Viruses Associated with Cassava Mosaic Disease in Senegal and Guinea Conakry

A survey in Senegal and Guinea Conakry established the presence and incidence of cassava mosaic virus disease (CMD) in both countries. CMD occurred in all the fields surveyed, although its incidence was higher in Senegal (83%) than in Guinea (64%). Populations of the whitefly vector, Bemisia tabaci, were low in both countries averaging 1.7 adults per shoot in Guinea and 3.2 in Senegal. Most infections were attributed to the use of infected cuttings, 86 and 83% in Senegal and Guinea, respectively, and there was no evidence of rapid current‐season, whitefly‐borne infection at any of the sampled locations. Disease severity was generally low in the two countries and averaged 2.5 in Guinea and 2.3 in Senegal. No plants with unusually severe CMD symptoms characteristic of the CMD pandemic in East and Central Africa were observed. Restriction fragment length polymorphism (RFLP)‐based diagnostics revealed that African cassava mosaic virus (ACMV) is exclusively associated with CMD in both the countries. Neither East African cassava mosaic virus (EACMV), nor the recombinant Uganda variant (EACMV‐UG2) was detected in any sample. These survey data indicate that CMD could be effectively controlled in both countries by phytosanitation, involving the use of CMD‐free planting material and the removal of diseased plants.     

Molecular Variability and Distribution of Cassava Mosaic Begomoviruses in Nigeria

Several begomovirus species and strains causing Cassava mosaic disease (CMD) have been reported from cassava in Africa. In Nigeria, African cassava mosaic virus (ACMV) was the predominant virus in this important crop, and East African cassava mosaic virus (EACMV), first reported from eastern Nigeria in 1999, was also found occasionally. A survey was conducted in 2002 to resolve the diversity of the virus types present in cassava in Nigeria and to further understand the increasing complexity of the viruses contributing to CMD. A total of 234 leaf samples from cassava with conspicuous CMD symptoms were collected in farmers’ fields across different agroecological zones of Nigeria and subjected to polymerase chain reaction (PCR) with type‐specific primers. In addition and, to provide a full characterization of the viruses present, DNA‐A genome components of several viruses and informative genome fragments were sequenced. In Nigeria, ACMV proved to be the dominant virus with 80% of all samples being positive for ACMV. The East African cassava mosaic Cameroon virus (EACMCV) prevalent in Cameroon and Ivory Coast was detected in single infections (2%) and in mixed infections (18%) with ACMV. There was no indication for other virus strains of EACMV present in the country. The EACMCV samples collected showed a high nucleotide sequence identity >98% and resembled the described sequence of a Cameroon isolate (EACMCV‐CM) more than an Ivory Coast isolate, EACMCV‐CM[CI]. Evidence is provided that the EACMCV has reached epidemiological significance in Nigeria.     

Spatial and Temporal Spread of Cassava Mosaic Virus Disease in Cassava Grown Alone and when Intercropped with Maize and/or Cowpea

The spread of cassava mosaic disease (CMD) and populations of the whitefly vector (Bemisia tabaci) were recorded in cassava when grown alone and when intercropped with maize and/or cowpea. The trials were planted under conditions of high inoculum pressure in 1995 and 1996 at a site in the lowland rainforest zone of southern Cameroon. In the 1995 experiment, the maize and cowpea intercrops reduced the final incidence of CMD in the cassava cvs. Dschang White and Dschang Violet, but not in the more resistant cv. Improved. In the 1996 experiment with cv. Dschang Violet, the maize and cowpea intercrops grown alone or together decreased adult whitefly populations on cassava by 50% and CMD incidence by 20%. The monomolecular population growth model generally provided the best fit for disease progress. Areas under the disease progress curves (AUDPCs) and incidences expressed as multiple infection units were significantly (P<0.05) less for cassava intercropped with maize and/or cowpea than in cassava alone; times to 50% CMD incidence were significantly (P<0.05) longer in all intercrop systems. In 1995 the basic infection rates (r) were similarly low (0.010 per month) in the moderately resistant cv. Dschang Violet intercropped with maize and in all treatments in the more resistant cv. Improved. By contrast, rates were significantly higherfor cv. Dschang Violet alone or with cowpea and in all treatments for the less resistant cv. Dschang White (0.030–0.060). In 1996, r values in cassava grown alone (0.077) were significantly larger (P<0.05) than in the other cropping systems (0.042–0.052). There were no significant differences in the symptom severity in the different cropping systems. Disease foci were isodametric and more compact in plots containing cowpea than in other cropping systems.     

