1.06 (Nyanga) and 5.25 w/p (Estuaire). The use of infected cuttings was the main mode of CMD spread (62.67%). These results highlight the need to identify the viruses responsible for the observed cases of CMD, to sanitise cultivated plant material, and to implement a CMD control strategy in Gabon.

Expression, Purification, and Characterisation of South African Cassava Mosaic Virus Cell-to-Cell Movement Protein.

South African cassava mosaic virus (SACMV) is a circular ssDNA bipartite begomovirus, whose genome comprises DNA-A (encodes six genes) and DNA-B (encodes BC1 cell-to-cell movement and BV1 nuclear shuttle proteins) components. A few secondary and tertiary structural and physicochemical characteristics of partial but not full-length begomovirus proteins have been elucidated to date. The full-length codon-optimised SACMV BC1 gene was cloned into a pET-28a (+) expression vector and transformed into expression host cells E. coli BL21 (DE3). The optimal expression of the full-length BC1-encoded movement protein (MP) was obtained via induction with 0.25 mM IPTG at an OD600 of ~0.45 at 37 °C for four hours. Denatured protein fractions (dialysed in 4 M urea), passed through an IMAC column, successfully bound to the nickel resin, and eluted using 250 mM imidazole. The protein was refolded using stepwise dialysis. The molecular weight of MP was confirmed to be 35 kDa using SDS–PAGE. The secondary structure of SACMV MP presented as predominantly β-strands. An ANS (1-anilino-8-naphthalene sulphonate)-binding assay confirmed that MP possesses hydrophobic pockets with the ability to bind ligands such as ANS (8-anilino-1-naphthalenesulphonic acid). A 2′ (3′)-N-methylanthraniloyl-ATP (mant-ATP) assay showed binding of mant-ATP to MP and indicated that, while hydrophobic pockets are present, MP also exhibits hydrophilic regions. Intrinsic tryptophan fluorescence indicated a significant conformational change in the denatured form of BC1 in the presence of ATP. In addition, a phosphatase assay showed that MP possessed ATPase activity.

Cassava mosaic disease and its whitefly vector in Cameroon: Incidence, severity and whitefly numbers from field surveys

Cassava plays a key role in the food security and economy of Cameroon, but its production is constrained by cassava mosaic disease (CMD). However, comprehensive surveys of CMD in Cameroon have been lacking. This study aimed at evaluating the current status of CMD and its whitefly vector. Field surveys were conducted in 2020 using a sampling, diagnostics and data storage protocol that has been harmonized across 10 West and Central African countries for ease of comparison. Thirty plants per field were assessed for CMD severity, whitefly abundance and source of infection. Surveys were conducted in 343 fields and confirmed the presence of CMD in all 10 regions of Cameroon. Among the 10,057 assessed plants, 33.07% were deemed healthy (asymptomatic). At the field level, only 6.7% fields were found to be healthy. The mean CMD incidence across the country was 66.93%, and the mean severity score was 2.28. The main mode of infection was likely through contaminated cuttings. The mean whitefly count per plant was 5.78. This study is the first countrywide survey of CMD in Cameroon and provides insights that can be useful for improving the country's CMD intervention and management strategies.

Characterization of the functional domains of nuclear shuttle protein (NSP) of Indian cassava mosaic virus using green fluorescent protein as reporter.

Indian cassava mosaic virus (ICMV), responsible for the cassava mosaic disease in India, harbours two circular genomic components, DNA-A and DNA-B; the former being responsible for the encapsidation and replication and the latter for intra- and inter-cellular movement of the viral DNA. Two proteins, encoded by DNA-B, the movement protein (MP) and the nuclear shuttle protein (NSP), act in concert on the newly replicated viral DNA to move it from the nucleus to the cell periphery. To map the functional domains of NSP, the intra-cellular localization of its full-length protein and deletion derivatives was studied in the epidermal cells of detached leaves of the laboratory host plant, Nicotiana benthamiana, where the target proteins were transiently expressed as GFP fusions. This analysis revealed domains for nuclear localization at the N-terminus, as well as for localization towards the cell periphery both at the C-terminus and center of the NSP.

