Cassava mosaic disease in Sub-Saharan Africa: a feasible solution for an unsolved problem

Cassava mosaic disease (CMD) is a widespread viral disease that causes high yield losses in susceptible genotypes in Sub-Saharan Africa. Effective control of CMD is beneficial and justifiable since cassava is an important source of carbohydrates for over 200 million people in most African countries where the crop is grown. Despite much research effort, CMD has not been completely brought under control, and this remains a tremendous challenge to scientists and other stakeholders. This paper highlights and discusses some of the challenges in controlling CMD and offers some solutions. The need to diversify cassava utilization as food, feed and industrial raw material is crucial for the commercialization of the crop, and this diversification will eventually lead to farmers making extensive use of improved CMD-resistant genotypes and other inputs

The status of cassava mosaic disease, cassava begomoviruses, and whitefly vector populations in Nigeria

A diagnostic survey was conducted in 2002 and 2003 to determine the status of cassava mosaic disease (CMD) and cassava mosaic begomoviruses in Nigeria and to ascertain if the virulent Ugandan variant of the East African cassava mosaic virus (EACMV-Ug2) is present in the country. The survey was an activity of a CMD project on measures to protect cassava production in Nigeria from potential devastating effects of a severe form of the disease. Routes were selected in states of the six geopolitical zones in Nigeria and 418 farmers’ cassava fields were visited. The CMD status in each field was rated as mild, moderately severe, or severe. Cassava leaf samples were collected from 1397 plants on which CMD severity was also rated on a five-point scale. Leaf samples of some weeds showing characteristic CMD-like mosaic were also collected. Whitefly counts were made in each farm and samples of this insect vector were collected. The leaf and whitefly samples were tested by polymerase chain reaction for African cassava mosaic virus (ACMV), EACMV, the Ugandan variant of EACMV (EACMV-Ug2), Indian cassava mosaic virus (ICMV), and South African cassava mosaic virus(SACMV). Cassava in most farms in the south–south and middle belt geopolitical zones showed mild CMD symptoms. On the contrary, most farms in the southeast and northeast zones showed either moderately severe or severe symptoms. The number of farms with cassava with either moderately severe or severe symptoms was about the same as the number of farms with cassava with mild symptoms in the southwest and northwest zones. Taking the entire country, 48% of the farms had cassava with either moderately severe or severe symptoms, which was about the same as the proportion of farms (52%) with cassava with mild symptoms. The farms appeared to be randomly distributed. Of the 1397 cassava leaf samples, 1106 had symptoms of which 74.1% tested positive for ACMV alone, 0.3% for EACMV alone, 24.4% for mixed infections by the two viruses, and 1.2% did not react with any of the primers used. The two viruses were also detected in 32% of the 291 symptomless plants. ACMV and EACMV were also detected in the whitefly vector samples. EACMV-Ug2, ICMV, and SACMV were not detected in any of the whitefly or leaf samples. Farms in which ACMV occurred in single infection as well as in mixed infections with EACMV predominated. Most doubly infected plants showed severe symptoms. Two biological variants of ACMV based on symptom expression on cassava in the field were identified. A high number of plants expressed symptoms indicative of mixed infections by the two biological variants and in most cases the symptoms were severe. ACMV and EACMV were detected in a leguminous plant Senna occidentalis (L.) Link and a weed Combretum confertum Lams., which are new natural hosts of the viruses. Most farms in southern Nigeria had whitefly populations of between 5 and 500 per plant compared to farms in the north, which had whitefly populations of between 0 and 4 per plant. A lot of the farms in the northeast and northwest had no whiteflies. The virulent Ugandan variant of CMD was not detected. However, the high proportion of mixed infections by ACMV and EACMV, which could result in recombination events such as the one that produced EACMV-Ug2, and the occurrence of variants of ACMV, demand appropriate measures to safeguard cassava production in the country. The CMD project is therefore timely to provide resistant cassava genotypes to Nigerian farmers to forestall any outbreak of severe CMD, either by natural spread from outside Nigeria or by spontaneous recombination between virus strains already present in the country

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

Genetic variation and genotype X environment interaction for yield and other agronomic traits in cassava in Nigeria