Evaluation of Cassava Cultivars for Resistance to Cassava Mosaic Disease and Yield Potential in Central African Republic

Eleven cassava genotypes were tested against cassava mosaic disease (CMD) and compared to a local susceptible cultivar in field conditions from June 2011 to July 2012 in Central African Republic (CAR) at two sites representative of the savanna (Damara) and forest (Pissa) zones of the country. The mean number of whiteflies observed on plants varied among genotypes within each site, but was found nearly three times higher at Damara than at Pissa, resulting in a CMD incidence nearly five times higher at Damara than at Pissa. However, no relation was observed between the number of insect on the plants and the level of susceptibility/resistance of the genotypes. The difference of disease pressure between the two sites revealed high level of resistance in several genotypes, while some other ones indicated rather only a partial resistance. Nevertheless, none of the genotypes tested was found immune, in the end, the virus being detected at least in one site in every genotype, including those ones presenting no symptoms in both sites. The impact of CMD on yield components was assessed on the local susceptible check and three partially resistant genotypes, showing that the disease has no significant effect on the tuberous roots number as well as their weight in both sites. The yield potential varied among different genotypes and between the two sites, the mean number of tuberous roots as well as their mean weight being higher in Damara than in Pissa. This study identified highly resistant genotypes such as ‘Gabon’ that performed well in both sites, and ‘91/02322’ that was symptomless and presented a yield potential equivalent to the local check. These genotypes could be distributed to growers with the main advantage to be resistant to CMD and, therefore, reducing the risk to spread sources of inoculum all over the cassava cropping areas in the country.     

Field Studies of Cross Protection with Cassava Mosaic Geminiviruses in Uganda

The effect of cassava mosaic virus disease (CMD) was compared on plants grown from cuttings that were initially virus‐free at planting and those infected with a mild strain of East African cassava mosaic virus‐Uganda (EACMV‐UG). All initially healthy plants developed CMD symptoms within 5 months of planting (MAP) at both trial sites in Uganda, although spread was more rapid at Kamuli than at Serere. Significantly (P < 0.001) higher symptom severity scores were recorded in initially healthy plants, which had average scores of 3.6 and 3.5 at Kamuli and Serere, respectively, compared with 2.8 for mildly diseased plants at each location. Severity scores of 4 and 5 were more frequent in initially healthy plants, accounting for 77 and 39% of the total infections recorded in comparison with 47 and 11% in mildly diseased plants at Kamuli and Serere, respectively. Mildly diseased plants were significantly taller than initially healthy plants 8 and 12 MAP at both locations. However, the converse was true 4 MAP although differences were significant at Serere but not at Kamuli. Mildly diseased plants yielded significantly more tuberous roots than initially healthy plants at Kamuli but not at Serere. Average total weights of tuberous roots per plant were 2.48 and 1.63 kg for mildly diseased and initially healthy plants at Kamuli and 4.46 and 4.61 kg at Serere, respectively. These results may help to explain the increased prevalence in recent years of mildly diseased plants of local CMD‐susceptible cultivars in eastern Uganda, from where these varieties virtually disappeared following the severe CMD epidemic in the 1990s. The results also provide the first field evidence of a cross protective effect of mild strains of a cassava mosaic geminivirus.     

Heritability estimates in contrasting environments as influenced by the adaptation level of barley germ plasm

Low heritability estimates in marginal or stress environments have often been used as one of the main justification for conducting selection work in environments with optimum or near‐optimum conditions for plant growth and grain yield. In this study, we have examined the relationships between grain yield and broad‐sense heritability in four groups of recombinant inbred lines (RILs) obtained from four barley crosses derived from parents differing in adaptation to stress. The RILs and the parents were grown in 13 combinations of years and locations (environments) in Syria and Jordan. Grain yields ranged from about 30 kg ha^?1 to nearly 4000 kg ha^?1 and genotype × environment interactions explained about half of the total variance for environmentally standardised data. Broad‐sense heritability in the individual year–location combinations varied from 0 to 0.68 and both the simple correlation and the rank correlation coefficients between grain yield and heritability were not significant. Genotype × years within individual locations, which measures the repeatability of a location in discriminating between genotypes, was also independent from the yield level, confirming that low‐yielding locations can be reliable selection environments. Also, there was no relationship between the type of cross and the magnitude of heritability in the various environments, but, as expected, the magnitude of heritability was significantly associated with the genetic distance between the parents. It is concluded that, holding all other factors affecting response to selection constant, concerns about the magnitude of heritability at low‐yielding locations are not justified and should not prevent them from being used as selection sites.     

