Banana is one of the most important economic crops for developing countries in tropical and sub-tropical areas [12] and is cultivated in about 120 countries [33]. Available estimates indicate that average global banana production rose from 69 million tonnes in 2000-2002 to 116 million tonnes in 2017-2019, at an approximate value of 31 billion USD. These values are an estimate as the bulk of banana production is conducted informally thus making it difficult to obtain accurate figures [11]. Domesticated bananas are thought to have originated somewhere in New Guinea, Indonesia, the Philippines or the Southeast Asia Peninsula 7 000 to 10 000 years ago [10, 26]. Banana cultivation spread to other parts of the world reaching Africa possibly 3 000 years ago. During the period between 700 and 1 500 years ago, different banana varieties were repeatedly introduced to Africa and the south-west Indian Ocean Islands [21, 30].
Banana belongs to the genus Musa in the family Musaceae of the order Zingiberales. The genus Musa is divided into two sections: Musa and Callimusa [28] Musa, formerly known as Eumusa, is the larger section of the genus. The edible Musa species, their hybrids and polyploids originated from the two main wild species of banana, viz. Musa acuminata Colla and M. balbisiana Colla [25]. Banana is divided into two main groups namely dessert and cooking bananas. ‘Gros Michel’, which belongs to the AAA genomic group, was the first exported banana cultivar to be planted over thousands of hectares. Despite its susceptibility to Fusarium wilt, it is still grown in many countries because of its superior flavour. Another important cultivar is ‘Cavendish’, which may have originated in South China [16]. ‘Cavendish’ cultivars yield the most common fruit and form the backbone of the domestic industries in countries like Australia, India, China and South Africa [32].
Banana bunchy top virus (BBTV), a multi-component, circular, single-stranded DNA virus that is 18-20 nm in diameter, is the type member of the genus Babuvirus and the family Nanoviridae [3]. The viral genome comprises at least six encapsidated components (DNA-R; DNA-S; DNA-M; DNA-N; DNA-C and DNA-U3) each approximately 1100 nucleotides in length [41]. Phylogenetic relationships, largely based on the DNA-R, -N and –S segments, grouped BBTV isolates worldwide as ‘South Pacific’ or ‘Asian’ origin[20]. This categorization was later modified into two different groups, viz. the Pacific Indian Ocean (PIO) and the South East Asian (SEA) groups based on their geographical delineation [4].
Banana bunchy top disease (BBTD), caused by BBTV is spread in a circulative manner by Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae) commonly known as the banana aphid [22]. The first symptom of the disease is the appearance of dark green streaks on the undersurface of the leaf [22]. As the disease progresses, infected leaves become progressively stunted and malformed and have an upright bearing eventually resulting in a ‘bunchy’ display. Yield losses of up to 100% can be experienced when plants are infected with BBTV and fail to produce bunches [35].
BBTV was reported for the first time in the Fiji Islands in 1889 [22] thereafter, it has been identified in 44 countries in Africa, Australia, Asia and the South Pacific Islands [6, 22]. In Africa, BBTD was first reported in 1901 in Egypt. Currently it has been reported in 17 African countries including Cameroon, Zambia, Mozambique, Malawi and Nigeria [1, 13, 23]. The presence of BBTV in South Africa was confirmed in 2016 from the banana production area located in the South Coast region of the KwaZulu-Natal Province [18]. The source of infection in this region is currently unknown. BBTV is a quarantine virus included in the South African Phytosanitary Services list of pathogens that must be absent in imported Musaceae propagation material under the Agricultural Pests Act, 1983 (Act no. 36 of 1983). BBTV has inflicted a devastating impact on the banana industry in Australia in the 20th century, but due to strict quarantine regulations and enforcement, this disease has mostly been contained [9, 15].
Banana is amongst the most important commercial subtropical fruit grown in South Africa mostly for home consumption. Subsistence farming of banana also contributes to the food source of poorer communities and income is generated through informal trade at local markets. Only a small fraction of all the bananas produced is sold on the world market [7]. Approximately 415 000 tonnes were harvested during the 2018/19 marketing season valuing the industry at approximately 137 million USD [8]. Based on the recent detection of BBTV in South Africa, the extent of the spread of BBTV in South African banana production areas was investigated in order to establish effective management strategies in the affected regions. Prior to the identification and confirmation of BBTV in South Africa in 2015, the last survey provided no evidence for the presence of this virus in the country [27].