This factsheet describes some considerations for the importer when dealing with germplasm that may contain infective banana streak viruses (BSVs) integrated into the banana plant genome.

Facts about the Musa genome and banana streak viruses (BSVs)

• Most edible bananas belong to the genus Musa; they have a genome originating from M. acuminata (A genome) and/or M. balbisiana (B genome).
• Several banana streak viruses (BSVs) have been described: they are related but distinct virus species, all causing a similar range of symptoms.
• B genomes almost universally carry sequences of one or more of the banana streak viruses within their chromosomes, some of these integrant sequences can be activated [1], e.g. they can trigger an infection with viral particle of BSV (see figure 1).
• So far, infective integrants of three BSV species have been reported in the Musa B genome: BSOLV, BSGFV and BSIMV.
• There is currently no evidence that other BSV species have infective counterparts in the B genome.
• Spontaneous BSV infections occur upon activation of infective BSV integrants in interspecific hybrids carrying one copy of the B genome (AB, AAB, AAAB), whether natural or created by breeding programmes. Viral particles can be detected[2] in the sap only once a plant becomes infected and this infection can cause typical BSV symptoms.
• Following spontaneous infection, natural resistance mechanisms in the plant are triggered. Virus loads and symptoms decrease, and plants naturally can recover from BSV infections after variable periods of time, but it depends on banana genotype and environmental conditions.
• All BSVs are transmitted by mealybugs but the rate of spread is very slow, and the virus from infective integrant is rarely transmitted to new plants. Most spread is a consequence of vegetative propagation practices.
• Yield losses from BSV infection are generally mild to negligible.

Factors triggering spontaneous infection and recovery are not yet well understood and vary between accessions, growth stage of the plant, environmental conditions and possibly other unknown factors. Plant tissue culture, which is commonly required for global movement of banana germplasm, is a stress that can cause spontaneous infection from BSV integrants. Accessions with a B genome are distributed by the International Transit Center (ITC)  with a disclaimer stating that “infective BSV integrants are present within the B genome of Musa spp. Accessions of AB, AAB and AAAB genotypes may spontaneously develop BSV infection and produce infectious and transmissible viral particles from the infective integrated counterpart, that might result in symptoms occurring at any stage of growth during cell culture, horticultural multiplication or in the field.”

Figure 1: Schematic representation of infection and recovery process following activation of infective BSVs.

ITC indexing scheme prior to germplasm distribution (see figure 2)

• Before release, ITC accessions are tested for the presence of all known Musa viruses including banana bunchy top virus (BBTV), banana bract mosaic virus (BBrMV), banana mild mosaic virus (BanMMV), cucumber mosaic virus (CMV) and banana streak viruses (BSVs).
• Accessions can also be tested for the presence of infective BSV integrants in their genome.          
• If BSV particles are detected, the accession will undergo therapy, whatever the BSV species
• A new therapy is carried out if BSV particles from BSV species without infective integrated sequence in the Musa genome are detected after therapy.
• In any other case (e.g. no detection or detection of particles from infective integrated species), the accession is made available according to 2 modalities:
  
  • Accession without infective integrant BSV sequence(s) in their genome: available
  • Accession with infective integrant BSV sequence(s) in their genome: available with disclaimer

Figure 2: ITC indexing scheme for BSVs prior to distribution of germplasm containing B genome. (*) Detection of BSV species means detection of BSV particles from this species in the accession; ** detection of BSV particles from BSV species without infective integrated sequence in the Musa genome.