Biological and Molecular Categorization of Strains of Banana bunchy top virus

Banana bunchy top virus (BBTV), a complex circular single-stranded DNA virus with multiple genomic components, is a destructive pathogen in banana-cultivating areas worldwide. Based on symptoms (such as vein clearing, green streak on pseudostem, leaf atrophy, bunchy top and stunting) as well as polymerase chain reaction (PCR) amplification patterns with different primer pairs, all BBTV isolates collected from Taiwan and other countries can be divided into five distinctive strains. Three primer pairs, C1, stem-loop common region (CR-SL) and TS were used for PCR amplifications. Strain 1, which induces conspicuous symptoms, is a common severe strain; it reacted positively with C1 and CR-SL but negatively with TS in the PCR assays, so its PCR pattern was indicated as +/+/) for C1, CR-SL and TS primer pairs, respectively. Strain 2 seemed to be a Taiwan-specific severe strain which induced severe symptoms, and its PCR pattern was +/+/+ as it showed positive reactions with all three primer pairs. Strain 3, causing the most severe symptoms, is a Malaysia-specific severe strain whose PCR pattern is )/+/) . Strain 4 induced moderately severe symptoms and is an intermediate strain whose PCR pattern is )/+/) . Strain 5 is a mild strain; it did not induce symptoms in banana and it reacted positively with C1, CR-SL and TS primer pairs. Interestingly, an additional 537-bp fragment was amplified from Strain 5 with the CR-SL primer pair. The PCR pattern of Strain 5 is therefore indicated as +/++/+ . This study demonstrates that various BBTV strains exist in nature and they differ biologically and also molecularly.