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The typical symptoms of bunchy top of banana are very distinctive and readily distinguished from those caused by other viruses of banana. Plants can become infected at any stage of growth and there are some initial differences between the symptoms produced in aphid-infected plants and those grown from infected planting material.
In aphid-inoculated plants, symptoms usually appear in the second leaf to emerge after inoculation and consist of a few dark-green streaks or dots on the minor veins on the lower portion of the lamina. The streaks form 'hooks' as they enter the midrib and are best seen from the underside of the leaf in transmitted light. The 'dot-dash' symptoms can sometimes also be seen on the petiole. The following leaf may display whitish streaks along the secondary veins when it is still rolled. These streaks become dark green as the leaf unfurls. Successive leaves become smaller, both in length and in width of the lamina, and often have chlorotic, upturned margins. The leaves become dry and brittle and stand more erect than normal giving the plant a rosetted and 'bunchy top' appearance.
Suckers from an infected stool can show severe symptoms in the first leaf to emerge. The leaves are rosetted and small with very chlorotic margins that tend to turn necrotic. Dark-green streaks are usually evident in the leaves.
Infected plants rarely produce a fruit bunch after infection and do not fruit in subsequent years. Plants infected late in the growing cycle may fruit once, but the bunch stalk and the fruit will be small and distorted. On plants infected very late, the only symptoms present may be a few dark green streaks on the tips of the flower bracts (Thomas et al., 1994).
Mild strains of BBTV, which induce only limited vein clearing and dark-green flecks, and symptomless strains have been reported in Cavendish plants from Taiwan (Su et al., 1993). Mild disease symptoms are expressed in some banana cultivars and Musa species. The dark-green leaf and petiole streaks, so diagnostic and characteristic of infection of cultivars in the Cavendish subgroup, can be rare or absent (Magee, 1953). Some plants of 'Veimama' (AAA, Cavendish subgroup), after initial severe symptoms, have been observed to recover and to display few if any symptoms.
BBTV has not been eradicated from any country where it occurs, but it is believed to have been eliminated from certain banana-growing districts in Australia. Here, the disease is kept in check by strict State Government legislation which controls the source and movement of planting material, controls the issue of planting permits and requires the destruction of feral plants and all plants with symptoms. Banana inspectors are also employed to police these regulations and locate diseased and feral plants. An ambitious programme of eradication is on-going which is based on replacing plantations where the disease regularly occurs, with BBTV-tested, tissue-cultured, planting material (Thomas et al., 1994). Another initiative to control the spread of BBTV in Africa, named ALLIANCE for BBTV control in Africa, aims to contain the spread of BBTV from disease-affected areas to new regions through quarantine regulations and the recovery of banana production by eradication of infected stools and replanting with health planting material (Kumar et al., 2016; www.bbtvalliance.org).
When little work had been undertaken on testing germplasm for resistance, it was thought that all cultivars were susceptible, although some may take longer to develop symptoms and others may escape infection because of aphid preferences or host morphological factors. However, work in Australia suggested that Musa coccinea, a wild species, and the cultivar Kluai Teparot (ABBB/ABB) may have physiological resistance (J Thomas, QDPI, Indooroopilly, Queensland, Australia, personal communication, 1995). Several studies have since been undertaken to evaluate Musa genotypes for BBTV resistance and found genotypes with tolerance (no symptoms and near normal performance of virus infected plants (e.g. Gros Michel), delayed expression of symptoms (e.g. Dwarf Apple, also known as Santa Catarina) and difficult to infect (e.g. Fugamou) (Espino et al., 1993; Hooks et al., 2009b; Niyongere et al. 2011; Ngatat et al., 2017).
Banana bunchy top disease can be effectively controlled by the eradication of diseased plants and the use of virus-tested planting material. Before destruction, diseased plants should first be sprayed with power kerosene or insecticide to kill all viruliferous aphids. The whole stool, including corm and all associated suckers, must then be destroyed by uprooting and chopping into small pieces or by herbicide treatment, as the virus will ultimately spread to all parts of the mat. Control must be practised across the whole production area to avoid the rapid re-infection of virus-tested planting material (Thomas et al., 1994; Kumar et al., 2016).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: