Whitefly Bemisia tabaci (Homoptera: Aleyrodidae) infestation on cassava genotypes grown at different ecozones in Nigeria

Large-scale screening of cassava, Manihot esculenta Crantz, genotypes for resistance to infestation by whitefly Bemisia tabaci Gennadius, the vector of cassava mosaic geminiviruses, is limited. A range of new cassava elite clones were therefore assessed for the whitefly infestation in the 1999/2000 and 2000/2001 cropping seasons in experimental fields of International Institute of Tropical Agriculture, Ibadan, Nigeria. On each scoring day, between 0600 and 0800 hours when the whiteflies were relatively immobile, adult whitefly populations on the five topmost expanded leaves of cassava cultivars were counted. All through the 6-mo scoring period, there was a highly significant difference in whitefly infestation among the new cassava elite clones. Vector population buildup was observed in Ibadan (forest-savanna transition zone) and Onne (humid forest), 2 mo after planting (MAP). Mean infestation across cassava genotypes was significantly highest (16.6 whiteflies per plant) in Ibadan and lowest in Zaria (0.2). Generally, whitefly infestation was very low in all locations at 5 and 6 MAP. During this period, cassava genotypes 96/1439 and 91/02324 significantly supported higher infestations than other genotypes. Plants of 96/1089A and TMS 30572 supported the lowest whitefly infestation across cassava genotypes in all locations. The preferential whitefly visitation, the differences between locations in relation to whitefly population, cassava mosaic disease, and the fresh root yield of cassava genotypes are discussed.     

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.     

Occurrence and distribution of cassava begomoviruses in Kenya

A survey for cassava mosaic disease (CMD) was conducted in Kenya, to investigate the factors contributing to the generally increased incidence and severity of CMD in the cassava growing regions and to study the distribution of the disease's causal begomoviruses, African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV) and their strains. Special emphasis was given to the occurrence of the destructive recombinant Uganda variant strain of EACMV (EACMV-UG2). Samples from 91 farmers' fields in the main cassava-growing areas of coastal and western Kenya were collected and subjected to ELISA and PCR for detection and typing of the begomoviruses present. CMD incidence was highest in western Kenya (80–100%) and lowest in the Coast province (25–50%). In Western and Nyanza provinces, 52% of the samples tested contained EACMV-UG2, 22% ACMV and 17% contained both ACMV and EACMV-UG2. EACMV was found in four cases at different sites. In cassava samples from the coast province, only EACMV with DNA-A sequences similar to EACMV strains present in Kenya and Tanzania was found. East African cassava mosaic Zanzibar virus (EACMZV) was present in several farms in the Kilifi district. In 15% of all cassava samples with CMD symptoms, flexuous, filamentous virus-like particles were also found, providing evidence for a more complex virus situation in cassava grown at the Kenyan coast. In western Kenya, where intense cassava cultivation takes place, CMD is rampant and EACMV-UG2 was found in mixed virus infections with ACMV driving the epidemics. In coastal areas, where farms are scattered and in isolation, EACMV is endemic, however, with a lower disease incidence and with a limited impact to cassava production.     

Genetic mapping of a dominant gene conferring resistance to cassava mosaic disease

Cassava mosaic disease (CMD) is the most-important disease of cassava (Manihot esculenta) in Africa, and is a potential threat to Latin American (LA) cassava production. Although this viral disease is still unknown in LA, its vector - the whitefly - has recently been found. The disease is best controlled through host-plant resistance, which was first found in third backcross derivatives of an interspecific cross between cassava and Manihot glaziovii, and is thought to be polygenic. Recently, high levels of resistance were also found in several Nigerian cassava landraces. Classical genetic analysis and molecular genetic-mapping of the landraces showed that a major dominant gene confers this resistance. Bulk segregant analysis (BSA) was used to quickly identify a simple sequence repeat (SSR) marker linked to the CMD-resistance gene. The marker, SSRY28, is located on linkage group R of the male-parent-derived molecular genetic map. The gene, designated as CMD2, is flanked by the SSR and RFLP marker GY1 at 9 and 8 cM, respectively. To our knowledge, this is the first report of qualitative virus resistance in cassava, and of molecular markers that tag CMD resistance in cassava. We discuss the use of markers linked to CMD2 for marker-assisted breeding of CMD resistance in Latin America and for increasing the cost-effectiveness of resistance breeding in Africa.     

