Genetic diversity of whitefly (Bemisia spp.) on crop and uncultivated plants in Uganda: implications for the control of this devastating pest species complex in Africa

Journal of Pest Science - Over the past three decades, highly increased whitefly (Bemisia tabaci) populations have been observed on the staple food crop cassava in eastern Africa and associated...     

Distribution and accumulation of cassava brown streak viruses within infected cassava (Manihot esculenta) plants

Cassava brown streak disease (CBSD), caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), ranks among the top seven biological threats to global food security. The disease poses a significant threat to cassava production in East and Central Africa (ECA). In Uganda, overall CBSD incidence increased by c. 20% since it re‐emerged in 2004, and the disease persistently reduces cassava yields and storage root qualities. The spread of CBSD has been studied spatially in fields in different agroecologies. However, within‐host distribution and accumulation of CBSV and UCBSV in naturally infected cassava plants is unknown. Therefore, within‐host CBSV and UCBSV distribution was studied to correlate CBSD symptoms with virus titre in organs of infected cassava. Leaf, stem and storage root samples, with and without symptoms, were collected from 10 genotypes of field‐grown cassava. Presence of CBSV and UCBSV was detected by RT‐PCR and virus levels determined by qRT‐PCR. CBSV was present in 100% of CBSD samples with symptoms, with 45·3% positive for presence of both CBSV and UCBSV. Tolerant cassava genotypes were infected with CBSV alone and accumulated higher titre in roots than in aerial organs. Susceptible genotypes were co‐infected with CBSV and UCBSV and exhibited variation in virus titre in each organ. Across genotypes, virus titre was lowest in the youngest leaves and highest in mature non‐senescing leaves. This information provides insight into the relationship between CBSV, UCBSV and their cassava host, and is valuable for CBSD resistance breeding, epidemiology studies and CBSD control.     

African Cassava Whitefly,Bemisia tabaci,Resistance in African and South American Cassava Genotypes

The whitefly, Bemisia tabaci, is a major pest of cassava, particularly in Africa where it is responsible both for the transmission of plant viruses and, increasingly, for direct damage due to feeding by high populations. To date, there have been no practical solutions to combat this emerging problem, due to the inability of the subsistence farmers that grow cassava to afford expensive inputs such as insecticides. A programme of research was carried out linking institutes in Africa, the UK and South America, to identify possible resistance sources in cassava to the whitefly, Bemisia tabaci. The South American genotype MEcu 72 and several Ugandan cassava landraces including Ofumba Chai, Nabwire 1 and Mercury showed good levels of resistance to B. tabaci. Field and screen-house experiments showed that all of the improved, high-yielding cassava mosaic disease (CMD) resistant cassava genotypes assessed were highly susceptible to B. tabaci and supported high populations of all life stages. These data support the hypothesis that the continuing high populations of cassava B. tabaci in Uganda are due, in part, to the widespread adoption of CMD-resistant cassava varieties during the CMD pandemic. They also show that the whitefly, Aleurotrachelus socialis, resistance present in the South American cassava genotypes could have broader applicability in the Old World.     

High throughput real‐time RT‐PCR assays for specific detection of cassava brown streak disease causal viruses,and their application to testing of planting material

Cassava brown streak disease (CBSD) caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) is causing severe losses in cassava production in Kenya, Tanzania and Uganda. Two real‐time RT‐PCR assays based on TaqMan chemistry capable of detecting and distinguishing these two viruses are described. These assays were used to screen 493 cassava samples collected from western and coastal Kenya, the main cassava regions of Uganda and inland Tanzania. Both viruses were found in all three countries and across regions therein. Association of CBSD leaf symptom status with CBSV and UCBSV assay results was weak, confirming the need for a diagnostic assay. For leaf samples that were observed with CBSD‐like leaf symptoms but shown as CBSV and UCBSV negative by the RT‐PCR assay, deep sequencing using a Roche 454 GS‐FLX was used to provide additional evidence for the absence of the viruses. The probability of the CBSD associated diagnostics detecting a single CBSV or UCBSV positive sample amongst other non‐CBSD samples was modelled. The results of this study are discussed in the context of the application of diagnostics of CBSD‐associated viruses under the Great Lakes Cassava Initiative and the need to minimize the risk of further spread of the viruses with cassava multiplication material. It is shown that high throughput testing undertaken at Fera of 300 cassava leaves taken from fields for seed multiplication, when analysed in pools of 10, has given a 95% probability of detecting 1% infected plants in the field.     

Symptoms, aetiology and serological analysis of sweet potato virus disease in Uganda

Sweet potato virus disease (SPVD) is the name used to describe a range of severe symptoms in different cultivars of sweet potato, comprising overall plant stunting combined with leaf narrowing and distortion, and chlorosis, mosaic or vein-clearing. Affected plants of various cultivars were collected from several regions of Uganda. All samples contained the aphid-borne sweet potato feathery mottle potyvirus (SPFMV) and almost all contained the whitefly-borne sweet potato chlorotic stunt closterovirus (SPCSV). SPCSV was detected by a mix of monoclonal antibodies (MAb) previously shown to react only to a Kenyan isolate of SPCSV, but not by a mixture of MAb that detected SPCSV isolates from Nigeria and other countries. Sweet potato chlorotic fleck virus (SPCFV) and sweet potato mild mottle ipomovirus (SPMMV) were seldom detected in SPVD-affected plants, while sweet potato latent virus (SPLV) was never detected. Isolates of SPFMV and SPCSV obtained by insect transmissions together induced typical symptoms of SPVD when graft-inoculated to virus-free sweet potato. SPCSV alone caused stunting and either purpling or yellowing of middle and lower leaves when graft-inoculated to virus-free plants of two cultivars. Similarly diseased naturally inoculated field plants were shown consistently to contain SPCSV. Both this disease and SPVD spread rapidly in a sweet potato crop.