The identification of superior genotypes and mega-environments on the basis of multiple traits is a key objective of multi-environment trials in cassava (Manihot esculenta Crantz). The objective of this study was to examine the genetic variation and genotype 3 environment interaction (GEI) effects for fresh root yield, six other agronomic traits, and severity ratings for cassava mosaic disease (CMD) and cassava green mite (CGM) in 40 genotypes of cassava. Experiments were conducted using a randomized complete-block design with four replications for 2 yr in three representative agro-ecological zones in Nigeria. Site regression (SREG) analysis revealed that GEI was a major source of fresh root yield variation and the different testing sites discriminated among the genotypes. Genotypes TMS 980581, TMS 974763, TMS 980002, TMS 993073, and M980068 were highest yielding at Otobi and Umudike, whereas TMS 980510, TMS 974779, and TMS 92B00068 yielded the most at Ishiagu. TMS 982226, TMS 920325, and M980028 had the poorest performance across all locations. Genotypes with the highest yield showed the lowest CMD scores, whereas very tall (well above 2 m) plants had low harvest index on the basis of multiple trait analysis. We identified optimally adapted genotypes for commercial cassava production in different areas in Nigeria

Resistance profile of improved cassava germplasm to cassava mosaic disease in Nigeria

Cassava mosaic disease (CMD) caused by a group of begomoviruses and transmitted by whitefly vector is a serious disease in all the cassava-growing areas of Africa. Field evaluation with replication was conducted in 2003 and 2004 in three agroecologies in Nigeria to study the response of 40 cassava genotypes to CMD and to investigate genotype• environment (GE) interactions on their reactions to CMD, using the rank-sum classification and site regression analysis model. The 40 genotypes were separated into resistant (n= 17), moderately resistant (n= 6), moderately susceptible (n= 2) and susceptible (n= 15) groups. Environments, genotypes and GE interactions were all highly significant (P< 0.0001) for the virus disease contributing 9.5%, 71.36% and 19.14%, respectively to total variation. More than 40% of the genotypes were identified as resistant to the disease. Genotypes TMS 980581, TMS 993073, TMS 974763, TMS M980040, TMS 980505, TMS 970211, TMS 974769, TMS 992123, TMS M980068 and TMS 970162 were shown to have high resistance to CMD. The study also identified Umudike, in south-east Nigeria, as having high disease severity and the most appropriate site for CMD resistance screening of genotypes. Most of the genotypes exhibited stable resistance to CMD. The implication that the availability of these resistant genotypes as identified in this study could be a source of CMD resistance for further breeding is discussed

Variants of East African cassava mosaic virus and its distribution in double infections with African cassava mosaic virus in Nigeria

In a survey for cassava mosaic begomoviruses conducted in 1997 and 1998 in Nigeria, East African cassava mosaic virus (EACMV) was detected by the polymerase chain reaction together with African cassava mosaic virus (ACMV) in 27 out of 290 cassava leaf samples of infected plants from 254 farmers' fields in five agroecological zones. One plant was infected with EACMV only. Five variant isolates of EACMV were observed based on their reactions to primers that could detect Cameroonian and East African strains of EACMV. Isolates of variants 1 and 3 occurred mostly in the derived or coastal and southern Guinea savannahs, while variants 4 and 5 predominated in the humid forest region. Isolates of variant 2 were widely distributed across the three agroecologies. EACMV was not detected in the northern Guinea savannah and arid and semiarid zones. Most doubly infected plants showed more severe symptoms than plants with single infection. Occurrence of EACMV variants together with ACMV detection and information about their distribution in Nigeria could be used for the selection of cassava clones in cassava mosaic disease resistance programs

Status of cassava mosaic virus diseases and cassava begomoviruses in subSaharan Africa

The current status of cassava Begomoviruses, the most serious constraint to the production of cassava, a major staple food crop in sub-Saharan Africa, is reviewed in relation to their distribution, effects, etiology, and epidemiology. It is concluded that control of the diseases would continue to depend on integrated management involving cultural practices and use of resistant cultivars. Current trends in diagnosis and control, including the production of transgenic plants, selection for resistance in cassava via molecular markers, and the determination of resistant profiles of cassava genotypes to a range of virus variants, are also discussed

