Cassava Mosaic Disease Incidence Research Articles

Gradual Emergence of East African cassava mosaic Cameroon virus in Cassava Farms in Côte d’Ivoire

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are endemic threats to cassava production, causing significant yield losses. They are caused respectively by begomoviruses and ipomoviruses that are transmitted by whiteflies and infected cuttings. This study aimed to monitor and characterize viruses responsible for these diseases in order to fill existing gaps in understanding their epidemiology in Côte d’Ivoire. Field surveys were conducted in 2016, 2017, and 2020, and no CBSD symptoms were observed. However, an increase in CMD incidence was noted from 45.95% in 2016 to 51.37% in 2020, while CMD severity remained moderate over the years with a mean value of 2.29. The number of whiteflies was relatively low and decreased over the years. Molecular diagnostics carried out on cassava leaf samples allowed the detection of East African cassava mosaic Cameroon virus (EACMCMV) that occurs in single as well as in mixed infection with African cassava mosaic virus (ACMV). Single EACMCMV infection, which was detected only in three agroecological zones of eastern Côte d’Ivoire in 2016, spread throughout the country in 2017 and became more widespread in 2020 with a preponderance in central and southern zones, whereas ACMV and EACMCMV coinfection has spread to the entire zones. Phylogenetic analysis of the viral isolates showed that they are closely related to those from Burkina Faso, Ghana, and Nigeria. This changing population of cassava virus species constitutes a serious threat to cassava cultivation. Understanding the evolutionary dynamics of these viruses could help in adopting better disease management strategies to control the disease.

Effects of seed treatment on cassava stake performance, whitefly population, disease incidence, and yield performance of cassava (Manihot esculenta Crantz) in Malawi

IntroductionWhitefly (Bemisia tabaci) is a pest of cassava (Manihot esculenta Crantz) and the vector for two of the crop’s major viral diseases – cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), causing severe economic losses for farmers. In this context, we conducted an on-farm experiment to study the efficiency of a seed treatment technology containing thiamethoxam, fludioxonil, and metalaxyl for rapid multiplication of superior cassava genotypes and early protection of the crop against whitefly at Salima and Nkhotakota, central Malawi in the 2019/2020 and 2020/2021 cropping seasons, respectively.MethodsThe trials were conducted using a randomized complete block design with four replicates. The effect of the application of the seed treatment on stake germination, whitefly population, CMD and CBSD incidences, and stem and storage root yields of three cassava varieties (Kalawe, Mbundumali, and Sauti) at three stake sizes (8 cm, 16 cm, and 25 cm) was compared with control – a 25 cm stake size of each of the cassava variety without seed treatment. A benefit-cost analysis was conducted to determine the profitability of the seed treatment technology for each stake size under certified and non-certified stem scenarios.Results and discussionRegardless of stake size and variety, plant germination was highest (96% by 16%) and mean whitefly population lowest (adult 0.4 vs. 3.0 plant-1; nymph 1.0 vs. 3.3 plant-1) with seed treatment application than without. Disease incidence measurements showed no significant effect of seed treatment on CBSD control (p = 0.31), but it reduced CMD incidence by 17% vs. 20% in the untreated. Stem and storage root yields across stake size and cassava variety were highest with seed treatment; 489 bundles ha-1 and 10 (DM) Mg ha-1, respectively. Using the 8 cm stake regardless of variety resulted in the highest average benefit-cost ratio for certified (18.3 USD USD-1) and non-certified (7.8 USD USD-1) cassava stem scenarios.ConclusionWe conclude that the application of the seed treatment tested in this study would offer protection to planted stakes, increase their germination, reduce whitefly population, increase stem and storage root yields in areas experiencing whitefly pressure, and result in high economic profits.

Cutting Dipping Application of Flupyradifurone against Cassava Whiteflies Bemisia tabaci and Impact on Its Parasitism in Cassava.

