Analysis of cassava brown streak viruses reveals the presence of distinct virus species causing cassava brown streak disease in East Africa

Abstract

Cassava brown streak virus (CBSV) isolates were analysed from symptomatic cassava collected between 1997 and 2008 in the major cultivation regions of East Africa. An analysis of complete RNA genomes of seven isolates from Kenya, Tanzania, Mozambique, Uganda and Malawi revealed a common genome structure, but the isolates clearly clustered in two distinct clades. The first comprised isolates from Kenya, Uganda, Malawi, north-western Tanzania and the CBSV described previously, and shared between 87 and 95 % nucleotide sequence identity, whilst the second included isolates from coastal regions of Mozambique and Tanzania, which shared only 70 % nucleotide sequence identities with isolates of the first clade. When the amino acid sequences of viral proteins were compared, identities as low as 47 % (Ham1) and 59 % (P1) between the two clades were found. An antiserum obtained against the capsid protein of a clade 1 isolate identified a 43 kDa protein in clade 1 isolates and a 45 kDa protein in clade 2 isolates. Several cassava cultivars were susceptible to isolates of clade 2 but resistant to those of clade 1. The differences observed both in biological behaviour and in genomic and protein sequences indicate that cassava brown streak disease in East Africa is caused by at least two distinct virus species. It is suggested that those of clade 1 retain the species name Cassava brown streak virus, whilst those of clade 2 be classified as Cassava brown streak Mozambique virus.

Characterization of Brown Streak Virus–Resistant Cassava

“…CBSV and UCBSV differ in their virulence on cassava cultivars (Winter et al, 2010;Mohammed et al, 2012). We therefore used different combinations of CBSV and UCBSV isolates to assess the stability of the CBSD resistance.…”

“…Leaf and root symptoms triggered by CBSD infection vary largely in terms of localization and intensity in susceptible and tolerant cassava varieties (Winter et al, 2010;Mohammed et al, 2012). Molecular tools available today for detecting and discriminating CBSV species offer new opportunities to better determine the tolerance and resistance levels to CBSVs as well as to investigate cassava response to CBSD (Abarshi et al, 2010;Mbanzibwa et al, 2011a;Moreno et al, 2011;Abarshi et al, 2012;Tomlinson et al, 2013).…”

“…Sequencing efforts of CBSD infected cassava samples from different regions in East Africa led to the identification of two viral species (Mbanzibwa et al, 2009b;Monger et al, 2010;Winter et al, 2010;Ndunguru et al, 2015) now referred to as Cassava brown streak virus (CBSV), and Ugandan cassava brown streak virus (UCBSV). Despite their initial identification from distinct geographical zones, co-occurrence of CBSV and UCBSV has been reported and recent outbreaks of CBSD are not uniquely associated with a particular virus species Mbanzibwa et al, 2011a).…”

Occurrence of cassava brown streak disease and associated Cassava brown streak virus and Ugandan cassava brown streak virus in the Comoros Islands

“…The expanding CBSD epidemic in East and Central Africa is caused by two ipomoviruses (family Potyviridae), Cassava brown streak virus (CBSV; Monger et al, 2010) and Ugandan cassava brown streak virus (UCBSV; Winter et al, 2010). In 2014, symptoms of yellowing on leaves and brown necrotic rot on tuberous roots on different cassava landraces on the west coast of Mayotte (Maoré) were associated with CBSD and UCBSV (Roux-Cuvelier et al, 2014).…”

RNAi-derived field resistance to Cassava brown streak disease persists across the vegetative cropping cycle

“…1 CBSD is caused by two distinct virus species: Cassava brown streak virus (CBSV) and Ugandan Cassava brown streak virus (UCBSV), both of which belong to the genus Ipomovirus, family Potyviridae, and possess a ssRNA genome of messenger sense. 2,3 Both viruses are transmitted by whiteflies (Bemisia tabaci), resulting in CBSDsymptomatic plants carrying infections by one or both viral species. Since its re-emergence in Uganda in 2004, 4 CBSD has become prevalent in East Africa, with new outbreaks of the disease resulting from dissemination of infected cassava planting materials and high whitefly vector populations.…”