Altitudinal and environmental variation in lifespan in the Copper butterfly Lycaena tityrus

1.  Variation in longevity within and between natural populations is widespread, and understanding the relative importance of environmental and genetic factors as well as their interactions in mediating such variation is crucial in longevity research.
2.  In this study lifespan of adult copper butterflies was examined in relation to altitude, temperature (20 and 27 °C), sex and adult feeding.
3.  As expected, longevity increased with decreasing temperature, and sucrose-fed butterflies had longer lifespans compared to water-fed and finally non-fed individuals. The impact of feeding, especially of having access to water or not, was larger at the higher compared to the lower temperature.
4.  Regarding altitudinal patterns, increased lifespan in high-altitude populations was largely restricted to beneficial feeding conditions, while under carbohydrate deprivation low-altitude animals lived longer, suggesting that low-altitude butterflies do better under food stress.
5.  Differences in longevity between sexes were small at 20 °C, while females lived substantially longer than males at the higher temperature. Consequently, females may be less susceptible to high temperature stress than males. Further, males suffered more from food stress than females, suggesting that females are generally more stress resistant than males.
6.  Using a full factorial design, this study demonstrates that variation in longevity is caused by several factors, and additionally by substantial interactive effects. Consequently, patterns of variation in longevity are complex, and one needs to be cautious when neglecting this source of variation, by focussing on individual factors only.     

Serological and biological variations of African cassava mosaic virus, in Nigeria

To determine the occurrence of variants of African cassava mosaic virus, 316 cassava leaf samples were collected from mosaic‐affected cassava plants in 254 farmers. fields in 1997 and 1998, covering the humid forest, coastal/derived, southern Guinea and northern Guinea savannas and arid and semi‐arid agroecologies of Nigeria. The samples were tested in triple antibody sandwich enzyme‐linked immunosorbent assay using a panel of 10 monoclonal antibodies (MAbs) against the virus in which 29 reaction patterns were observed. In cluster analysis, nine serotypes were obtained at 0.80 Jaccard similarity coefficient index in which at least 50% of isolates of each serotype reacted alike. The serotypes ranged between two extremes: serotype 1 with 90% isolates reacting with the 10 MAbs and serotype 8 in which 90% of its isolates failed to react with the antibodies. Isolates of serotypes 1, 2, 4 and 8 were widely distributed while those of the other serotypes were estricted to certain agroecologies. Four representative isolates 227 (serotype 1), 231 (serotype 2), 235 and 283 (serotype 8) elicited different responses in Nicotiana, benthamiana, with isolate 283 not able to infect this and other test plants used. The serological variations did not necessarily reflect the biological variations. In polymerase chain reaction tests, one out of the five pairs of ACMV primers tested distinguished only isolate 283. The humid forest, derived/coastal and southern Guinea savannas where most of the crop is grown in Nigeria had a high number of variants, which makes the agroecologies suitable for the selection of resistant cassava clones against ACMV.     

Resistance to Ergot in Self-incompatible Germplasm Resources of Winter Rye

The poor progress in breeding for resistance to ergot ( Claviceps purpurea [Fr.] Tul.) in rye ( Secale cereale L.) is attributed to the lack of appreciable genotypic variation for this trait. The present study was, therefore, undertaken to evaluate 52 indigenous and exotic genetic resources and 13 open-pollinated cultivars for ergot resistance. These 65 self-incompatible populations were evaluated in four environments with each entry plot surrounded by wheat plots on four sides. Inoculation with a pathogen mixture was done thrice during flowering and harvesting was done early by hand. Resistance traits were ergot incidence and severity. Logit-transformed data were subjected to analyses of variance. The lowest disease severity in any environment was 0.18%, and that was still higher than the official threshold of 0.1% for feed. In the pooled analysis across environments, significant genotypic differences as well as genotype × environment interaction were present. The correlation between ergot severity and pollen shedding was not significant indicating that genotypic differences were not affected by pollen shedding. Broad-sense heritability ( h ² ) estimates were moderate (0.54–0.65). 'Halo', an old German cultivar, and the genetic resources 'Schmidt-Roggen' and 'Dukat', had low ergot severity (Halo having the lowest severity) as well as low disease incidence. The results document a significant, but moderate genotypic variance for ergot resistance among self-incompatible rye that can be exploited to breed ergot-resistant rye.     