The cassava whitefly Bemisia tabaci causes damage in cassava through the feeding and vectoring of plant viruses that cause cassava mosaic and cassava brown streak diseases. This study sought to explore the efficacy of cutting dipping in flupyradifurone for whitefly control and the impact of the mode of application on whitefly parasitism under farmer field conditions. The insecticide treatment significantly reduced adult whiteflies by 41%, nymphs by 64%, and cassava mosaic disease (CMD) incidence by 16% and increased root yield by 49%. The whitefly parasitism rate by Encarsia spp. parasitoids was 27.3 and 21.1%, while Eretmocerus spp. had 26.7 and 18.0% in control and flupyradifurone, respectively, and these differences were not significant. Electropenetrography recordings of whitefly feeding behaviour on flupyradifurone-treated plants showed significantly reduced probing activity and a delay in reaching the phloem as compared to the control. The findings from this study demonstrated that cassava cutting dipping in flupyradifurone significantly reduces whitefly numbers and cassava mosaic disease incidence, thus contributing to a significant root yield increase in cassava. Flupyradifurone applied through cutting dips does not significantly impact parasitism rates in cassava fields. Routine monitoring of parasitoids and predators in insecticide-treated versus control fields should be emphasized to determine the impact of pesticides on these beneficial non-target organisms.

Viruses associated with cassava mosaic disease and their alternative hosts along Nigeria-Cameroon border

Cassava mosaic disease (CMD) threatens cassava production across the African continent. Because CMD is widespread in sub-Saharan Africa (SSA), it is particularly important to monitor border regions to prevent the introduction of other cassava (Manihot esculenta Crantz) viruses and strains into regions otherwise considered still free of the viruses. The objective of this study was to establish the occurrence of viruses associated with CMD and their alternate host plants along the Nigeria-Cameroon border. One hundred leaf samples from cassava plants and weed species were collected across 21 locations, along the border regions of Cross River State; and assessed for CMD incidence and severity. Nucleic acid extracts were obtained and used to test for African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV), using polymerase chain reactions. Plants from ten, seven and four locations had mild, moderate and severe CMD symptoms, respectively. Of the 100 samples collected, 68 were symptomatic and 32 were asymptomatic for CMD; while five weed species were symptomatic. ACMV alone was detected in six symptomatic and three asymptomatic cassava leaves. EACMV was singly detected in one symptomatic and two asymptomatic cassava leaves; while 13 cassava leaves from ten locations had mixed ACMV/EACMV infections. One asymptomatic cassava leaf from Ikang tested positive for ACMV and EACMV. EACMV alone was detected on a symptomatic weed species, Sclerocarpus africanus. The occurrence of viruses causing CMD in Nigeria, including the border regions, has now been further documented, and S. africanus is now verified as a host of EACMV. There is a need for further studies on other plants that may be putative reservoirs for cassava mosaic viruses, towards the development of integrated management strategies.

Occurrence of cassava mosaic begomoviruses in national cassava germplasm preserved in two agro‐ecological zones of Ivory Coast

AbstractCassava production in Ivory Coast is hampered by cassava mosaic disease (CMD), which is caused by several begomovirus species. To increase cassava production and conserve genetic resources, the Centre National de Recherche Agronomique (CNRA) maintains a germplasm collection of 610 cassava accessions of various origins, which is kept in open fields in Bouaké and Man. We carried out an epidemiological assessment of the collection and a molecular characterization of the viruses infecting the CNRA cassava germplasm. The field in Man was less affected by CMD (incidence 49.78%, severity 2.23, whitefly infection 2.23%), despite the abundant whitefly populations (1.34 per plant), compared to Bouaké (incidence 74.54%, severity 2.92, whitefly infection 8.04% and whitefly population of 0.34 per plant). However, the predominant mode of CMD transmission was through infected cassava cuttings at both sites. PCR analysis showed that the East African cassava mosaic Cameroon virus (EACMCMV) was always found in co‐infection with African cassava mosaic virus (ACMV), which also occurred in single infection. Co‐infections of EACMCMV and ACMV were more prevalent in Bouaké (52.64%) than Man (13.87%). Eleven complete genome sequences of cassava mosaic begomoviruses infecting the CNRA germplasm were obtained, and phylogenetic analysis showed that they are closely related to ACMV and EACMCMV isolates from Ghana, Burkina Faso and Nigeria. The results of this study will assist breeders to screen accurately for CMD resistance in progeny resulting from parental lines, enabling the strategic distribution of new clean cassava planting materials to reduce the impact of CMD.

