Occurrence and distribution of cassava begomoviruses in Kenya

A survey for cassava mosaic disease (CMD) was conducted in Kenya, to investigate the factors contributing to the generally increased incidence and severity of CMD in the cassava growing regions and to study the distribution of the disease's causal begomoviruses, African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV) and their strains. Special emphasis was given to the occurrence of the destructive recombinant Uganda variant strain of EACMV (EACMV-UG2). Samples from 91 farmers' fields in the main cassava-growing areas of coastal and western Kenya were collected and subjected to ELISA and PCR for detection and typing of the begomoviruses present. CMD incidence was highest in western Kenya (80–100%) and lowest in the Coast province (25–50%). In Western and Nyanza provinces, 52% of the samples tested contained EACMV-UG2, 22% ACMV and 17% contained both ACMV and EACMV-UG2. EACMV was found in four cases at different sites. In cassava samples from the coast province, only EACMV with DNA-A sequences similar to EACMV strains present in Kenya and Tanzania was found. East African cassava mosaic Zanzibar virus (EACMZV) was present in several farms in the Kilifi district. In 15% of all cassava samples with CMD symptoms, flexuous, filamentous virus-like particles were also found, providing evidence for a more complex virus situation in cassava grown at the Kenyan coast. In western Kenya, where intense cassava cultivation takes place, CMD is rampant and EACMV-UG2 was found in mixed virus infections with ACMV driving the epidemics. In coastal areas, where farms are scattered and in isolation, EACMV is endemic, however, with a lower disease incidence and with a limited impact to cassava production.     

Efficacy of chemotherapy and thermotherapy in elimination of East African cassava mosaic virus from Tanzanian cassava landrace

Cassava mosaic disease is caused by cassava mosaic begomoviruses (CMBs) and can result in crop losses up to 100% in cassava (Manihot esculenta) in Tanzania. We investigated the efficacy of chemotherapy and thermotherapy for elimination of East African cassava mosaic virus (EACMV) of Tanzanian cassava. In vitro plantlets from EACMV‐infected plants obtained from coastal Tanzania were established in the greenhouse. Leaves were sampled from the plants and tested to confirm the presence of EACMV. Plantlets of plants positive for EACMV were initiated in Murashige and Skoog (MS) medium. On the second subculture, they were subjected into chemical treatment in the medium containing salicylic acid (0, 10, 20, 30 and 40 mg/L) and ribavirin (0, 5, 10, 15 and 20 mg/L). In the second experiment, EACMV‐infected plantlets were subjected to temperatures between 35 and 40°C with 28°C as the control. After 42 days of growth, DNA was extracted from plant leaves and PCR amplification was performed using EACMV specific primers. It was found that plant survival decreased with increasing levels of both salicylic acid and ribavirin concentrations. In general, plants treated with salicylic acid exhibited a lower plant survival % than those treated with ribavirin. However, the percentage of virus‐free plants increased with an increase in the concentration of both ribavirin and salicylic acid. The most effective concentrations were 20 mg/L of ribavirin and 30 mg/L of salicylic acid; these resulted in 85.0% and 88.9% virus‐free plantlets, respectively. With regard to thermotherapy, 35°C resulted in 79.5% virus‐free plantlets compared to 69.5% at 40°C. Based on virus elimination, ribavirin at 20 mg/L, salicylic acid 30 mg/L and thermotherapy at 35°C are recommended for production of EACMV free cassava plantlets from infected cassava landraces.