Disease resistance characterisation of improved cassava genotypes to cassava mosaic disease at different ecozones

Twenty-two cassava genotypes and eight controls were evaluated in two cropping seasons for resistance to cassava mosaic disease (CMD) at the International Institute of Tropical Agriculture (IITA) fields, located at different ecozones of Nigeria. Disease incidence (DI) and index of symptom severity data were obtained monthly at each location and genotype. Symptomatic leaves were also collected during evaluation at each location, and virus was indexed by amplification in polymerase chain reaction. Significant differences within and across locations were observed in the reactions of cassava genotypes to CMD. DI across cassava genotypes was significantly (p = 0.05) highest in the Ibadan (22.6%), followed by Onne (19.3%). Generally, plants of clones 96/0860, 96/1439, 96/0160, 96/1089A, 96/1632, 96/1613, 96/1708, 96/0191, 96/0249 and 96/1565 had significantly lower values of DI in each location. African cassava mosaic virus in single infection was the predominant causal agent of CMD in IITA experimental fields under study.     

茄子花叶病新毒原

１９９１年从沈阳地区茄子花叶病叶片中分离到了ＳＹ－Ｉｓ分离物。经汁液摩擦接种７个科１６种植物,这一分离物可浸染４个科８种植物,但不侵染黄瓜、两葫芦和蚕豆,也不能经桃蚜和萝卜蚜传播。该分离物体外抗性的致死温度（ＴＩＰ）为９０－９５℃;稀释限点（ＤＥＰ）为１０￣（－６）－１０￣（－７）;体外保毒期（Ｌ）为一个月以上。光学显微镜下可见晶状胞质内含体。叶滴法电镜观察可见大小为±３００×１８ｎｍ的杆状病毒粒子。该病病叶汁液与烟草花叶病毒（ＴＭＶ）普通株抗血清呈明显阳性反应。用ＴＭＶ的一对引物进行多聚酶链反应（ＰＣＲ）,可扩增出一特异性核酸片段。根据上述实验结果,我们认为引起沈阳地区茄子花叶病的毒原种类为ＴＭＶ,是茄子花叶病的新毒原。     

利用美国SMV株系初步鉴定中国大豆鉴别寄主的抗性基因

参照美国的大豆花叶病毒(SMV)株系鉴别系统,利用G1、G2、G3、G5和G7 5个株系对24个中国鉴别寄主进行接种鉴定,初步对中国SMV大豆鉴别寄主的抗性基因进行了推断.结果表明:'Davis'、'文丰5号'、'齐黄1号'、'齐黄10号'、'徐豆1号'、'徐豆2号'、'吉林26'(PI 612720A,B)、'铁丰25'和'诱变30'的表现型与'York'相同,可能携带相同的抗性基因Rsv1-y;'合丰25'可能携带一个新的Rsv1-n基因;'吉林26'(PI 597411A)和'1138-2'(PI 592914)可能携带Rsv3基因;'吉林26'(PI 597411B)、'早18'、'早熟18'、'吉林21'、'鲁豆4号'抗所有鉴定的SMV株系,可能携带Rsv1-h、Rsv4、Rsv1+3、Rsv1+4或Rsv3+4基因.利用SMV接种鉴定的反应型是对大豆抗性基因进行初步筛选的有效方法,本研究结果对构建中国SMV株系通用鉴别系统有一定借鉴意义.     

