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Ndyetabula I.L.,Maruku Agricultural Research Institute | Merumba S.M.,Maruku Agricultural Research Institute | Jeremiah S.C.,Ukiriguru Agricultural Research Institute | Kasele S.,Ukiriguru Agricultural Research Institute | And 3 more authors.
Plant Disease | Year: 2016

Cassava brown streak disease (CBSD), caused by cassava brown streak viruses, is recognized as one of the most important plant disease threats to African food security. This study describes the incidence and severity of the different symptom types caused by CBSD, derived from extensive surveys in the country most severely affected by the disease: Tanzania. Total plant incidence and mean root severity of CBSD, recorded from 341 farmers’ fields, were both greater in the Coast Zone (49.5% and 3.05), than in the Lake Zone (32.7% and 2.57). Overall, the differing incidences recorded declined in the following order: total plant incidence (39.1%), plant shoot incidence (33.4%), plant root incidence (19.3%), root incidence (10.5%), and unusable root incidence (5.4%). The much lower-than-anticipated loss due to the root necrosis that is characteristic of CBSD was offset by large reductions of root number in plants expressing foliar symptoms of CBSD (15.7% in the Coast Zone and 5.5% in the Lake Zone). These data suggest that the effects of CBSD on the growth of affected plants are greater than those due to root spoilage. Based on these two factors, annual losses due to CBSD in the parts of Tanzania surveyed were estimated at >860,000 t, equivalent to more than U.S.$51 million. A novel approach to using farm-derived data on the responses of the most frequently cultivated varieties to CBSD infection allowed the grouping of the varieties into four categories, based on their relative resistance or tolerance to infection. This tool should be of value to breeders in identifying and selecting for sources of resistance or tolerance in both local and exotic germplasm, and should ultimately contribute to enhancing the management of one of Africa’s most damaging crop diseases. © 2016 The American Phytopathological Society.

Kulembeka H.P.,Ukiriguru Agricultural Research Institute | Ferguson M.,International Institute Of Tropical Agriculture | Herselman L.,University of the Free State | Kanju E.,International Institute Of Tropical Agriculture | And 4 more authors.
Euphytica | Year: 2012

Cassava brown streak disease (CBSD) is an economically important virus disease causing significant losses to cassava root yield and quality in east, central and southern Africa. Breeding for resistance in cassava requires an understanding of the underlying genetic control of CBSD resistance. Sources of CBSD resistance are available but little is known on the value of those varieties as parents for CBSD resistance breeding. Two resistant and two susceptible varieties were crossed in a half diallel design and 35 F 1 progeny from each of the six families, plus parents, were screened at two locations in a randomised complete block design with four replications in warm sub-humid environments of coastal Tanzania in 2008. Screening for CBSD field resistance was done using disease severity scoring on a scale of 1-5. Significant variations in disease severity were observed for crosses, general combining ability (GCA) and specific combining ability (SCA) at both locations. The CBSD shoot symptom severity scores ranged from 1 to 4.4 while root necrosis ranged from 1. 3 to 4.5. The contribution of GCA to the total sum of squares of crosses for disease scores ranged from 86.9 to 95.2 % compared to SCA that ranged from 4.8 to 14.2 %. Additive gene effects were more important than non-additive effects indicating that CBSD resistance is genetically determined and that selection should be successful to improve resistance. Selection of parents with good GCA effects will be important for success in CBSD resistance breeding. © 2012 Springer Science+Business Media B.V.

Kabanza A.K.,Naliendele Agricultural Research Institute | Kabanza A.K.,Catholic University of Leuven | Dondeyne S.,Catholic University of Leuven | Kimaro D.N.,Sokoine University of Agriculture | And 3 more authors.
Zeitschrift fur Geomorphologie | Year: 2013