Genetic diversity and geographic distribution of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) genotypes associated with cassava in East Africa

The genetic variability of whitefly (Bemisia tabaci) species, the vectors of cassava mosaic begomoviruses (CMBs) in cassava growing areas of Kenya, Tanzania, and Uganda, was investigated through comparison of partial sequences of the mitochondria cytochrome oxidase I (mtCOI) DNA in 2010/11. Two distinct species were obtained including sub‐Saharan Africa 1 (SSA1), comprising of two sub‐clades (I and II), and a South West Indian Ocean Islands (SWIO) species. Among the SSA1, sub‐clade I sequences shared a similarity of 97.8–99.7% with the published Uganda 1 genotypes, and diverged by 0.3–2.2%. A pairwise comparison of SSA1 sub‐clade II sequences revealed a similarity of 97.2–99.5% with reference southern Africa genotypes, and diverged by 0.5–2.8%. The SSA1 sub‐clade I whiteflies were widely distributed in East Africa (EA). In comparison, the SSA1 sub‐clade II whiteflies were detected for the first time in the EA region, and occurred predominantly in the coast regions of Kenya, southern and coast Tanzania. They occurred in low abundance in the Lake Victoria Basin of Tanzania and were widespread in all four regions in Uganda. The SWIO species had a sequence similarity of 97.2–97.7% with the published Reunion sequence and diverged by 2.3–2.8%. The SWIO whiteflies occurred in coast Kenya only. The sub‐Saharan Africa 2 whitefly species (Ug2) that was associated with the severe CMD pandemic in Uganda was not detected in our study.     

Loss of CMD2‐mediated resistance to cassava mosaic disease in plants regenerated through somatic embryogenesis

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two most important viral diseases affecting cassava production in Africa. Three sources of resistance are employed to combat CMD: polygenic recessive resistance, termed CMD1, the dominant monogenic type, named CMD2, and the recently characterized CMD3. The farmer‐preferred cultivar TME 204 carries inherent resistance to CMD mediated by CMD2, but is highly susceptible to CBSD. Selected plants of TME 204 produced for RNA interference (RNAi)‐mediated resistance to CBSD were regenerated via somatic embryogenesis and tested in confined field trials in East Africa. Although micropropagated, wild‐type TME 204 plants exhibited the expected levels of resistance, all plants regenerated via somatic embryogenesis were found to be highly susceptible to CMD. Glasshouse studies using infectious clones of East African cassava mosaic virus conclusively demonstrated that the process of somatic embryogenesis used to regenerate cassava caused the resulting plants to become susceptible to CMD. This phenomenon could be replicated in the two additional CMD2‐type varieties TME 3 and TME 7, but the CMD1‐type cultivar TMS 30572 and the CMD3‐type cultivar TMS 98/0505 maintained resistance to CMD after passage through somatic embryogenesis. Data are presented to define the specific tissue culture step at which the loss of CMD resistance occurs and to show that the loss of CMD2‐mediated resistance is maintained across vegetative generations. These findings reveal new aspects of the widely used technique of somatic embryogenesis, and the stability of field‐level resistance in CMD2‐type cultivars presently grown by farmers in East Africa, where CMD pressure is high.     

Progress of cassava mosaic virus disease and whitefly vector populations in single and mixed stands of four cassava varieties grown under epidemic conditions in Uganda

Progress curves of cassava mosaic virus disease (CMD) and populations of the whitefly vector (Bemisia tabaci) were assessed using four cassava varieties grown alone and as a random mixture in two experiments established under epidemic conditions at a site near Kampala in southern Uganda. There were significant differences in final CMD incidence and in the areas under the disease progress curves between varieties when grown alone and as a mixture in both experiments. Variety Ebwanateraka had the highest incidence and SS4 the lowest, even though it supported the largest populations of adult whiteflies. The overall incidence of CMD in the mixture was similar to that in pure stands of the partially resistant Nase 2 and greater than in the resistant Migyera and SS4. Compared to pure stands, incidence of CMD in each component of the mixture was reduced significantly only in Ebwanateraka, whereas vector populations were less only in SS4 and Nase 2. On several observation dates the actual incidence of CMD and populations of adult whiteflies in the mixture were significantly less than expected values estimated from the results for the four varieties when each was grown alone. A highly significant positive relationship was established for each variety between peak populations of adult whitefly and leaf area index at the time. The implications of the findings and the scope for future research on the use of varietal mixtures for the management of CMD are discussed.     