The occurrence of African cassava mosaic virus and East African cassava mosaic Cameroon virus in natural hosts other than cassava in Nigeria

Cassava mosaic disease (CMD) in Africa is caused by six distinct cassava mosaic viruses (family eminiviridae, genus Begomovirus). Among them, African cassava mosaic virus (ACMV) and East African cassava mosaicCameroon virus (EACMCV) have been reported in CMD-infected cassava plants in Nigeria. Based on PCR tests using virus-specific primers, a leguminous plant Senna occidentalis and the weed Combreturn confertumwere previously identified as natural hosts for ACMV and EACMCV and castor oil plant Ricinus cornmunis as a host for ACMV. In a recent study, we identified a second leguminous plant Leucana leucocephala as a host for ACMV and EACMCV. In order to confirm the PCR results, we have amplified 1 kbp DNA fragments specific to ACI and AV2 open reading frames (OW) of ACMV and AC3 and ACI ORFs of EACMCV. The amplicons were cloned and nucleotide sequences compared with corresponding sequences of ACMV and EACMCV isolates from cassava. All ACMV isolates from the four natural hosts showed sequence identity between 95 and 96 percent with cassava isolates. Similarly, all EACMCV isolates from the three natural hosts showed nucleotide sequence identity above 98 percent with isolates from cassava. The results confirm the natural occurrence of ACMV and EACMCV in four plant species other than cassava and indicate that theymay play a role in the epidemiology of CMD in Nigeria

Alternate hosts of African cassava mosaic virus and East African cassava mosaic Cameroon virus in Nigeria

Published online: 26 July 2008Cassava mosaic disease (CMD) caused by African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) is the major constraint to cassava production in Nigeria. Sequences of the DNA-A component of ACMV and EACMCV isolates from leguminous plant species (Senna occidentalis, Leucana leucocephala and Glycine max), castor oil plant (Ricinus communis), a weed host (Combretum confertum) and a wild species of cassava (Manihot glaziovii) were determined. All ACMV isolates from these hosts showed 96–98% nucleotide sequence identity with cassava isolates from West Africa. EACMCV was found only in four hosts (S. occidentalis, L. leucocephala, C. confertum, M. glaziovii), and sequences of these isolates showed 96–99% identity with cassava isolates from West Africa. These results provide definitive evidence for the natural occurrence of ACMV and EACMCV in plant species besides cassava

Growth, yield and disease responses of 12 cassava genotypes evaluated for two cropping seasons in a derived savannah zone of southeastern Nigeria

Ten new cassava genotypes selected for high yield and tolerance to some biotic stresses were evaluated alongside the local best, Otupam and ‘TMS 30572\' (a widely cultivated hybrid genotype, as an improved check) for two cropping seasons with a view to obtaining suitable genotype(s) that could be recommended for release to farmers. The results revealed that only TMS-96/0304, TMS-96/1672 and NR-930255 had above 90% sprout survival at 12 months after planting. Severity of cassava mosaic disease (CMD), cassava anthracnose disease (CAD) and cassava bacterial blight (CBB) varied among genotypes, plant age, and the cropping year. Symptom expressions of CMD and CBB were generally higher on the local best, Otupam and NR-930255. The severity of cassava green mite was moderate and statistically similar on all the genotypes while severity of cassava mealybug was mild. Hybrids NR930255, TMS96/1672 and TMS96/0304 produced the highest number of tubers. These genotypes also significantly (P< 0.05) produced the highest harvested tuber yield per hectare: NR 930255 (31.0 t/ha), TMS 96/1672 (28.0 t/ha) and TMS 96/0304 (29.4 t/ha). The three genotypes had the least percent yield difference between the potential and the actual yield. Mean rank-sum for all traits measured suggested that the most adaptable genotypes to Nsukka agro-ecology after two years of evaluation were NR-930255, TMS 96/0304 and TMS96/1672, but the poor disease ratings for NR-930255 and TMS 96/0304 precluded their recommendation for release while TMS 96/1672 could be recommended for release to farmers