RUSLE, the revised universal soil loss equation, is widely used for estimating potential soil erosion by water. Field measured model factors are however scarce for tropical regions. We derived RUSLE factors for several soil conservation measures based on three seasons of field plot measurements (2008-2010), in two contrasting landscape units of South Eastern Tanzania, the country's most important cashew growing area. Whereas the derived factors are useful for applying RUSLE in similar environments, this study points to the importance of understanding differences between soil types and landscape units when assessing potential soil erosion. On the Makonde plateau, rainfall erosivity was higher than on the inland plains (7,130 vs 5,783 MJmmha -1 h-1 year-1). The soil erodility K factor was also higher (0.014th MJ-1mm-1) on the sandy Cutanic Acrisols of the Makonde plateau than on the clayey Acric Ferralsols (0.006th MJ-1mm-1) of the inland plains. Likewise, soil loss on the Makonde plateau was much higher than on the inland plain (e. g. for maize 33-127 t ha-1 season-1 vs 3-10 t ha-1 season-1). The differences between the C factor for "maize", and for "maize with crop residues", as well as the differences between the P factors for "lemon grass strips" and "ridges and furrows" indicate that although soils of the Makonde plateau are more susceptible to soil erosion, these soils are also more responsive to soil conservation measures compared to soils of the inland plains. Farmers' local technique of making "ridges and furrows" is particularly effective. Furthermore, the C factors for cashew groves are one order of magnitude lower (0.08-0.09) than for "maize", or for "maize with crop residues" (0.2-0.7), but are still much higher than for "bush fallow" (0.001). Deforestation for agriculture hence bears the risk of increasing soil erosion rates; however, this risk can be minimised by growing cashew trees. © 2013 Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany.

Bredeson J.V.,University of California at Berkeley | Egesi C.N.,National Root Crops Research Institute NRCRI | Esuma W.,National Crops Resources Research Institute NaCCRI | Ezenwaka L.,National Root Crops Research Institute NRCRI | And 37 more authors.
G3: Genes, Genomes, Genetics | Year: 2015

Cassava (Manihot esculenta Crantz) is a major staple crop in Africa, Asia, and South America, and its starchy roots provide nourishment for 800 million people worldwide. Although native to South America, cassava was brought to Africa 400-500 years ago and is now widely cultivated across sub-Saharan Africa, but it is subject to biotic and abiotic stresses. To assist in the rapid identification of markers for pathogen resistance and crop traits, and to accelerate breeding programs, we generated a framework map for M. esculenta Crantz from reduced representation sequencing [genotyping-by-sequencing (GBS)]. The composite 2412-cM map integrates 10 biparental maps (comprising 3480 meioses) and organizes 22,403 genetic markers on 18 chromosomes, in agreement with the observed karyotype. We used the map to anchor 71.9% of the draft genome assembly and 90.7% of the predicted protein-coding genes. The chromosome-anchored genome sequence will be useful for breeding improvement by assisting in the rapid identification of markers linked to important traits, and in providing a framework for genomic selectionenhanced breeding of this important crop. © 2015 International Cassava Genetic Map Consortium (ICGMC).

Legg J.P.,IITA Tanzania | Jeremiah S.C.,IITA Tanzania | Jeremiah S.C.,Ukiriguru Agricultural Research Institute LZARDI | Obiero H.M.,Kenya Agricultural Research Institute | And 10 more authors.
Virus Research | Year: 2011

The rapid geographical expansion of the cassava mosaic disease (CMD) pandemic, caused by cassava mosaic geminiviruses, has devastated cassava crops in 12 countries of East and Central Africa since the late 1980s. Region-level surveys have revealed a continuing pattern of annual spread westward and southward along a contiguous 'front'. More recently, outbreaks of cassava brown streak disease (CBSD) were reported from Uganda and other parts of East Africa that had been hitherto unaffected by the disease. Recent survey data reveal several significant contrasts between the regional epidemiology of these two pandemics: (i) severe CMD radiates out from an initial centre of origin, whilst CBSD seems to be spreading from independent 'hot-spots'; (ii) the severe CMD pandemic has arisen from recombination and synergy between virus species, whilst the CBSD pandemic seems to be a 'new encounter' situation between host and pathogen; (iii) CMD pandemic spread has been tightly linked with the appearance of super-abundant Bemisia tabaci whitefly vector populations, in contrast to CBSD, where outbreaks have occurred 3-12 years after whitefly population increases; (iv) the CMGs causing CMD are transmitted in a persistent manner, whilst the two cassava brown streak viruses appear to be semi-persistently transmitted; and (v) different patterns of symptom expression mean that phytosanitary measures could be implemented easily for CMD but have limited effectiveness, whereas similar measures are difficult to apply for CBSD but are potentially very effective. An important similarity between the pandemics is that the viruses occurring in pandemic-affected areas are also found elsewhere, indicating that contrary to earlier published conclusions, the viruses per se are unlikely to be the key factors driving the two pandemics. A diagrammatic representation illustrates the temporal relationship between B. tabaci abundance and changing incidences of both CMD and CBSD in the Great Lakes region. This emphasizes the pivotal role played by the vector in both pandemics and the urgent need to identify effective and sustainable strategies for controlling whiteflies on cassava. © 2011 Elsevier B.V.

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