Comparing the regional epidemiology of the cassava mosaic and cassava brown streak virus pandemics in Africa

During November 2004, veinal chlorosis on mature cassava leaves, typical of cassava brown streak disease (CBSD), was observed at Mukono in central Uganda. Five out of 11 cultivars at the site showed CBSD symptoms (incidence range 4 to 64%). In a survey of farmers' fields, CBSD was observed in Wakiso and Mukono districts. Incidence of cassava mosaic disease was also recorded and averaged 60% for landraces (range 16.7 to 100%) and 20% for resistant varieties (range 0 to 65%). Leaf samples of plants with CBSD symptoms produced an amplicon of 222 bp using reverse transcription-polymerase chain reaction with primers that amplify a fragment of the coat protein (CP) gene of Cassava brown streak virus. Sequence comparisons based on the amplified CP gene fragment indicated that the isolates have 77 to 82.9% nucleotide and 43.9 to 56.8% amino acid identity with those from Mozambique and Tanzania. There was 95.9 to 99.5% nucleotide and 85.1 to 90.5% amino acid identity among the Ugandan isolates. These results confirm the re-emergence of CBSD in Uganda after it was first observed in the 1930s in cassava introduced from Tanzania and controlled by eradication. Prior to this report, CBSD was known to be restricted to the coastal lowlands of East Africa.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PD-91-0024

Re-emergence of Cassava Brown Streak Disease in Uganda.

Publisher Summary This chapter provides a detailed description of the character, pattern of spread and impact of the pandemic and reviews management initiatives implemented to mitigate against its effects. It assesses the status of the pandemic and recommendations for its improved management. Although cassava mosaic disease (CMD) has been an important constraint to cassava production in Africa for more than a century, changes in the nature of the disease during the last two decades have led to losses on a hitherto unprecedented scale. Strategic epidemiological studies that have traced the development and spread of what is now known as the African CMD pandemic have provided vital insights into the mechanisms and pattern through which this disease is spread and the critical interactions with its whitefly vector, B. tabaci. Based on this new knowledge, an effective and wide-ranging management programme has been implemented utilizing each of the principal virus management tools, although the primary focus has been on the deployment of host-plant resistance. There has been a substantial impact in areas where management programmes have run for several years. The best example of this is Uganda, where more than a third of the cassava crop is under CMD-resistant varieties.

Cassava mosaic virus disease in East and Central Africa: epidemiology and management of a regional pandemic.

Two whitefly species, Bemisia afer (Priesner & Hosny) and B. tabaci (Gennadius) were used in transmission experiments with Cassava brown streak virus (CBSV) (Ipomovirus; Potyviridae). Adults of whiteflies were given access to CBSV by containing them in clip cages on CBSV-infected cassava plants. Whiteflies were then transferred, together or separately, to CBSD-susceptible cassava plants of var. 'Albert' in a controlled environment. In glasshouse experiments, whiteflies were caged with CBSV-infected and virus-free cassava plants. Transmission of CBSV was sporadic and occurred in three of seven experiments when inoculated by B. afer and B. tabaci or B. tabaci alone, but not by B. afer alone. However, rate of transmission was low (maximum 22%) even when using high whitefly numbers of up to 120 per target plant. Successful transmission was confirmed by the detection of CBSV by reverse transcription-polymerase chain reaction. Spread of cassava brown streak disease (CBSD) in the field in Tanzania coincided with increases in whitefly numbers; further supporting the evidence that B. tabaci is a vector of CBSV. The results of this study establish for the first time that B. tabaci is the vector of CBSV, similar to other ipomoviruses.

Transmission of Cassava brown streak virus by Bemisia tabaci (Gennadius)

One stop mycology

A survey was conducted in southern Tanzania to determine the relationship between above‐ground symptoms of cassava brown streak virus (CBSV) infection and root necrosis which is often associated with the disease. CBSV symptoms were found in 62 of the 64 fields sampled. Seventy‐nine per cent of plants with above‐ground symptoms of CBS also exhibited root necrosis, compared with only 18% of plants with no visible symptoms of CBSV. No fungi were isolated on acidified water agar from more than half the root samples taken from plants with root necrosis. Where fungi were isolated they represented a range of common soil‐borne fungi, and no single fungus predominated. No evidence could therefore be found to indicate a cause of root necrosis other than brown streak virus. Some varietal differences in response to CBSV were apparent from the survey and at least one variety developed above‐ground symptoms but not root necrosis.

M. Raya

Papers

The association between root necrosis and above‐ground symptoms of brown streak virus infection of cassava in southern Tanzania