Symptom severity of cassava mosaic disease in relation to concentration of African cassava mosaic virus in different cassava genotypes

The concentration of African cassava mosaic virus (ACMV) was assessed by enzyme-linked immunosorbent assay in relation to symptom severity among resistant, moderately resistant and susceptible cassava genotypes. Resistant genotype NR 8083 had significantly lower symptom severity scores ( P  < 0·05) than the susceptible genotype TMS 91934, but the two genotypes contained similar levels of virus concentration. The moderately resistant genotypes TMS 30572 and NR 8082 expressed significantly lower symptom severities ( P  < 0·05) than the susceptible genotypes TMS 91934 and TME 117, but they contained significantly higher virus concentrations ( P  < 0·05) than TMS 91934 and similar virus concentration as in TME 117. However, two other resistant genotypes, TME 1 and TME 8, had low symptom severity scores and virus concentrations. There was significant interaction ( P  ≤ 0·05) between cropping season and virus concentration in all the genotypes except TMS 30572. The resistant and moderately resistant genotypes that had high virus concentrations sustained storage root yield losses. The severity of symptoms expressed was not necessarily a reflection of the virus concentration in some of the genotypes. In addition to the use of symptom severity scores to group genotypes into resistant classes, it is recommended that virus concentration should also be considered. Genotypes displaying mild symptoms, but with high levels of virus accumulation, could be an important source of inoculum in the spread of ACMV by the whitefly vectors. This suggests that each genotype should be tested for virus accumulation prior to its release to the farmers.     

Components of resistance of cassava to African cassava mosaic virus

Components of resistance of cassava (Manihot esculenta) to African cassava mosaic virus (ACMV) and their interrelationships were confirmed and quantified in a series of experiments at Adiopodoumé (Ivory Coast, West-Africa). The response to virus infection and toBemisia tabaci infestation of a large collection of cassava, including local cultivars and others derived from inter-specificM. glaziovii hybrids was assessed. A consistent correlation was found between virus titre, symptom intensity, disease incidence and non-systemicity (recovery) which suggests that they are different expressions of the same genetic resistance. By contrast, there was no correlation between whitefly infestation and incidence of ACMV, suggesting that resistance to virus and vector are determined by two distinct genetic mechanisms. Several improved cultivars derived from inter-crossing cassava withM. glaziovii as well as some local cultivars were highly resistant and combined low susceptibility, low symptom intensity, low virus content and high level of recovery. Although yield losses ranged from 10% to 30% in such resistant cultivars, the combined effect of high field resistance and high rate of recovery lead to low disease incidence and limited yield losses, even in areas of high infection pressure such as Adiopodoumé.     

Cassava common mosaic virus infection causes alterations in chloroplast ultrastructure,function, and carbohydrate metabolism of cassava plants

Cassava common mosaic virus (CsCMV) is a potexvirus that causes systemic infections in cassava plants, leading to chlorotic mosaic and producing significant yield losses. To date, the physiological alterations and the mechanism underlying biotic stress during the cassava–CsCMV compatible interaction remains unknown. In this study, we found that CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts in source leaves during the course of viral infection. Extrusion of the chloroplast membrane with amoeboid-shaped appearance and disorganized grana stacks were observed in infected mesophyll cells. These alterations were associated with up to 35% reduction of relative chlorophyll content, and a decline of CO₂ fixation (13.5% and 24.2% at 90 and 210 days after planting, respectively). The effects of CsCMV infection on the performance index on absorption basis dropped up to 37%. The analysis of chlorophyll a fluorescence showed a progressive loss of both oxygen evolving complex activity and “connectivity” within the tripartite system (core antenna-LHCII-reaction centre). Here, we report the latter phenomenon for the first time in a viral infection. The oxidative stress process was observed in CsCMV-infected plants (20.8% reduction of antioxidant capacity with respect to noninfected plants). Other effects of the pathogen included reduction of starch and maltose content in source leaves, and a significant increase (24.7%) of the sucrose:starch ratio, which indicates an altered pattern of carbon allocation. Our results suggest that CsCMV induces chloroplast distortion associated with progressive chloroplast function loss and diversion of carbon flux in source leaf tissue, leading to the loss of cassava tuber yield.     