Abundance,diversity and geographic distribution of cassava mosaic disease pandemic‐associated Bemisia tabaci in Tanzania

Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), one of the most economically important agricultural pests worldwide, is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD). In East and Central Africa, a severe CMD pandemic that spread from Uganda in the late 1980s still continues to devastate cassava crops. To assess the association of distinct B. tabaci genetic groups with the CMD pandemic, mitochondrial cytochrome oxidase I gene sequences were analysed from whiteflies collected during surveys conducted from 2010 to 2013 in Tanzania. Four genetic groups – Sub‐Saharan Africa 1 (SSA1), Mediterranean, Indian Ocean and East Africa 1, and a group of unknown whitefly species were identified. SSA1 comprised four subgroups: SSA1‐SG1, SSA1‐SG2, SSA1‐SG1/2 and SSA1‐SG3. SSA1‐SG1 was confined to the pandemic‐affected north‐western parts of Tanzania whilst SSA1‐SG2 and SSA1‐SG3 were found in the central and eastern parts not yet affected by the pandemic. The CMD pandemic front was estimated to lie in Geita Region, north‐western Tanzania, and to be spreading south‐east at a rate of ca 26 km/year. The pandemic‐associated B. tabaci SSA1‐SG1 predominated up to 180 km ahead of the CMD front indicating that changes in whitefly population characteristics precede changes in disease characteristics.     

Temporal spread of cassava mosaic virus disease in a range of cassava cultivars in different agro-ecological regions of Uganda

The spread of cassava mosaic disease (CMD) in a range of cassava cultivars was studied in experiments and on-farm trials in different agro-ecological regions of Uganda in 1989–1990 and 1990–1991. No spread occurred in either experiment at the southernmost site near Kampala, but there was considerable spread at the four sites elsewhere and also in the on-farm trials in Luwero district. There were significant differences in the final incidence of disease between locations and between cultivars at each location. Where spread occurred it was more rapid in the Ugandan cvs Ebwanateraka, Senyonjo and Bao than in four of the five improved TMS cultivars introduced from Nigeria. These usually showed an apparent decline in incidence of CMD after reaching maxima 4 to 8 months after planting (MAP). The areas under the disease progress curves (AUDPCs) differed significantly between locations and cultivars and were less for cvs TMS 30572, TMS 30395, TMS 30337 and TMS 60142 than for cvs Ebwanateraka, Senyonjo, Bao and TMS 30786. Overall, the mean AUDPCs were greatest at Migyera in Luwero district in 1989–1990 and at Kagando in Kasese district in 1990–1991. They were significantly less at Mubuku in Kasese district in 1989–1990 than at the other two experimental sites where spread occurred. Adult whitefly vector populations were highest at Migyera and Kagando in the 1989–1990 and 1990–1991 trials, respectively, and they were higher on cvs Bao, Ebwanateraka and TMS 30786 than on other varieties. Mean numbers of adults increased until 3–5 MAP and then declined, but CMD incidence increased progressively to reach maxima at or near crop maturity. Locations with the largest numbers of adults also had a relatively high incidence of CMD. Symptoms of CMD were usually more severe on cvs Ebwanateraka, Bao and Bukalasa 11 than on the TMS cultivars, on which symptoms remained slight throughout growth and usually decreased from 5 MAP. The differences between sites, the resistance of the cultivars and the relationship between CMD incidence and whitefly populations are discussed.     

Reaction of Cassava Genotypes to the Cassava Mosaic Disease in Three Distinct Agroecologies in Nigeria

Nine cassava genotypes were grown at six representative sites in Nigeria for 3 years to study their response to cassava mosaic disease (CMD), investigate the influence of genotype × environment (G × E) interactions on their reactions to the disease, and identify genotypes with stability to the disease, using the Additive Main Effects and Multiplicative Interaction statistical model. Environments, genotypes and G × E interactions were highly significant (P < 0.01) for the disease. The G × E interactions accounted for 19.5% of the treatment sums of squares for CMD and influenced the relative ranking of genotypes across environments. The magnitude of the G × E interaction effect for CMD was larger than that of genotypes. Examination of the G × E interaction structure revealed specific areas where screening of cassava genotypes for resistance to CMD could be performed best. The study identified genotypes such as TMS 30001 and 63397 with resistance to CMD and CMD‐stable clone U/41044, which could be distributed to growers, and sites such as Ibadan and Ubiaja with high CMD severity for screening genotypes for reaction to CMD.     

