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Bunchy top


Bunchy top

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Common names of the disease
Bunchy top (recommended[1])
Banana Bunchy Top Disease (BBTD)
Cabbage top

Curly top

Causal agent

Banana bunchy top virus

Distribution
Asia, Pacific and Africa

Bunchy top is a viral disease caused by the Banana bunchy top virus (BBTV). The disease, often called BBTD for banana bunchy top disease, gets its name from the bunchy appearance of infected plants. But because the first symptoms usually go unnoticed, smallholder banana farmers tend to be caught unaware. By the time the signs of an advanced infection become apparent, the virus will most likely have spread to other plants. Infected plants cannot recover, so unless they are destroyed, they will serve as a source of viral particles that can be transmitted from plant to plant by the banana aphid, Pentalonia nigronervosa. The virus is also spread through infected planting material.

There are no known sources of resistance to the disease. All cultivars are believed to be susceptible. To the extent they vary, it's in the time taken for the symptoms to develop. Bunchy top is considered to be the most devastating viral disease affecting bananas. The IUCN's Species Survival Commission has listed it among the world's 100 worst invasive alien species[2].

The disease most likely originates from the centre of origin of the genus Musa, which extends from India to the western Pacific. Although the disease was first reported in Fiji in 1879[3], the virus itself was only isolated in the late 1980s[4][5].

Distribution

Bunchy top is widespread in Southeast Asia and the South Pacific, and is present in parts of India and Africa. It was observed for the first time in Fiji in 1879, Egypt circa 1900, and Sri Lanka and Australia in 1913[6]. Ocfemia also mentions 1925 reports of a similar disease affecting abaca (Manila hemp) in the Philippines[7].

In Africa, it was subsequently reported in the Democratic Republic of Congo in 1958[8], Eritrea[9], Equatorial Guinea[10], Gabon in 1981[11], Congo-Brazzaville, Burundi and Rwanda in 1987[12], Central African Republic[13], Malawi in 1994[14], Zambia and Mozambique in 2007[15], Angola[16] and Cameroon in 2008[17], Benin in 2011[18], Nigeria in 2012[19] and South Africa in 2015[20]. In Eritrea and Mozambique, cases of bunchy top have not been confirmed since the first reports.

In addition to Sri Lanka, it has been reported in parts of China, Indonesia, Malaysia, Vietnam[21], Philippines, Taiwan, Japan[22], India, Iran[23], Nepal and Pakistan[24], Bangladesh, Myanmar, and Thailand.

Besides Australia and Fiji, it has been reported in Hawaii, Guam, Samoa, American Samoa, Tonga, Tuvalu, Kiribati and New Caledonia[25]. The American continent (Central and South America and the Caribbean) is reportedly free of bunchy top.

Symptoms

Narrow and short leaves and bleached (chlorotic) margins.
Narrow and short leaves and bleached (chlorotic) margins.

Bunchy top 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. Cabbage top[26] and curly top[27] (for abaca) have also been used to term the disease.

Initial symptoms may be difficult to detect for a novice due to their inconspicuous nature. 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 dot-dash patterns, the so-called 'Morse code', 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 the transmission from an infected mother plant to her suckers. These plants are typically stunted (under 1 m) and rarely produce fruit.

The Morse code symptom consists of dark green dots and dashes along the veins of the leaf. They also appear on the midrib and petiole.
The Morse code symptom consists of dark green dots and dashes along the veins of the leaf. They also appear on the midrib and petiole.
The J-hook symptom consists in dark green streaks that terminate at the midrib. J-hooks are often associated with the 'Morse code' symptom.
The J-hook symptom consists in dark green streaks that terminate at the midrib. J-hooks are often associated with the 'Morse code' symptom.
Short, narrow and upright leaves give infected plants the bunchy top appearance after which the disease is named.
Short, narrow and upright leaves give infected plants the bunchy top appearance after which the disease is named.

When aphids carrying the virus (also called viruliferous aphids) feed on a healthy plant, 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. Each new infected leaf is narrower and shorter and also stands more upright than the previous one. 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 terms primary and secondary infections are used to differentiate the infections caused by aphids from those passed on by the mother plant to her suckers. Depending on authors, their ascription varies.

Although bunchy top symptoms are usually very distinctive across all banana cultivars, symptomless and mild infections have been reported in Taiwan[28] and attenuation of initially severe symptoms have been reported in a cultivar known as Veimama (also called Vi-mia-mia or Middle finger) in Fiji[3]. Leaf margin bleaching, yellowing or necrosis can also be caracterisitic of nutrient deficiencies or environmental stresses.

Symptoms of bunchy top in leaves that emerged after the plant was infected with the virus by an aphid.
Symptoms of bunchy top in leaves that emerged after the plant was infected with the virus by an aphid.
Mottled inflorescence of a plant infected at a late growth stage. The mottling pattern is formed by the discontinuous streaks in the veins of the bracts.
Mottled inflorescence of a plant infected at a late growth stage. The mottling pattern is formed by the discontinuous streaks in the veins of the bracts.
When a plant infected with the virus at a late stage of growth produces a bunch, the latter is typically small and will not grow normally.
When a plant infected with the virus at a late stage of growth produces a bunch, the latter is typically small and will not grow normally.