African cassava whitefly and viral disease management through timed application of imidacloprid

Cassava whitefly are a group of cryptic species within the Bemisia tabaci sensu lato complex that causes significant damage to cassava in Africa. B. tabaci Sub-Saharan Africa 1 (SSA1) is the major species in the study region which transmits plant-virus diseases to cassava and causes direct feeding damage. Benefits from management of cassava viral diseases through deployment of resistant varieties are being undermined by their susceptibility to B. tabaci SSA1 and the problem is exacerbated by high populations of B. tabaci in cassava fields. To develop a clean seed system that is disease-free, the judicious use of insecticides is required to manage this pest and vector. This study evaluated the effect of timing of insecticide application on controlling B. tabaci SSA1 population abundance and damage to cassava. Field trials were carried out between 2017 and 2018 in Kamuli and Wakiso districts of Uganda using the widely available insecticide Imidacloprid. Three cassava cultivars (NAROCASS1, SAUTI, KBH/2002/026) were planted in a randomized complete block trial design replicated three times. The insecticide was applied in split-plots designated as: (i) no protection (NP), no insecticide applied (control), (ii) early protection (EP), insecticide applied 2 times between 0 and 4 months after planting, (iii) long protection (LP), insecticide applied 3 times between 0 and 8 months after planting, and (iv) total protection (TP), insecticide applied 4 times between 0 and 12 months after planting. Except for control plots, stem cuttings were first dipped into a diluted solution of Imidacloprid prior to planting. Bi-monthly data were collected on the B. tabaci population, feeding damage, cassava mosaic disease and cassava brown streak disease. Stem cuttings and root yields were determined at harvest. The marginal rate of return for each treatment was calculated to determine cost effectiveness. Insecticide application effectively reduced B. tabaci populations, feeding damage and disease spread. Cassava yield differed significantly with insecticide treatment and amongst cultivars. Root and stem yield losses of >60% was recorded in unprotected control plots. The marginal rate of return of the early protection (0–4 MAP) program was 163%. We discuss these findings and the implication for judicious use of insecticides as part of a cassava clean seed system.

Hotspot siRNA Confers Plant Resistance against Viral Infection.

Simple SummaryA hallmark of antiviral RNAi is the production of viral siRNA (vsiRNA). Profiling of vsiRNAs indicates that certain hotspot regions of viral genome or transcribed viral RNAs are more prone to RNAi-mediated cleavage. However, the biological relevance of hotspot vsiRNAs to the host innate defence remains to be elucidated. Here, we show that direct targeting a hotspot by synthetic vsiRNA confers plant resistance to virus infection. Hotspot and coldspot vsiRNAs, based on vsiRNA profile of the African cassava mosaic virus (ACMV), were synthesised. However, only the double-stranded hotspot vsiRNA protected plants from ACMV infection with undetectable levels of viral DNA replication and viral mRNA. We further demonstrated that the hotspot vsiRNA-mediated virus resistance had a threshold effect and required an active RDR6. These data show that hotspot vsiRNAs bear a functional significance on antiviral RNAi, suggesting that they may have the potential as exogenous protection agents for controlling destructive plant viral diseases.A hallmark of antiviral RNA interference (RNAi) is the production of viral small interfering RNA (vsiRNA). Profiling of vsiRNAs indicates that certain regions of viral RNA genome or transcribed viral RNA, dubbed vsiRNA hotspots, are more prone to RNAi-mediated cleavage for vsiRNA biogenesis. However, the biological relevance of hotspot vsiRNAs to the host innate defence against pathogens remains to be elucidated. Here, we show that direct targeting a hotspot by a synthetic vsiRNA confers host resistance to virus infection. Using Northern blotting and RNAseq, we obtained a profile of vsiRNAs of the African cassava mosaic virus (ACMV), a single-stranded DNA virus. Sense and anti-sense strands of small RNAs corresponding to a hotspot and a coldspot vsiRNA were synthesised. Co-inoculation of Nicotiana benthamiana with the double-stranded hotspot siRNA protected plants from ACMV infection, where viral DNA replication and accumulation of viral mRNA were undetectable. The sense or anti-sense strand of this hotspot vsiRNA, and the coldspot vsiRNA in both double-stranded and single-stranded formats possessed no activity in viral protection. We further demonstrated that the hotspot vsiRNA-mediated virus resistance had a threshold effect and required an active RDR6. These data show that hotspot vsiRNAs bear a functional significance on antiviral RNAi, suggesting that they may have the potential as an exogenous protection agent for controlling destructive viral diseases in plants.

