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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.

Night G.,Institute des science Agronomiques du Rwanda | Asiimwe P.,University of Arizona | Gashaka G.,Institute des science Agronomiques du Rwanda | Nkezabahizi D.,Institute des science Agronomiques du Rwanda | And 8 more authors.
Agriculture, Ecosystems and Environment | Year: 2011

A survey was conducted in 2007 to obtain information on the distribution, incidence and severity of cassava pests and diseases in Rwanda, and determine how these parameters relate with cassava varieties and intercropping. Local (unimproved) cultivars predominated in most farmers' fields (over 83%) and 78% of the fields were intercropped. Cassava green mite, Mononychellus tanajoa and cassava whitefly, Bemisia tabaci, were the most abundant pests. Within-field incidence of green mite averaged 42% but damage was mild (average score of 2.3 on a scale of 5). Typhlodromalus aripo, the mite predator of green mite, was found in 28% of the fields surveyed, with a mean incidence of 5.7% within fields. The mean number of B. tabaci whitefly adults on the apical five leaves was 0.92 whereas the mean number of whitefly nymphs on a middle leaf was 5.2. Incidence of cassava mosaic disease within sites averaged 33.2%. Cutting infection accounted for 66% of infected plants but the relative contribution of cutting and whitefly infection varied among the major varieties. Mite damage as well as mosaic disease incidence and severity were higher on local varieties. However, whitefly populations were higher on improved varieties. Intercropping was associated with lower pest populations and disease incidence and severity. © 2011 Elsevier B.V.

Night G.,Cornell University | Night G.,Institute des science Agronomiques du Rwanda | Gold C.S.,Centro Internacional Of Agricultura Tropical Ciat | Power A.G.,Cornell University
International Journal of Tropical Insect Science | Year: 2010

Host plant resistance is an important tool in the management of the banana weevil Cosmopolites sordidus (Germar). Although ovipositing females do not discriminate between resistant and susceptible cultivars, plants of resistant cultivars have smaller larval populations and sustain less damage in the field. These observations suggest that lower damage levels observed in resistant cultivars reflect larval success. This study was carried out to evaluate laboratory screening of banana cultivars as a rapid screening method for resistance to the banana weevil by determining the influence of cultivars on weevil survivorship, development duration and adult weight. Larvae were raised on corm pieces of two susceptible, three intermediate and four resistant cultivars, resistance categories having been determined from a previous field screening trial. The developmental period was prolonged in resistant cultivars. However, cultivars had less influence on survivorship and adult weight. Moreover, the ranking of survivorship did not correlate with resistance levels observed in the field. The implications of these findings for laboratory screening of cultivars for resistance to C. sordidus are highlighted. Copyright © ICIPE 2010.

Night G.,Cornell University | Night G.,Institute des science Agronomiques du Rwanda | Gold C.S.,Centro Internacional Of Agricultura Tropical | Power A.G.,Cornell University
International Journal of Tropical Insect Science | Year: 2010

Use of host plant resistance for the management of the banana weevil Cosmopolites sordidus (Germar) requires development of rapid cultivar screening methods. A previous study showed prolonged development of larvae feeding on excised plant material of resistant cultivars and suggested partial breakdown of resistance in such plant material. In the current study, development and survivorship of larvae feeding on potted plants of banana cultivars with differing levels of resistance were evaluated. Weevil performance was evaluated on one susceptible (Atwalira), one intermediate (Kabula) and four resistant cultivars (FHIA 17, Kayinja, Kisubi and Yangambi Km5). Differences in development of larvae feeding on the susceptible Atwalira and intermediate/resistant cultivars became apparent at 15 days after infestation of plants. Differences in survivorship of larvae feeding on different cultivars were not statistically significant. Results of this study show that experiments using potted plants can be used to screen for resistance to banana weevil based on the development of larvae. The influence of previous feeding damage to banana plants on larval development and survivorship in a susceptible (Atwalira), an intermediate (Kabula) and a resistant cultivar (Kisubi) was also examined. While no effects on larval development rate were apparent, survivorship was reduced in previously attacked plants for the intermediate cultivar, indicating that resistance may be induced by previous weevil attack of plants. © 2010 icipe.

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