Epidemiological Assessment of Cassava Mosaic Disease in a Savanna Region of the Democratic Republic of Congo

An epidemiological survey was conducted, from September 2009 to January 2010 in 206 famers’ fields located across 21 cassava-growing localities of Ngandajika territory in Lomami province (central part of Democratic Republic of Congo (DRC), to determine distribution and status of Cassava Mosaic Disease (CMD). Parameters related to the identification and the evaluation of CMD (incidence, severity and gravity) and number of adult whitefly vector were assessed. CMD was present in all localities surveyed and varied according to the localities and varieties. CMD incidence was low at the INERA station (4.33%) but high in Kafumbu (74.55%). Disease severity was low at the INERA station (1.09) but high in Kafumbu (2.67). Gravity was low at the INERA station (2.99%) and elevated in Quartier Congo (67.82%). The mean of adult whitefly populations varied with sites. However, the whiteflies were more abundant in Mpunga (3.65) compared to the INERA station (1.09). Overall, 71 % of varieties showed varying degrees of sensitivity to CMD. The results of this study revealed that the health status of cassava in Ngandajika is alarming and deserves sustained intervention. Adequate and effective control methods must to be in place to reduce the inoculum levels and the speed of CMD propagation and to limit yield losses caused by this disease.

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.

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.

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.

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.Significance: With an aim towards reducing the food insecurity rate in Africa, we report on the use of genetic tools for a fast and efficient release of new cassava varieties to benefit breeders, farmers and consumers, given the food and industrial importance of this staple crop. This study adds tremendous knowledge to phenotypic and molecular screening for CMD resistance. The outcome will encourage breeders in various cassava breeding programmes to accelerate genetic gains as well as increase breeding accuracy and efficiency for CMD resistance.

Evaluation and Characterization of High Yielding Cassava Mosaic Resistant Variety YTP2

The cassava variety YTP2 (Me 681) has been developed through selection from Thondamuthur type at Tapioca and Castor Research Station, TNAU, Yethapur. The performance of YTP2 in the Adaptive Research Trial (ART) and On Farm Trial (OFT) in the farmer’s field inferred that this new variety is well adapted to cassava growing districts of Tamil Nadu. In addition to the above, YTP2 was found to be resistant to cassava mosaic disease incidence (CMD). Plants are erect, medium growing and non-branching type and suitable for growing under irrigated and rainfed conditions. The internodal length is shorter and the leaf size is medium with sufficient canopy. The leaves of the plants droop down to reduce the transpiration loss which is more advantageous to overcome or escape from drought and heat stress during summer season. It is a dual purpose variety wherein the tubers contain high starch content which is much favourable for the manufacture of starch, sago and also suited for table purpose. The overall performance of this variety showed higher tuber yield (42.20 t ha-1) and starch content (28.40%) which is 15.94% and 18.20% increase over the check varieties YTP1 and H226 respectively. The results of DNA fingerprint data involving SSR markers (SSRY235, NS169 and NS928) showed that it is genetically distinct from the existing commercial varieties viz., YTP1, H226 and Sree Athulya.

Survey and molecular detection of Sri Lankan cassava mosaic virus in Thailand.

