Dual begomovirus infections and high Bemisia tabaci populations: two factors driving the spread of a cassava mosaic disease pandemic

A cassava mosaic disease (CMD) pandemic currently affects much of East and Central Africa. To understand the factors driving the pandemic’s continued spread, complementary data sets were collected from cassava plots, planted with healthy cuttings, at eight sites along a north–south transect in southern Uganda, through the pandemic’s leading edge. Data were collected on virus incidence, symptom severity, populations of the whitefly vector, Bemisia tabaci , their infectivity and ability to transmit different viruses. In 1996, 6 months after planting, CMD incidences were highest at sites 1 and 2, then decreased progressively until site 6, and remained low at sites 7 and 8. The largest B. tabaci populations also occurred at northernmost sites, 1–3. In 1997, CMD incidence increased significantly at sites 5–8 and this was associated with significant increases in the B. tabaci populations. The pandemic’s spread was also associated with significant increases in the percentage of dual infections of East African cassava mosaic virus-Uganda and African cassava mosaic virus, which caused the severest symptoms and the greatest reduction in leaf area. Whitefly adults collected from within the pandemic area were infective, whereas those collected ahead of the pandemic were not. The transmission rate of African cassava mosaic virus from plants with dual infections was significantly less than that of East African cassava mosaic virus-Uganda, which may explain the latter’s predominance within the pandemic. These results show that the arrival of East African cassava mosaic virus-Uganda into areas affected previously only by African cassava mosaic virus, has resulted in novel virus/vector/host–plant interactions that drive the pandemic’s continued spread

In vitro microtuberisation of two improved Ugandan Solanum potato varieties

The performance of explants derived from three and six week-old green house-grown potato (Victoria and Kisoro) mother plants was evaluated. The mother plants were fertilized with different combinations of nitrogen, phosphorus, and potassium fertilizers. Explants were cultured on modified Murashige and Skoog's(MS) basal salts, supplemented with appropriate vitamins and growth regulators, for shoot and microtuber induction. Microtubers were induced directly from nodal cuttings inoculated on tuberisation medium and from defoliated in vitro shoots previously grown on shoot propagation medium, before transfer onto the tuberisation medium. In vitro shoot length and node number were significantly (P<0.05) increased by sole N application and retarded by sole P, or K application. Application of N along with P or K showed intermediate effects, but improved shoot growth. Single N application was inimical to microtuber forn1ation, whereas P and K promoted early tuberisation, characterised with significantly (P<0.05) laql'e diameters and fresh weights. Victoria produced bigger and fewer microtubers than Kisoro. Increased number of micro tubers occurred due to combined application ofN,P and K. Microtubers derived from defoliated in vitro shoots showed no significant (P>O.OS) difference in size due to mother plant pre-treatment, unlike the number of microtubers per explant. The six week-old nodal cuttings barely regenerated (5-15%) on the tuber induction medium, and had 30-70% regeneration on shoot induction medium. Better regeneration (60-70%) occurred in theN fertilized mother plants than with P and K (30-50%). Microtubers derived from defoliated in vitro shoots. irrcspecth·e of genotype and pre-treatment produced very few, tiny microtubers and others did not tuberize. Attempts to chemically break dormancy using gibberellic acid gave eratlc results. No significant variation was observed between the conventional mother clones and the in vitro cultures and microtubers.Keywords: Solanum tuberosum, nitrogen, phosphorus, potassium, microtubers, dormancy, gibberellic acid

Improvement of local cassava germ plasm in Uganda

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Dual infection by cassava begomoviruses in two leguminous species (Fabaceae) in Yangambi, Northeastern Democratic Republic of Congo

A study on cassava mosaic begomoviruses was conducted around Yangambi (DR Congo) by sampling 10 different leguminous species with or without symptoms similar to cassava mosaic disease. DNA was isolated to amplify CMBs using primers targeting AC2 and AC4 genes for virus detection by PCR. The results showed a dual infection by ACMV and EACMV in two weed species, Centrosema pubescens and Pueraria javanica, associated with mosaic symptoms. The DNA-A genome component of ACMV and EACMV from the infested weeds was sequenced. Seven ACMV and four EACMV isolates are reported. The major ACMV strains were closely related to ACMV-NGogo, ACMV-IC and ACMV-UGMld, whereas all EACMV strains were closely related to a Uganda variant, the most prevalent virus. This study shows that whiteflies may transmit CMBs to non-cassava plants under high epidemic pressure