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.