Cassava plantations in an area of 458 hectares spanning five provinces along the Thailand-Cambodia border were surveyed from October 2018 to July 2019 to determine the prevalence of cassava mosaic disease (CMD) caused by Sri Lankan cassava mosaic virus (SLCMV) in the region. CMD prevalence was 40% in the whole area and 80% in Prachinburi, 43% in Sakaeo, 37% in Burium, 25% in Surin, and 19% in Sisaket provinces. Disease incidence of CMD was highest 43.08% in Sakaeo, followed by 26.78% in Prachinburi, 7% in Burium, 2.58% in Surin, and 1.25% in Sisaket provinces. Disease severity of CMD symptoms was mild chlorosis to moderate mosaic (2-3). The greatest disease severity was recorded in Prachinburi and Sakaeo provinces. Asymptomatic plants were identified in Surin (12%), Prachinburi (5%), Sakaeo (0.2%), and Buriram (0.1%) by PCR analysis. Cassava cultivars CMR-89 and Huai Bong 80 were susceptible to CMD. In 95% of cases, the infection was transmitted by whiteflies (Bemisia tabaci), which were abundant in Sakaeo, Buriram, and Prachinburi but were sparse in Surin; their densities were highest in May and June 2019. Nucleotide sequencing of the mitochondrial cytochrome oxidase 1 (mtCO1) gene of whiteflies in Thailand revealed that it was similar to the mtCO1 gene of Asia II 1 whitefly. Furthermore, the AV1 gene of SLCMV-which encodes the capsid protein-showed 90% nucleotide identity with SLCMV. Phylogenetic analysis of completed nucleotide sequences of DNA-A and DNA-B components of the SLCMV genome determined by rolling circle amplification (RCA) indicated that they were similar to the nucleotide sequence of SLCMV isolates from Thailand, Vietnam, and Cambodia. These results provide important insights into the distribution, impact, and spread of CMD and SLCMV in Thailand.

Epidemiological assessment of cassava mosaic disease in Burkina Faso.

Surveys were conducted in 2016 and 2017 across the main cassava‐growing regions of Burkina Faso to assess the status of cassava mosaic disease (CMD) and to determine the virus strains causing the disease, using field observation and phylogenetic analysis. CMD incidence varied between regions and across years but was lowest in Hauts‐Bassins (6.0%, 2016 and 5.4%, 2017) and highest in Centre‐Sud (18.5%, 2016) and in Boucle du Mouhoun (51.7%, 2017). The lowest CMD severity was found in Est region (2.0) for both years and the highest in Sud‐Ouest region (3.3, 2016) and Centre‐Sud region (2.8, 2017). The CMD infection was primarily associated with contaminated cuttings in all regions except in Hauts‐Bassins, where whitefly‐borne infection was higher than cuttings‐borne infection in 2016. PCR screening of 687 samples coupled with sequence analysis revealed the presence of African cassava mosaic‐like (ACMV‐like) viruses and East African cassava mosaic‐like (EACMV‐like) viruses as single infections at 79.5% and 1.1%, respectively. Co‐infections of ACMV‐like and EACMV‐like viruses were detected in 19.4% of the tested samples. In addition, 86.7% of the samples positive for EACMV‐like virus were found to be positive for East African cassava mosaic Cameroon virus (EACMCMV). Phylogenetic analysis revealed the segregation of cassava mosaic geminiviruses (CMGs) from Burkina Faso into three clades specific to ACMV, African cassava mosaic Burkina Faso virus (ACMBFV), and EACMCMV, confirming the presence of these viruses. The results of this study show that EACMCMV occurrence may be more prevalent in Burkina Faso than previously thought.

A Combined Nutrient/Biocontrol Agent Mixture Improve Cassava Tuber Yield and Cassava Mosaic Disease

Cassava (Manihot esculenta Crantz) is an important tropical root crop and a major dietary energy source for more than 500 million people. The major production constraints in cassava are the occurrence of nutrient deficiency and cassava mosaic disease (CMD). Hence to increase the cassava yield, it is critical to develop a technology to overcome the problems associated with nutrient deficiencies and CMD. Series of field experiments were conducted to evaluate and validate a new mixture containing biocontrol agent and nutrients on different genotypes and locations. The result indicated that foliar spray of combined nutrient/biocontrol agent mixture at 21 d interval from one to five-month after planting (MAP) had significantly decreased the incidence of nutrient deficiency symptom and CMD incidence resulting in an increased tuber yield. There were significant differences among the cassava genotypes for CMD reaction and foliar spray of combined nutrient/biocontrol agent mixture at 21 d interval from 1 to 5 MAP. The genotype H226 had lower CMD incidence and higher tuber yield. The multilocation trial indicated that foliar spray of combined nutrient/biocontrol agent mixture at 21 d interval from 1 to 5 MAP significantly improved the tuber yield (24%) and decreased the CMD incidence (65%) than unsprayed control. Metabolomic study indicates that foliar spray of combined nutrient/biocontrol agent mixture has altered lipid biosynthesis and metabolism, as evidenced by increased accumulation of octadecatrienoic acid (2.28-fold) trilinolein (126.3-fold) in combined nutrient/biocontrol agent mixture sprayed plants over unsprayed control. Overall, it is evident that foliar spray of combined nutrient/biocontrol agent mixture from 1 to 5 MAP has decreased CMD incidence and increased the tuber yield.