Restriction of Virus Movement into Axillary Buds is an Important Aspect of Resistance in Cassava to African cassava mosaic virus

Axillary buds and bark samples of resistant, moderately resistant and susceptible (control) cassava genotypes either naturally infected under field conditions or experimentally inoculated by grafting were indexed for African cassava mosaic virus (ACMV). Virus detection was carried out using enzyme‐linked immunosorbent assay and polymerase chain reactions to determine the distribution of the virus within the plant and elucidate the genotypes response to virus movement. Significantly more bud and bark samples were positive for virus on the susceptible genotype TME 117 than resistant genotypes TMS 30001 and TMS 91/02319, or the moderately resistant genotype TMS 30572. Detectable virus concentration was significantly lower in the buds of moderately resistant and resistant genotypes than the susceptible control. Under field conditions, it was significant that more primary stem buds were infected than the buds of secondary and tertiary stems but such a gradient was not obvious with bark samples. Shoots that had asymptomic new leaves after the initial symptomatic leaves had no virus in their buds, but some of the bark samples from the same plants tested positive. A significant interaction was observed between year and stem type, and among year, genotype and stem type with respect to virus detection in bud and bark samples. Restriction of virus movement into axillary buds occurred in all the resistant and moderately resistant genotypes. This may explain ACMV‐infected stem cuttings of resistant genotypes producing healthy plants in subsequent generation.     

Current characteristics of cassava mosaic disease in postepidemic areas increase the range of possible management options

As a case study to document the current characteristics of cassava mosaic virus disease (CMD) in postepidemic areas, surveys were carried out, in 2003 and 2004, in Siaya District of western Kenya. This was an area affected by a severe CMD pandemic in the late 1990s. Data recorded on cassava varieties were CMD incidence, severity index and number of adult whiteflies. Farmers (174) were interviewed on their understanding of the disease and their knowledge and practice of management interventions. Cassava cultivation was being re‐established, but local landraces predominated. Resistant varieties were present 13% in 2003, and 4% in 2004, of the surveyed fields. Adhiambolera was the most common variety, occurring in 35% and 40% of fields in 2003 and 2004, respectively, and had an average CMD incidence of 82% in 2003 and 73% in 2004. By contrast, the CMD‐resistant variety Migyera had a low mean incidence (28% in 2003). The overall incidence for both years was 71%, consisting of 61% as a result of infection through planting diseased cuttings and 10% as a result of whitefly infection. In 2003, the total incidence was 72% and the average severity 2.7 (severity index), while in 2004 the incidence was 78% and the severity 2.6. There were significant severity variations in each division of the Siaya District during the 2 years except for Karemo and Ukwala. The abundance of whiteflies on the top five leaves of plants was low in 2003 but high in 2004, with means of 1 and 16, respectively, over the same seven divisions in both years, although this variation was thought to be because of seasonal factors. East African cassava mosaic virus‐Uganda was the predominant geminivirus present in every division. Phytosanitation by farmers was minimal, as evidenced by 29% of farmers using a selection of CMD‐free stems for planting and 15% using hand‐roguing for CMD management. Occurrence of more than 25% CMD‐free plants in 2004, moderate CMD severity and limited spread provide a conducive environment for the use of phytosanitation as a CMD control measure that can be immediately used by farmers growing their own cassava varieties.     

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.     

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.     

Virus content of leaves of cassava infected by African cassava mosaic virus

African cassava mosaic virus (ACMV) was detected in cassava leaves by ELISA. Some normal constituents of cassava leaves interfered with virus detection but leaf extracts of Nicotiana benthamiana did not. The symptom pattern was determined early in the growth of a leaf and subsequently changed little. ACMV was found only in the yellow or yellow green areas of the mosaic pattern. Virus content of the leaves increased with increasing symptom intensity, but decreased with leaf age and ACMV was not detected in mature leaves. Most whiteflies were found on young growing cassava leaves and the number decreased progressively with leaf age. This distribution will aid both the acquisition and inoculation of the virus.     