Agromorphological Characterization and Evaluation of the Susceptibility of 19 Cassava Clones (Manihot esculenta crantz) to Cassava Mosaic Disease and Cassava Bacteriosis in Togo

Cassava (Manihot esculenta Crantz) is an important tuber crop grown in many countries around the world that contributes to people's food security and is a source of income for producers. But it is clear that cassava cultivation encounters enormous constraints, among which cassava mosaic disease (CMD) and cassava bacteriosis are the most important. Thus, knowledge and exploitation of the diversity of cassava is of particular importance for maintaining and improving its productivity. It is in this perspective that the present study is carried out, the objective of which is to identify the morphological characteristics of 19 cassava clones, and to evaluate the susceptibility of the clones to cassava mosaic disease (CMD) and cassava bacteriosis at the Lomé Agronomic Experimentation Station. The results of this study revealed significant phenotypic variability within the clones. The evaluation of the phytosanitary state of the clones showed that the mean attack severity varied from 1.07 to 2.70 for CMD and from 1.81 to 2.11 for bacteriosis. Fresh tuber yields varied from 12.86 t/ha to 75.71 t/ha. Seven clones (GB20, GA24, D24, N22, Cm, C04, C02) obtained a higher yield than the control clone C01 (40 t/ha). The GB20 clone (75.71 t/ha) was the most productive, 89.26% more than the control clone.

Effects of growth media on growth and commercialisable percentage of disease-free cassava cultivar KM140 (Manihot esculenta Crantz “KM140”)

Cassava mosaic disease (CMD) is one of the most serious diseases that has caused heavy losses in starch yield. In vitro propagation from CMD disease-free cassava is the optimal method to produce healthy seedlings. Identification of a suitable growth medium for the development of ex vitro plantlets during the acclimation stage is an important step in order to obtain healthy plant. In this study, common substrates such as coir, rice husk ash, and vermicompost were mixed in different proportions to create formulations for the autotrophic stage of disease-free KM140 cassava plants. The parameters measured including plant height, number of leaves, stem diameter, number of branches, root length, dry matter ratio of leaves, roots, survival rate, and percentage of nurse stage cassava plantlets were evaluated. The results showed that cassava plants were transferred from in vitro to ex vitro environments gained a survival rate of 84.5% in coir substrate after 1 week. The growth medium contained a mix of 85% coir + 10% rice husk ash + 5% vermicompost was suitable for the growth and development of KM140 cassava plantlets. In this condition, the plantlets reached height of 38.8 cm with 22.0 leaves and the percentage of commercialisable plantlets reached 85.3%.

High-Throughput Virus Screening in Crosses of South American and African Cassava Germplasm Reveals Broad-Spectrum Resistance against Viruses Causing Cassava Brown Streak Disease and Cassava Mosaic Virus Disease

Screening cassava lines for resistance against viruses causing cassava brown streak disease (CBSD) is cumbersome because of the unpredictable and erratic virus infections in the slow plant infection processes that are frequently not associated with distinct leaf symptoms and because of the reliance on the assessment of root necrosis as an indicator of plant resistance/tolerance. The selection of resistant candidates thus extends over several growing cycles and is still associated with uncertainties about the stage of virus infection. To reduce the time for selection of resistant crosses and the uncertainties associated with field screening, we have developed a fast-forward virus screening workflow to assess cassava seedlings from crosses of cassava brown streak- and cassava mosaic virus-resistant parents. After passing through an intensive and precise virus infection routine, cassava seedlings that carried resistance against cassava brown streak and mosaic viruses were identified. Taking the results of 195 seedlings from 18 crossing families together, it became evident that resistance against the viruses causing CBSD is a dominant trait. The protocol developed for virus resistance screening in cassava can be readily adopted. It shifts resistance evaluation from the field to the nursery and replaces the erroneous and lengthy virus infection and screening process with a method of precision and speed.

Cassava mosaic disease (CMD) in Benin: Incidence, severity and its whitefly abundance from field surveys in 2020

Cassava mosaic disease (CMD) is the main threat to cassava (Manihot esculenta Crantz) production in Benin. This study was conducted to assess CMD incidence, disease severity, and adult whitefly (Bemisia tabaci) populations in 11 regions of Benin. A total of 180 cassava fields across the country were assessed during June–December 2020 following the harmonized protocol of the Central and West African Virus Epidemiology program. Based on symptoms observation, CMD was present in all surveyed fields in Ouémé and Alibori regions. The highest disease incidence levels were observed in Malanville (100%), Kpomassè (86.67%), Kandi and Zagnanado (both 81.67%), Ségbanan (80%), and Avrankou (76.67%) districts. The highest mean severity scores were in Couffo (3.68), Mono (3.63), and Atlantique (3.33) regions, while the lowest was in Alibori (2.37). Adult whitefly populations (mean number/plant) were highest in Couffo (15.88) and Mono (13.00) regions and lowest in Donga (0.06). Significant relationships were found between CMD severity and whitefly abundance (P = 0.0010) but there was no significant relationship between whitefly numbers and CMD incidence (P = 0.0577). These findings indicate that CMD has expanded its range across Benin. They also provide a basis for designing a response strategy for the control of cassava virus diseases such as CMD.