Kasule, F Farmer Preference of Cassava Cultivars in Eastern Uganda: A Choice Beyond Disease Resistance

The cassava breeding program in Uganda has released many improved cultivars resistant/tolerant to cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). However, many farmers have continued to use cultivars that are susceptible to these major viral diseases but with diverse attributes. There is a need to understand farmers’ cassava cultivar attribute preferences and CBSD, CMD prevalence on the preferred cultivars. A total of 150 cassava farmer fields (74 in Bukedea district and 76 in Kumi district) located in eastern Uganda were evaluated for farmers’ cultivars and attribute preferences, as well as prevalence of CBSD and CMD on the farmer selected cultivars. The eastern region was of interest in the study, due to it-being the major cassava producing area in the country. In addition, 30 cassava plants of different genotypes were chosen randomly along transects of each field and assessed for CBSD/CMD incidence and severity on preferred cultivars. Results showed that more than 64% of the farmers in eastern Uganda preferred NASE 03 due to its sweet taste and high yields, followed by NASE 14 (21%) because of its high yields. There was a significant (P≤0.001) correlation between cassava cultivars preferred and CBSD severity (r = -0.56), CBSD incidence (r = -0.53), CMD severity (r = -0.51) and CMD incidence (r = -0.39). In corroboration, the most preferred cultivar, NASE 03 was found most susceptible to CBSD and CMD in both Bukedea and Kumi districts with CBSD incidence of 62.2% and 52.7% and CMD incidence of 56.9% and 34.3% respectively. The results showed that CBSD and CMD are prevalent on farmer preferred cassava cultivars, and that farmer cultivar preference depends not only on disease resistance but also other attributes.

Genetic Diversity of Mitochondrial DNA of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Associated with Cassava and the Occurrence of Cassava Mosaic Disease in Zambia.

Simple SummaryBemisia tabaci is an important vector that transmits cassava brown streak viruses and cassava mosaic begomoviruses that cause cassava brown streak and cassava mosaic diseases, respectively. In 2013 and 2015 we carried out a study to determine the genetic variability within the Bemisia tabaci complex associated with cassava in Zambia. This investigation made use of mitochondrial cytochrome oxidase I gene sequences of samples collected from selected provinces of Zambia. We found three population subgroups (SGs): SSA1-SG1, SSA1-SG2 and SSA1-SG3 within the sub-Saharan Africa 1 (SSA1) genetic group. Whitefly abundance and the incidence of cassava mosaic disease were both greatest in Western Province, in which the SSA1-SG1 subgroup predominated. Establishing which genetic groups and populations of the B. tabaci species complex are associated with cassava mosaic disease and their distribution in the country is key to guiding the strategic deployment of resources to monitor disease spread and ensure food security for millions of cassava-dependent households.Bemisia tabaci is an important vector of cassava brown streak viruses and cassava mosaic begomoviruses, the causal agents of cassava brown streak disease and cassava mosaic disease (CMD), respectively. A study was carried out to determine the genetic variability of B. tabaci associated with cassava and the occurrence of CMD in Zambia in 2013 and 2015. Phylogenetic analysis showed the presence of only the sub-Saharan Africa 1 (SSA1) genetic group in Zambia. The SSA1 population had three population subgroups (SGs): SSA1-SG1, SSA1-SG2 and SSA1-SG3. All three SSA1 population subgroups occurred in Western Province. However, only SSA1-SG3 occurred in Eastern Province, while only SSA1-SG1 occurred in North Western and Luapula Provinces. Adult B. tabaci were most abundant in Western Province in 2013 (11.1/plant) and 2015 (10.8/plant), and least abundant (0.2/plant) in Northern Province in both 2013 and 2015. CMD was prevalent in all seven provinces surveyed, with the highest incidence recorded in Lusaka Province in both 2013 (78%) and 2015 (83.6%), and the lowest in Northern Province in both 2013 (26.6%) and 2015 (29.3%). Although SSA1-SG1 occurred at greater abundances than the other subgroups, there was no direct association demonstrated between whitefly subgroup and incidence of CMD. Establishing which B. tabaci genetic groups and populations are associated with CMD and their distribution in the country is a key factor in guiding the development of CMD control strategies for cassava-dependent households.