Cassava common mosaic virus infection affects growth and yield components of cassava plants (Manihot esculenta) in Argentina

Cassava is an important crop with great economic and social significance in many countries. Most of its biomass is usable: storage roots can be destined for industry or fresh consumption, leaves are a source of protein, vitamins, and minerals, and stems can be used as propagation material. Due to its vegetative propagation, cassava yield is greatly constrained by viral diseases. Cassava common mosaic virus (CsCMV), one of the most widely spread viruses in Latin America, is the only cassava-infecting potexvirus able to cause disease in a single infection: cassava common mosaic disease (CCMD). Here, we evaluated the effect of an Argentinian strain of CsCMV on several agronomic traits. Field trials were established in north-eastern Argentina for three successive cropping seasons, 2016/17, 2017/18, and 2018/19. CsCMV presence was confirmed by PCR or ELISA tests in mechanically inoculated cassava plants. The evaluated traits were plant height (PH), total fresh weight (TFW), shoot fresh weight (SFW), diameter and FW of tuberous root per plant (TRD and FWTR), FW of each individual tuberous root (FWiTR), length and number of tuberous roots (TRL and TRN), harvest index (HI), and estimated yield of tuberous root (EY) in both infected and noninfected plants. Significant yield losses were recorded in infected plants. The most affected traits were EY and FWTR (both showing 44.3% of losses), followed by TFW, SFW, TRN, and FWiTR (37.8%, 33.5%, 29.9%, and 24.8% of losses, respectively). These losses are extremely high for Argentina, where this crop provides food security and demand for the raw material is unsatisfied.     

Strategies for controlling cassava mosaic virus disease in Africa

Cassava mosaic disease (CMD) is caused by whiteflyborne viruses of the genus Begomovirus (family Geminiviridae ). The disease has long been regarded as the most important of those affecting cassava in sub-Saharan Africa, and has been the subject of much research, especially since the onset of the current very damaging pandemic in eastern and central Africa. This review considers the main features of CMD and the various possible means of control. The main emphasis to date has been on the development and deployment of virus-resistant varieties. These are widely adopted in countries where CMD has caused serious problems, and provided a powerful incentive for farmers to abandon some of the most susceptible of their traditional varieties. Only limited use has been made of phytosanitation involving CMD-free planting material and the removal (roguing) of diseased plants. Cultural methods of control using varietal mixtures, intercrops or other cropping practices have also been neglected, and there is a need for much additional research before they can be deployed effectively. Nevertheless, the severe losses now being caused by CMD in many parts of sub-Saharan Africa could be greatly decreased through the application of existing knowledge.     

Distinct strains of the re‐emergent Cassava common mosaic virus (genus: Potexvirus) infecting cassava in Argentina

Cassava common mosaic disease (CCMD) has been reported in all regions where cassava is grown in the Americas and the causal agent, Cassava common mosaic virus (CsCMV), has been identified as a mechanically transmitted potexvirus (Alphaflexiviridae). In Argentina, cassava is grown mainly in the northeast (NEA) region that shares borders with Brazil and Paraguay. Increasing incidences of CCMD were observed during the years 2014 to 2016 associated with severe leaf mosaic symptoms and yield reductions where the occurrence of CsCMV was confirmed by RT‐PCR and sequencing. In this work, the virus has been successfully purified and a double‐antibody sandwich (DAS‐) ELISA test has been developed from an Argentinean isolate of CsCMV to extend the diagnostics of the disease. A collection of 726 samples was screened and CsCMV was detected with 100% prevalence in the NEA region. Additional co‐infecting viruses were detected in some plants (64.4%); in these, CCMD symptoms correlated with CsCMV only, although more severe symptoms could be observed in mixed infected plants. Sequence analysis of the conserved RdRp domain showed a wider diversity of CsCMV isolates. Interestingly, a separate phylogenetic cluster was formed by isolates from the NEA region that only shared 77.1% to 80.3% nucleotide identity with the other clusters. These results indicate the presence of mixed strains occurring in the NEA region and suggest the presence of geographically distinct strains of CsCMV in South America.     

Nigerian tobacco latent virus: a new Tobamovirus from tobacco in Nigeria

Field-grown tobacco plants in Nigeria showing chlorotic mottle and marginal veinbanding on the leaves apparently contained several viruses. One of them proved to be a new Tobamovirus for which we suggest the name Nigerian tobacco latent virus (NTLV), because it did not produce systemic symptoms on various cultivars of Nicotiana tabacum. Sequence analyses of the coat and movement protein genes and their translation products, as well as serological studies, revealed that NTLV is only distantly related to known Tobamoviruses from which it also differs in host range and symptomatology. Its closest relationship was found to Tobacco mild green mosaic virus (TMGMV). The percentages of amino acid sequence identity amounted to 73% for the coat proteins and to 64% for the movement proteins of the two viruses. The total sequence of 1415 nucleotides analysed share 63% identity with the corresponding region of TMGMV. In the immunoelectron microscopical decoration test using antisera at a dilution of 1 : 50, reactions of NTLV were observed only with its own antiserum and one out of two antisera to TMGMV. An antiserum to NTLV diluted 1 : 2 failed to react with TMGMV. NTLV induces the formation of characteristic inclusions in infected cells.     

