Banana bunchy top disease (BBTD) is the most important viral disease of bananas (Musa spp) caused by the Banana bunchy top virus (BBTV). The disease was named after the symptoms seen in its natural hosts, Musa spp., where the infected plants are stunted and have “bunchy” leaves at the top (Kumar et al., 2015; Stephen et al., 1007). The virus isolated in the late 1980s was assigned to the Nanoviridae family in the genus Babuvirus (Dale 1987; Kumar et al., 2015). The disease was first identified in Fiji in 1889, and known at present (as of April 2018) to occur in about 37 countries in Africa, Asia, Australia and South Pacific islands. It is regarded as A1 quarantine pathogen and considered among the top 100 invasive species because of the severe threat the virus poses to Musa biodiversity (Global Invasive Species Database, 2005).
In Africa, the disease occurrence was confirmed in Angola, Benin, Burundi Cameroon, Congo Brazzaville, Central African Republic, Democratic Republic of Congo, Egypt, Equatorial Guinea, Gabon, Malawi, Mozambique, Nigeria, Rwanda, South Africa, Uganda, Tanzania and Zambia. BBTV was confirmed in all these countries, but not in Equatorial Guinea (Kumar et al., 211; 2015). The BBTV was detected in Togo in 2018 in a few fields, but eradicated in the same year
Transmission and epidemiology
BBTV is transmitted by the banana aphid (Pentalonia nigronervosa) in a persistent manner. The virus is also transmitted through infected plant suckers and other plant components used in banana propagation but are not sap transmissible. Banana aphids have a worldwide distribution with a host range almost exclusive to Musa spp. Past reports of occurrence of banana aphid on several closely related plant families including the Araceae (Alocasia sp., Caladium spp., Dieffenbachia spp., Xanthosoma sp.), Cannaceae (Canna spp.), Heliconiaceae (Heliconia spp.), Strelitzeaceae (Strelitzia spp.) and Zingiberaceae (Alpinia spp., Costus sp., Hedychium spp.) (Wardlaw, 1961) were recently proven as a different aphid species named as ‘Pentalonia caladii van der Goot (Foottit et al, 2010). Banana aphids are found at the base of the pseudostem at soil level, in the apical portion, beneath the outer leaf sheaths and on newly emerging suckers.
Transmission efficiency for individual aphids has been reported as ranging from 46 to 67% (Magee, 1927; Wu and Su., 1990a; Hu et al., 1996) and the virus is more efficiently acquired by nymphs than by adults (Magee, 1940). In studies of outbreaks of bunchy top in commercial banana plantations, Allen (1978b, 1987) showed that the average distance of secondary spread of the disease by aphids was only 15.5-17.2 m. Nearly two-thirds of new infections were within 20 m of the nearest source of infection and 99% were within 86 m. Allen and Barnier (1977) showed that if a new plantation was located adjacent to a diseased plantation, the chance of spread of bunchy top into the new plantation within the first 12 months was 88%. This chance was reduced to 27% if the plantations were separated by 50-1000 m and to less than 5% if they were 1000 m apart. On average, the interval between infection of a plant and movement of aphids from this plant to initiate new infections elsewhere (the disease latent period) was equivalent to the time taken for 3.7 new leaves to emerge. The rate of leaf emergence varied seasonally with a maximum in summer (Allen, 1987).
Disease symptoms usually appear about a month after infection. The disease is named after one of the most characteristic symptoms of an advanced infection, when the leaves become progressively dwarfed, upright and bunched at the top of the plant, with wavy and chlorotic margins that tend to turn necrotic. Initial symptoms are more difficult to detect. The first symptoms are dark green streaks on the lower portion of the leaf’s midrib and later on the secondary veins. Removing the waxy white coating on the midrib makes it easier to see the streaking. The streaks consist of dots and short lines, the so-called ‘Morse code’ pattern, the most diagnostic symptom of bunchy top. As infection progresses, streak symptoms become more evident on the leaf blade. Dark-green hook-like extensions of the veins can also be seen in the narrow, light-green zone between the midrib and the lamina. The short hooks point down along the midrib toward the petiole. These hooks are best observed from the underside of the leaf by holding the leaf to the light.
The symptoms are most severe and distinctive when the infection arises from infected planting material (primary infection). These plants are typically stunted (less than 1 m) and rarely produce fruit.
When a plant gets infected by aphids after a period of BBTV-free growth (secondary infection), the symptoms are usually milder and only appear in tissues formed after the infection. The first symptoms usually appear at the earliest in the second leaf to emerge after a plant has become infected. The leaf will look normal to untrained eyes but on closer inspection it will have the dots and dashes on the midrib, petiole and blade. The margin also curls up. If the plant flowers, the veins of the bracts of the inflorescence may show discontinuous streaks that resemble the ‘Morse code’ symptoms on petioles and leaves (mottled inflorescence). If a bunch is produced, its growth will be stunted and the hands and fingers are likely to be distorted and twisted. On plants infected very late in their life cycle, the only symptoms present may be a few dark green streaks on the tips of the flower bract.
The early stages of a secondary infection may be confused with signs of nutrient deficiency or environmental stress.
BBTD is identified based on the disease symptoms and use of diagnostic tools. A range of diagnostic methods based on the Enzyme-linked Immunosorbent Assay (ELISA), Polymerase Chain reaction (PCR), Loop-mediated Isothermal Amplification) and RPA (recombinase polymerase amplification) have been established for the detection of BBTV in plants and aphid vectors.
The most important factors in controlling banana bunchy top virus are killing the aphid vector (disease carrier) and rogueing (removing and destroying) infected banana plants. Killing the aphids on the banana prevents spread of viruliferous aphids to the healthy plants and spread the disease. Since banana is the only host of BBTV, rogueing infected matts reduces spread of the virus by reducing the opportunity for aphids to acquire the virus or for people to obtain and transport infected suckers or planting material.
Banana aphids can be treated by either injecting the infested banana plant with imidacloprid; or by spraying the entire infested banana plant with paraffinic oil. All banana plants within 10 metres of the infested banana plant should also be sprayed with paraffinic oil to reduce aphid populations in the area.
As part of the BBTV ALLIANCE project a multi-pronged approach is being tested to eradicate BBTV infected matts and recover banana production in the affected regions (Kumar et al., 2016). Several strategies are being used, including awareness raising to prevent the spread of the disease in Africa and worldwide.
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