Application of a Simplified Molecular Protocol to Reveal Mixed Infections with Begomoviruses in Cassava

Samples of cassava leaves exhibiting severe symptoms of cassava mosaic disease (CMD) were collected with the PhytoPASS kit in fields surrounding the city of Bujumbura (Burundi). These materials were then sent to Belgium for polymerase chain reaction determination of the CMD begomoviruses inducing the observed symptoms. Different pairs of specific primers were used to amplify DNA sequences specific to African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV), the Uganda variant of East African cassava mosaic virus (EACMV-UG) and South African cassava mosaic virus (SACMV). It was revealed that mixed infections were prevailing in the analyzed materials. Most of the samples submitted to this analysis were found to be co-infected by three different begomoviruses (ACMV + EACMV + EACMV-UG). The so revealed mixed infections could explain the high severity of CMD symptoms noticed on cassava in the region of Bujumbura while the diversity within the CMD causal agents illustrates the importance to take this parameter into consideration for a successful use of plant genetic resistance to control the disease.     

桑树对花叶病抗性遗传规律的研究

用７个母本、５个父本品种,按ＮＣＩＩ杂交设计,进行桑树对花叶病抗性的遗传分析。首次证明：桑树对花叶病的抗性具有数量遗传特点,符合加性－显性模型,主要受加性基因控制,广义遗传力和狭义遗传力中等,一般配合力显示绝对优势,具有超显性现象。母本品种含有比父本品种更多的显性基因     

Changes in the incidence and severity of Cassava mosaic virus disease, varietal diversity and cassava production in Uganda

A survey in 1994 assessed the incidence and severity of Cassava mosaic virus disease (CMD) in Uganda and the changes that had occurred since the previous survey in 1990–92. Three counties in each of 27 districts were selected and 15 fields were assessed per county. CMD occurred in each of the 1215 fields surveyed and the overall incidence within fields was 64% compared with 52% in the same districts in 1990–92. Mean incidence differed significantly between districts and between counties within some districts. The incidence and severity of CMD had increased in 20 and 23 of the 27 districts, respectively, compared with 1990–92, indicating the continued progress of the epidemic that was first reported in Uganda in 1988. In all but three districts, there had been a decrease in the total area of cassava. The number of different varieties that predominated in one or more fields had increased in 17 districts compared with 1990–92, although the total number of varieties encountered was little changed. Ebwanateraka was again the most widely grown variety but it was recorded less frequently and predominated or was co‐dominant in only two districts in 1994 compared with seven in 1990–92. The biggest decrease in the cultivation of Ebwanateraka had occurred in districts where there was a high incidence of CMD in 1990–92.     

Temperature-mediated patterns of local adaptation in a natural plant-pathogen metapopulation

There have been numerous investigations of parasite local adaptation, a phenomenon important from the perspectives of both basic and applied evolutionary ecology. Recent work has demonstrated that temperature has striking effects on parasite performance by mediating trade-offs in parasite life history and through genotype × environment interactions. To test whether parasite local adaptation is mediated by temperature, I measured the performance of sympatric populations against allopatric populations of a fungal pathogen, Podosphaera plantaginis , on its host Plantago lanceolata , across a temperature gradient. I used data on parasite life history and epidemiology to derive fitness estimates to measure local adaptation. The results demonstrate unambiguously that trajectories of host–parasite co-evolution are tightly coupled with parasite adaptation to the abiotic habitat, as the strength, and even direction, of local adaptation varied with temperature. Patterns of local adaptation further depended on how parasite fitness was estimated, highlighting the importance of choosing relevant fitness measures in studies of local adaptation.     

Discrimination of cassava-associated Bemisia tabaci in Africa from polyphagous populations, by PCR–RFLP of the internal transcribed spacer regions of ribosomal DNA

Restriction fragment length polymorphism (RFLP) analysis of the ribosomal DNA internal transcribed spacer regions of Bemisia tabaci was used to distinguish cassava‐associated populations from other host‐associated populations. Endonuclease restriction profile analysis indicated that cassava‐associated populations from Africa represent a distinct group, with a significant level of separation into subgroups that were not linked to geographical origin. Analysis of molecular variance (amova ) revealed that a high proportion of the total genetic variation (47%) was attributable to among‐population differences within the host‐associated groups. Principal coordinate analysis supported the differentiation between the cassava and the non‐cassava group, a result which was in agreement with the cluster analysis of the restriction fragment profile. Internal transcribed spacer RFLP markers, especially SmaI, identified in this study can be used to monitor the spread of B. tabaci biotypes, especially of the more virulent biotype B that has so far not been reported in the cassava‐growing belt of Africa.