Field experiments in Cameroon on cassava mosaic virus disease and the reversion phenomenon in susceptible and resistant cassava cultivars

Cassava is mainly propagated through stem-cuttings. There is evidence that some cuttings propagated from plants infected by cassava mosaic geminiviruses may produce virus-free plants. Field experiments were conducted under contrasting ecological conditions at two sites in southern Cameroon to study the reversion phenomenon, in particular the extent to which the virus causing cassava mosaic disease (CMD) was absent from cuttings collected from CMD-affected plants. Cuttings were taken from the basal, middle and upper portions of the stems of diseased plants of susceptible and partially resistant cassava cultivars. The proportions of disease-free shoots developing from the different batches of cuttings were recorded to indicate the extent of reversion. This was significantly less frequent in cv. Red Local (susceptible) (5-43%) than in the susceptible cv. Bambui Local (65%) and cv. Improved (70%), which is partially resistant to infection. Reversion was significantly higher with cuttings collected from the youngest portions of the stem compared with older portions and with short cuttings (5, 10 cm) compared with those 20 or 25 cm long. There was a significant negative correlation between symptom severity in different cultivars and the extent of reversion. This was greater at the lowland Mile 17 site, near Buea (ca 400 masl) than at the mid-altitude site at Dschang (ca 1300 masl) where conditions were cooler, suggesting an influence of temperature. In order to assess the effect of CMD in cassava the Disease Index (DI) was developed. The result of DI analyses showed that the DI could be used to identify cultivars on the basis of their ability to revert from CMD and the severity of CMD on the non-reverted plants.     

Tentative description of Hippeastrum latent virus in Hippeastrum hybridum plants and differentiation from Hippeastrum mosaic virus

In mosaic-diseased plants ofHippeastrum hybridum two viruses were found. One virus with a normal length of 706 nm caused local lesions onHyoscyamus niger test plants and mosaic symptoms in the leaves ofH. hybridum. This virus was identified with theHippeastrum mosaic virus (HMV) (*/*∶*/*∶E/E∶S/*) and had a dilution end point between 10^?3 and 10^?4, a thermal inactivation point between 55–60°C and a longevity at room temperature of 28–32 hours. The second virus had a normal length between 584 and 611 nm depending on the method used. It caused local lesions onGomphrena globosa andChenopodium quinoa leaves, and after inoculation ofH. hybridum was found to be present without showing symptoms. It was readily purified from inoculated leaf tissue ofC. quinoa andNicotiana clevelandii by differential centrifugation and ofH. hybridum by density-gradient centrifugation. Purified virus had an absorption minimum at 242 nm, a maximum at 262 nm and a 260/280 absorption ratio of 1.19. The dilution end point was between 10^?3 and 10^?4, the thermal inactivation point between 70 and 80°C and the longevity in vitro at room temperature 28–32 hours. Although no direct comparisons have been made with other members of the potexvirus group, the virus seems to be a new one now namedHippeastrum latent virus. Both viruses were not seed-borne.     

Molecular epidemiology of cassava mosaic disease in Madagascar

Cassava is the staple food for hundreds of millions of people in Africa but its cultivation is seriously constrained by cassava mosaic disease (CMD) in Madagascar, and in Africa in general. This study identified the cassava mosaic geminiviruses (CMGs) involved in CMD in Madagascar and their associated epidemiological characteristics from countrywide surveys. Molecular characterization of CMGs in Madagascar revealed an unprecedented diversity and co‐occurrence of six viruses: African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Kenya virus (EACMKV), East African cassava mosaic virus (EACMV), South African cassava mosaic virus (SACMV) and the recently described Cassava mosaic Madagascar virus (CMMGV). Distinct geographical distributions were observed for the six viruses. While ACMV was more prevalent in the central highlands, EACMV and EACMKV were prevalent in lowlands and coastal regions. Both EACMCV and SACMV occurred in almost all the localities visited. PCR diagnosis revealed that mixed infection (up to four co‐infected viruses) occurred in 21% of the samples and were associated with higher symptom severity scores. Pairwise comparisons of virus associations showed that EACMCV was found in mixed infections more often than expected while ACMV and SACMV were mostly found in single infections. A greater abundance of whiteflies was observed in lowland and coastal areas. Nevertheless, infected cuttings remain the primary source of CMD propagation (95%) in Madagascar.     