Evaluation of whitefly population and weather effect of cassava mosaic incidence on commonly grown cassava in Benue State, Nigeria

Vector population and weather are critical in virus disease incidence and could be strategic for its management in agriculture. To investigate the influence of whitefly population and weather on Cassava Mosaic Disease (CMD) in cassava, TMS 419 and TME 30572, commonly grown in the state were planted. The varieties were laid out in Randomized Complete Block Design in four replicates and left to natural infection by CMD viruses. At two weeks after planting and fortnightly, whitefly population counts were taken, CMD incidence was estimated and severity scored on 5- point scale. Daily temperature, humidity, wind speed and wind direction were obtained as secondary data. Data were analysed using ANOVA at p=0.05. Whitefly population (18/plant) was significant (P≤0.05) in the first 30 days at 26.9-27.1oC, 42% humidity, 10 Km/hr wind speed and when the wind direction was in the East or North-East. The vector population then declined until the end of experiment. Cassava mosaic incidence changed with whitefly population, humidity and wind speed. Disease incidence in cassava plots was low (3.6%). TME30572 maintained lower disease incidence (3.0%) and severity (2.0) during the study. R2 indicated that 27, 42 and 57% of whitefly population could be explained by temperature, humidity and wind speed, respectively while 4% of the disease incidence could be explained by the whitefly population. The number of CMD infected plants peaked (4%) when population of whitefly was 18/plant, above which the disease reduced at 0.0017 rate per unit change in the vector population. Mosaic disease incidence in cassava plots was influenced by vector population as conditioned by the weather.
 Keywords: Bemisia tabaci, Cassava, Cassava Mosaic Disease, Nigeria, Weather, Whitefly vector

South West and North Central Nigeria: Assessment of cassava mosaic disease and field status of African cassava mosaic virus and East African cassava mosaic virus.

Cassava mosaic disease (CMD), caused by cassava mosaic begomoviruses (CMBs), is a major threat to cassava production in Nigeria. The predominant CMBs in Nigeria are African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV) and East African cassava mosaic Cameroon virus (EACMCV), which are transmitted through infected stem cuttings and whitefly vectors. This study was conducted in 2015 and 2017 to assess the epidemiology of CMD and the current distribution of CMBs in cassava farms in South West (SW) and North Central (NC) Nigeria. A survey of cassava farms was undertaken, and samples representative of disease symptoms were collected and assessed using molecular techniques. A total of 184 and 328 cassava farms were sampled in 2015 and 2017, respectively. CMD incidence for both regions surveyed was 43.80 and 12.25% in 2015 and 2017, respectively. Fields in SW recorded a higher incidence rate in 2015 (SW: 45.11%, NC: 42.47%), while the reverse occurred in 2017 (SW: 10.90%, NC: 14.01%). Overall, the CMD incidence in Benue State (NC) was significantly higher than other locations surveyed in both years. CMD symptom severity and mean whitefly population were higher in SW Nigeria in the two survey years. ACMV was widespread across both zones, occurring in 79.1% (453/613) and 54.8% (386/704) of cassava leaf samples analysed in 2015 and 2017, respectively. EACMV was detected in only 6.0% (37/613) and 4.7% (33/704) of all cassava leaf samples analysed in 2015 and 2017, respectively. Overall, a higher proportion of infected samples were found in NC in both 2015 (NC: 85.2%, SW: 75.4%) and 2017 (NC: 73.6%, SW: 45.2%). Detection using strain‐specific primers revealed that 97% of EACMV positive samples were indeed infected by the EACMCV strain of the virus. As previously reported, samples with mixed infections showed a higher symptom severity than samples with single ACMV or EACMV infections. This study provides an update to the distribution of CMBs in SW and NC Nigeria and will be useful in development of monitoring and management strategies for the disease in both regions.

