Diversity of Banana Streak Virus In Kenya

Abstract

Banana streak virus (BSV) is a serious constraint to the production of banana and plantain (Musa spp.) In Sub-Saharan Africa. BSV has been found to be present in virtually all the commercial banana cultivars in Kenya. Various studies have reported that the banana streak virus is caused by a range of BSV isolates. The ability to quickly and reliably detect BSV is a prerequisite to the management of the virus. A variability study of BSV in Kenya was carried out to develop suitable diagnostic techniques. Transmission of the disease is through several species of mealy-bug (Planococcus spp) but a more significant transmission is through infected vegetative propagation materials. In some Musa hybrids and derived cultivars, integration of BSV into the plant genome can cause subsequent infection when triggered by stress factors such as drought, tissue culture, temperature, and genomic hybridization. /11 Filro propagation has been identified as one of the main activation triggers of integrated sequences to cause systemic (episoma l) banana streak disease (BSD). This was especially observed in the B genome containing AAAB hybrids (FHIA 18 and FHIA 0 I/Go ld finger). The heterogeneous nature of the virus in the field hinders reliable detection by serological techniques. Integrated BSV sequences cause problems in direct molecular detection of virus particles by polymerase chain reaction (PCR). Hence, deoxyribonucleic acid (DNA) -based techniques, following serological trapping of the virus, immune-capture polymerase chain reaction (IC-PCR) has been found most reliable in the detection of BSV. The main objective of this study is to identify BSV isolates in Kenyan banana cultivars. Factors affecting the infection and expression of BSV was restudied to optimize the diagnostic protocol in order to help with virus indexing and consequently mass production of BSV-free banana seed li ngs. The work was accomplished through random collection of liclc1 banana material from various banana growing regions, serological analysis using Enzyme Iinked Immunosorbent Assay (ELISA) kit which confirmed 87% BSV infection. Isolation of nucleic acid from infected leaf samples, molecular characterization of BSV and sequence analysis of the clone enabled identification or about seven isolates. Three of the seven isolates were new isolates. Genome-based methods of detection such as nucleic acid hybridization and PCR amplification were used. Through screening of various antibodies and primers, their effectiveness in detecting BSV was determined. Tissue culture materials of 15 cultivars were evaluated for BSV re- infection and expression so as to determine the tolerant cultivars under field and greenhouse conditions and stage of infection and expression. Environmental factors such as nutrition that affect the expression of BSV were also studied. Statistical analysis system (SAS) for ANOVA and various bioinformatics packages were used for analysis of data for the experiments. Results indicated that BSV is associated with an increase of growth time, decrease of plant height, girth, number of leaves and high nitrogen leve ls. Cultivar, location and season also played a critical role in the expression of BSV. The study also confirmed that detection of new isolates was influenced by antibody type, concentration and antigen concentration. The available antibodies have limitations in the comprehensive detection of BSV. However, uses of genomic techniques were more effective than serological techniques. Based on amplification, cloning, sequencing, phylogenetic analyses, and pairwise comparison of the nucleotide sequence similarities of 80% and above (proposed criteria by the International Committee on Taxonomy of viruses (ICTV), three new isolates of BSV were identified and confirmed for the first time in Kenya. The isolates were Khabusi2 from Mt. Elgon region (EAH - AAA genome), Lisulya from Kisii region (EAH - AAA), and Nshule from Kisii region (EAH - AAA).