Occurrence and molecular characterization of begomoviruses associated with cassava across agro-ecological zones in Cross River State, Nigeria

Cassava (Manihot esculenta Crantz) is a major root crop in sub-Saharan Africa, including Cross River State, Nigeria, where the crop is cultivated as a staple food in many communities. Cassava mosaic begomoviruses (CMBs) have limited cassava production across Africa. Thus, between March and August 2021, forty-five samples of symptomatic cassava leaves across the state were collected and evaluated for CMBs using molecular techniques. Partial sequencing of the African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV) coat protein genes produced sequence lengths of 620 bp for representative isolates from Okpoma (MZ507578), Ababene (MZ507579) and Idundu (MZ507580), and 554 bp for an Ovonum isolate, respectively. BLASTn results for MZ507578 and MZ507580 showed a percentage similarity of 99.35 and 99.03%, respectively with the nucleotide sequence of ACMV (MH251339) from Nigeria for MZ507579, percentage similarity was 99.84% with ACMV (MG250088) from Ghana. Conversely, the Ovonum isolate showed a 100% percentage similarity with EACMV (MW826364) from Nigeria. Phylogenetic analysis clustered the Calabar isolates (MZ507578, MZ507579 and MZ507580) with MH251339 from Nigeria, MG250088 from Ghana and EU155147 from Togo while the Ovonum isolate was grouped with MW826364 and MT571462, both from Nigeria. The two main CMBs in Nigeria, ACMV and EACMV, were identified and characterized from Cross River state in this study, indicating that disease monitoring should be continuous and more virus-resistant cultivars should be introduced for proper virus management.

Agronomic Performance and Carotenoid Content of Kenyan Yellow-Fleshed Cassava Clones

Vitamin A deficiency is common in Kenya; it has been reported among populations that depend on cassava as their main staple. Clonal evaluation of 324 locally developed yellow-fleshed cassava clones was carried out in two sites, Thika and Kiboko located in central and eastern Kenya respectively. Data was collected on agronomic performance, yield quality and reaction to biotic constraints. The clones rated well for the evaluated traits. The overall performance was better in Kiboko than Thika. Cassava Mosaic Disease, Cassava Brown Streak Disease (CBSD) and Cassava Green Mite (CGM) severity was scored on a scale of 1-5. The mean CBSD score was 1.17 in Kiboko and 1.46 in Thika, CMD severity was 1.15 in Kiboko but the disease was not observed at Thika. The CGM damage severity was 1.63 in Kiboko and 1.97 in Thika. The weight of roots per plant was 3.49 and 2.25 kg at Kiboko and Thika respectively. The dry, matter content of the roots was 49.86% at Kiboko and 29.06 at Thika. The cyanogenic potential was assessed using the pictrate test and rated on a scale of 1-9 and Kiboko had a score of 2.79 compared to 4.14 in Thika. A total of 91 samples were analyzed for carotenoid content using the spectrophotometry method. The Kiboko site had a higher mean carotenoid content (1736.96 µg/100 g) compared to Thika (1105.85 µg/100 g). Most of the samples (71%) analyzed had recorded a higher carotenoid content in Kiboko than Thika. The beta-carotene content was higher in Thika (374.16 µg/100 g) than Kiboko (291.30 µg/100 g). Promising clones adaptable to the two sites were identified.

The role of pathogen-mediated insect superabundance in the East African emergence of a plant virus.

One of the major crops for food security is cassava. Superabundant Bemisia tabaci whitefly, comprising unusually high landscape populations of the insect, have been implicated in cassava virus emergence. Studies have been unable to select from several hypotheses, however, as to the dynamic drivers of superabundant whitefly associated with the emergence in East Africa of severe cassava mosaic disease. One possibility is that pathogenic modification of infected plants can itself increase the growth of insect vector colonies on infected plants.Through the modelling of population processes at the landscape scale we introduce a framework for analysing patterns in the association of disease and insect waves.Our analyses demonstrate the role of pathogen‐mediated insect superabundance in a plant disease invasion. Synthesis. An elevated abundance of insects at the landscape scale is frequently implicated in invasions of the plant pathogens that they carry. We advance ecological understanding of plant disease invasions by showing how landscape data can be used to investigate the causes of insect vector superabundance.