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.     

Comparison of disease patterns assessed by three independent surveys of cassava mosaic virus disease in Rwanda and Burundi

Cassava mosaic disease (CMD) seriously affects cassava yields in Africa. This study compared the spatial distribution of CMD using three independent surveys in Rwanda and Burundi. Geostatistical techniques were used to interpolate the point‐based surveys and predict the spatial distributions of different measures of the disease. Correlative relationships were examined for 35 environmental and socio‐economic spatial variables of which 31 were correlated to CMD intensity, with the highest correlation coefficients for latitude (?0·47), altitude (?0·36) and temperature (+0·36). The most significant explanatory variables were entered in separate linear regression models for each of the surveys. The models explained 54%, 44% and 22% of the variation in CMD. The residuals of the regression models were interpolated using kriging and added to the regression models to map CMD across both countries. Significant differences were calculated in some areas after correcting for interpolation error. An important explanation of the differences is interaction between the CMD pandemic and the dates of the three surveys. Large relative prediction errors obtained in the regression kriging procedure show the need to improve the survey design and decrease measurement error. Improved maps of crop diseases such as CMD could aid targeting of control interventions and thereby contribute to increasing crop yields. This study validated the unique character of each of the survey approaches adopted and underlines the importance of specific interpretation of results for CMD management. The study emphasizes the need for optimization of sampling designs and survey protocols to maximize the potential of regression kriging.     

Economic impact of Turnip mosaic virus, Cauliflower mosaic virus and Beet mosaic virus in three Kenyan vegetables

Screenhouse experiments conducted in Kenya showed that inoculation of cabbage seedlings with Turnip mosaic virus (TuMV), either alone, or in combination with Cauliflower mosaic virus (CaMV), reduced the number and weight of marketable harvested heads. When viruses were inoculated simultaneously, 25% of cabbage heads were non-marketable, representing 20-fold loss compared with control. By contrast, inoculation with CaMV alone had insignificant effects on cabbage yield. This suggests that TuMV is the more detrimental of these pathogens, and its management should be a priority. Early exposure to TuMV produced cabbages that were 50% lighter than non-infected plants, but later infection was less damaging suggesting that controlling virus infection at the seedling stage is more important. TuMV was far less damaging to kale than it was to cabbage; although high proportions of TuMV-inoculated kale plants showed symptoms (>90%), the marketability and quality of leaves were not significantly reduced, and no clear relationship existed between timing of infection and subsequent crop losses. Early inoculation of Swiss chard with Beet mosaic virus (BtMV) significantly impaired leaf quality (∼50% reduction in marketable leaf production), but the impact of disease was greatest in plants that had been inoculated at maturity, where average leaf losses were two and a half times those recorded in virus-free plants. Disease-management of BtMV in Swiss chard is important, therefore, not only at the seedling stage, but particularly when plants are transplanted from nursery to field.     

Electron microscopical investigations of mosaic diseased wheat plants found in Italy

Summary For some years past at several places in the province of Brescia a mosaic disease of wheat has been detected which probably must be attributede to a virus (Grancini, 1959). Presumably the same disease has recently been constatated in other provinces of North Italy.Electron microscopical study of diseased wheat leaves showed that the preparations which were obtained with the dipping method ofBrandes (1957) all contained rod-shaped virus particles with a length of 300 m. These particles were absent from healthy plants. Whether or not this virus is directly responsible for the mosaic disease could not be established, since we did not succeed in transmitting the virus artificially.Samenvatting Sedert einige jaren wordt op verschillende plaatsen in de provincie Brescia een mozaïekziekte bij de tarwe gevonden, waarvna de oorzaak waarschijnlijk aan een virus moet worden toegeschreven (Grancini, 1959).Bij elektronemicroscopisch onderzoek van zieke bladeren is gevonden, dat de preparaten, welke met de doop-methode vanBrandes (1957) verkregen waren, alle staafvormige virusdeeltjes ter lengte van 300 m bevatten (fig. 1). In preparaten van gezond bladmateriaal kwamen deze deeltjes niet voor. Of dit virus direct aansprakelijk is voor de mozaïekziekte kon nog niet aangetoond worden, aangezien het tot nu toe niet gelukt is het virus langs kunstmatige weg over te brengen.