Assessment of yield components of some cassava (Manihot esculenta Crantz) genotypes using multivariate analysis such as path coefficients

Abstract Cassava, which is a tropical storage root crop, serves as a veritable source of carbohydrate for people living in Sub-Saharan Africa. It is also an important source of industrial starch, biopolymers, animal feeds and ethanol. However, the genetic improvement of the crop is impeded by dearth in information on its genetic variability. A field experiment was carried out at the National Root Crops Research Institute, Umudike (05° 29′ N; 07° 33′ E; 122 m a.s.l.), Nigeria, aimed at assessing the interrelationship between fresh root yield and thirteen quantitative yield-related characters of twenty-eight (28) newly developed cassava genotypes. The rain-fed experiment was laid out in a randomized complete block design with dual replications in 2016/2017 cropping seasons. The results indicated that the newly developed cassava genotypes [2] NR110315 followed by [20] B1-5 and [1] NR110238 exhibited (P < 0.05) highest fresh root yield among the twenty-eight genotypes tested. The fresh root yield of [2] NR110315 (18.77 Mt ha−1) was relatively higher compared with [18] NR050080, which gave the least root yield of 5.38 Mt ha−1. The correlation coefficients obtained from the associations among the variables were positive and exhibited highly significant (P ≤ 0.01) association, especially between leaf area index (LAI) and root diameter as well as with fresh root yield of cassava. Among the nine variables subjected to the path analysis, LAI, starch content, above ground dry matter (AGDM), weight of fresh marketable roots per plant and root length that contributed positively and directly to higher fresh root yield require greater attention during selection and breeding in cassava studies. The direct effect of the variables indicating their magnitudes in decreasing order showed that LAI exhibited the greatest effect followed by starch content, AGDM, weight of fresh marketable roots plant−1, root length, % cassava bacterial blight incidence and % Cassava mosaic disease incidence, which had the lowest direct effect on root yield of cassava. Cyanide potential exhibited a direct, weak and negative effect on fresh root yield. The results further suggested that cassava improvement could be achieved through these characters that have positive and highly significant magnitude effect on fresh root yield.

Evaluation of Cassava Genotypes for Agronomic Performance, Correlation with CMD and CBSD Parameters and Stability across Alupe, Kakamega and Kibos in Western Kenya

Cassava (Manihot esculenta Crantz) production in sub-Saharan Africa (SSA) is constrained by the two biotic constraints namely, cassava mosaic disease (CMD) and cassava brown streak disease (CBSD). The aim of this study was to evaluate elite cassava genotypes for variation in agronomical traits, correlate them to CMD and CBSD parameters and identify stable genotypes in Alupe, Kakamega and Kibos in Western Kenya. Twenty three (23) elite cassava genotypes that had shown resistance to either one or both of CMD and CBSD in Eastern Africa were evaluated. The trial was conducted using an alpha lattice balanced design with twenty three (23) genotypes, replicated three times at Alupe, Kakamega and Kibos in Western Kenya for an extended cropping cycle between 2016 and 2017. Results showed significant differences (P ≤ 0.05) between genotypes and location (or agro-ecology), but not interaction (P ≥ 0.05), for all the agronomic performance parameters evaluated. All the 23 cassava genotypes evaluated across the three locations had mean cyanide potential levels ranging from of 3.00–6.00 and were therefore, sweet and not bitter. The significant but negative relationship between CMD and CBSD incidence and severity with agronomic performance implied that their relationship was inverse. Confirmation of stability for agronomic performance was achieved through AMMI analysis, using AMMI stability value (ASV). Stable genotypes based on AMMI stability values (ASV) for fresh root yield across Alupe, Kakamega and Kibos were KBH/2002/066, Kibandameno (a local standard check), NASE-18, Kizimbani and NASE-3. These genotypes need to be further evaluated in more environments to assess their wider adaptability and stability.