1IITA, lbadan, Nigeria; 2University of Minnesota, St Paul, MN, USA

Background and objectives
Banana and plantain (Musa spp.) are a major source of carbohydrate in sub-Saharan Africa. They are affected by many pests, fungi and bacteria. However, one of the major constraints to improving Musa germplasm by the international transfer of improved hybrids is the presence of viruses. Two viruses are known to occur throughout West Africa: banana streak badnavirus (BSV) and cucumber mosaic cucumovirus (CMV). In addition, a virus which has been tentatively named banana die-back virus (BDBV) has been reported.

Materials and methods
Banana and plantain leaf samples from the humid forest region of Nigeria with virus-like symptoms were collected in addition to some symptomless samples. Diagnosis of the causal agent(s) of the symptoms was initially done using enzyme-linked immunosorbent assay (ELISA) for BSV and CMV. When required, confirmatory tests of mechanical inoculation to Nicotiana tabacum (for CMV) and immunosorbent electron microscopy (ISEM) for BSV were done. Diagnosis of BDBV was initially based on symptom expression, and confirmed by ELISA and mechanical inoculation to Vigna unguiculata.

Results and conclusions
BSV was consistently found associated with symptomatic leaf samples showing distinctive white streaking and was also found, at low frequency, in some asymptomatic samples. The disease was identified in most plantain-growing areas. It commonly occurred in some plantain hybrids, but the incidence was low in most landraces. An isolate of the virus from southern Nigeria was found to be serologically distinct from Dioscorea alata badnavirus in yams and cocoa swollen shoot badnavirus in cocoa.

While the symptoms of CMV are usually distinct, they can be confused with those exhibited by plants infected with BSV. The presence of CMV was confirmed in both banana and plantain, but was less common than BSV. CMV was also found in mixed infection with BSV in plantain. CMV in banana and plantain was found to be distinct from that found to be infecting adjacent cowpeas, causing less severe symptom expression after mechanical inoculation and vector transmission to N. tabacum.

The presence of BDBV was confirmed in banana by ELISA but was not found to be widely distributed. It was found in mixed infection with BSV in some cases. The vector of the virus is unknown but it is putatively a member of the nepovirus group, exhibiting a serological relationship with tobacco ringspot, tomato ringspot and cacao necrosis nepoviruses.

The presence of these viruses in banana and plantain, and the failure to eliminate BSV by meristem culture, emphasises the need for care in selecting planting material as these viruses can all be transmitted vegetatively through suckers used as planting material. With regard to their spread, no natural vector has been identified for BSV although mealybugs are implicated; aphids are known to be vectors of CMV and no vector has been found for BDBV.

The epidemiology of these viruses in Nigeria is not known. Despite the presence of large numbers of mealybugs, there appears to be no natural spread of BSV. Spread of CMV through vector transmission appears to be very localised, as does the occurrence of BDBV. However, care must be taken as these three viruses are transmitted vegetatively through suckers and can often be widely distributed by farmers and national/international germplasm movement.