Factors determining recovery and reversion in mosaic-affected African cassava mosaic virus resistant cassava

The severity and persistence of symptoms of mosaic virus disease were monitored during the first six months of two growing seasons in cassava of the African cassava mosaic virus (ACMV)-resistant cv. TMS 30572 either inoculated by grafting with a mild or severe strain or infected from the planted cutting. Symptomless shoots developed between January and March 1995 in two field trials differing in age by c. 6 months; this recovery occurred during particularly hot weather. Recovery was often only temporary in the plants inoculated with the severe strain and occurred later compared with those inoculated with the mild. In 1996, the weather was cooler and recovery that year was delayed until flowering, c. 7 months after planting, when recovered shoots were often produced from buds in the axils of symptomless leaves produced amongst diseased leaves. Most cuttings taken from the upper parts of diseased plants produced symptomless (reverted) progenies whereas most cuttings taken from the base of diseased plants produced diseased progenies. Reversion seemed to be associated with the recovery that had already occurred in the upper stems of the parent plants.     

Efficacy of chemotherapy and thermotherapy in elimination of East African cassava mosaic virus from Tanzanian cassava landrace

Cassava mosaic disease is caused by cassava mosaic begomoviruses (CMBs) and can result in crop losses up to 100% in cassava (Manihot esculenta) in Tanzania. We investigated the efficacy of chemotherapy and thermotherapy for elimination of East African cassava mosaic virus (EACMV) of Tanzanian cassava. In vitro plantlets from EACMV‐infected plants obtained from coastal Tanzania were established in the greenhouse. Leaves were sampled from the plants and tested to confirm the presence of EACMV. Plantlets of plants positive for EACMV were initiated in Murashige and Skoog (MS) medium. On the second subculture, they were subjected into chemical treatment in the medium containing salicylic acid (0, 10, 20, 30 and 40 mg/L) and ribavirin (0, 5, 10, 15 and 20 mg/L). In the second experiment, EACMV‐infected plantlets were subjected to temperatures between 35 and 40°C with 28°C as the control. After 42 days of growth, DNA was extracted from plant leaves and PCR amplification was performed using EACMV specific primers. It was found that plant survival decreased with increasing levels of both salicylic acid and ribavirin concentrations. In general, plants treated with salicylic acid exhibited a lower plant survival % than those treated with ribavirin. However, the percentage of virus‐free plants increased with an increase in the concentration of both ribavirin and salicylic acid. The most effective concentrations were 20 mg/L of ribavirin and 30 mg/L of salicylic acid; these resulted in 85.0% and 88.9% virus‐free plantlets, respectively. With regard to thermotherapy, 35°C resulted in 79.5% virus‐free plantlets compared to 69.5% at 40°C. Based on virus elimination, ribavirin at 20 mg/L, salicylic acid 30 mg/L and thermotherapy at 35°C are recommended for production of EACMV free cassava plantlets from infected cassava landraces.     

Exploiting the Combination of Natural and Genetically Engineered Resistance to Cassava Mosaic and Cassava Brown Streak Viruses Impacting Cassava Production in Africa

Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) are currently two major viral diseases that severely reduce cassava production in large areas of Sub-Saharan Africa. Natural resistance has so far only been reported for CMD in cassava. CBSD is caused by two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). A sequence of the CBSV coat protein (CP) highly conserved between the two virus species was used to demonstrate that a CBSV-CP hairpin construct sufficed to generate immunity against both viral species in the cassava model cultivar (cv. 60444). Most of the transgenic lines showed high levels of resistance under increasing viral loads using a stringent top-grafting method of inoculation. No viral replication was observed in the resistant transgenic lines and they remained free of typical CBSD root symptoms 7 month post-infection. To generate transgenic cassava lines combining resistance to both CBSD and CMD the hairpin construct was transferred to a CMD-resistant farmer-preferred Nigerian landrace TME 7 (Oko-Iyawo). An adapted protocol allowed the efficient Agrobacterium-based transformation of TME 7 and the regeneration of transgenic lines with high levels of CBSV-CP hairpin-derived small RNAs. All transgenic TME 7 lines were immune to both CBSV and UCBSV infections. Further evaluation of the transgenic TME 7 lines revealed that CBSD resistance was maintained when plants were co-inoculated with East African cassava mosaic virus (EACMV), a geminivirus causing CMD. The innovative combination of natural and engineered virus resistance in farmer-preferred landraces will be particularly important to reducing the increasing impact of cassava viral diseases in Africa.