Management

There is no cure for bunchy top. Regular inspections to detect and remove infected plants, and replanting with virus-indexed tissue-culture plantlets, which have been screened in a lab and certified 'virus-free', are the basis for good management[29].

In the 1980s, infected plants in Australian commercial banana farms were doused with kerosene (paraffin) and chopped up, until it was realized that the practice contributed to the dispersal of aphids as they flee for cover (in addition to being environmentally unfriendly). It has since been replaced by a dual strategy[29]. Firstly an herbicide is injected directly into the plant to destroy it followed by a systemic insecticide in order to kill the aphids. Contact insecticides are inefficient as aphids often hide under leaf sheaths or other plant parts. An additional measure to control aphids (such as spraying oil) may be necessary in winter when plants injected with herbicide take longer to die[30]. As part of a national effort started in 2009 to eradicate the disease from commercial plantations, hobby farms and backyard gardens, Australian scientists developed a mapping approach to detect banana plants in backyard gardens to facilitate the work of inspectors charged with destroying infected plants[31].

In the case of smallholder banana farmers who do not have access to spraying oils, herbicides and insecticides, infection levels can be reduced by regular inspections and prompt removal of infected plants, detrashing (removal of dead leaves hanging down the plant), use of clean planting material and varietal mixtures, and opening new fields as far away as possible from existing plantations[32].

Farmers should also watch out for alternative hosts of the virus (such as Canna, Heliconia, Strelitzia), as well as for alternative hosts of the aphids. Another thing to remember about bunchy top is the management of the ants, which tend the banana aphids. Ant colonies may be found in nests beneath the dead leaves that cling to the pseudostem. Removal of these leaves will reduce nesting areas for the ants and subsequently aphid populations.

References

1. Common names of banana diseases on the website of the International Society for Plant Pathology
3. Darnell-Smith, G.P. 1924. "Bunchy top" disease in banana. Queensland Agricultural Journal 21:169-179.
5. Harding, R.M., Burns, T.M., Dale, J.L. 1991. Virus-like particles associated with banana bunchy top disease contain small single-stranded DNA. J. Gen. Virol. 72(2):225-230.
6. Magee, C.J.P. 1927. Investigation on the Bunchy Top Disease of the Banana. Council for Scientific and Industrial Research Bulletin, Melbourne. 86 pp.
7. Ocfemia, G.O. 1926. Progress report on bunchy-top of abaca or Manila hemp. Phytopathology 16:894.
8. Wardlaw, C.W. 1961. The virus diseases: bunchy top. p68-115. In: Wardlaw, C.W. (ed.). Banana diseases including plantains and abaca. Longmans, London, UK.
9. Saverio, B. 1964. Banana cultivation in Eritrea and its problems. Edagricole 8:5–56.
10. Banana bunchy top virus in CABI's Invasive Species Compendium
11. Manser, P.D. 1982. Bunchy top disease of plantain in Gabon. FAO Plant Protection Bulletin 30(2):78-79.
12. Sebasigari, K. and Stover, R.H. 1988. Banana diseases and pests in East Africa. Report of a Survey in November 1987. INIBAP, Montpellier (FRA). 33p.
13. Diekmann, M. and Putter, C.A.J. (eds.). 1996. FAO/IPGRI technical guidelines for the safe movement of germplasm. No. 15. Musa spp. 2. ed.. IPGRI, FAO, Rome, Italy. 28p.
14. Kenyon, L., Brown, M. and Khonje, P.T. 1997. First report of banana bunchy top virus in Malawi. Plant Disease 81: 1096.
15. Gondwe, W.T., Mwenebanda, B.M.L. and Natha, E. 2007. Banana bunchy top disease in Mozambique and Zambia. InfoMusa 16(1&2):38-39.
16. Pillay, M., Blomme, G., Rodrigues, E. and Ferreira, L. 2005. First report of BBTV in Angola. InfoMusa 14(1):44-45.
17. Oben, T.T., Hanna, R., Ngeve, J., Alabi, O.J., Naidu, R.A. and Lava Kumar, P. 2009. Occurrence of Banana Bunchy Top Disease Caused by the Banana bunchy top virus on Banana and Plantain (Musa sp.) in Cameroon. Plant Disease 93(10):1076.
18. Lokossou, B., Gnanvossou, D., Ayodeji, O., Akplogan, F., Safiore, A., Migan, D.Z., Pefoura, A.M., Hanna, R. and Lava Kumar, P. 2012. Occurrence of Banana bunchy top virus in banana and plantain (Musa sp.) in Benin. New Disease Reports 25:13.
19. Adegbola, R.O., Ayodeji, O., Awosusi, O.O., Atiri, G.I. and Lava Kumar, P. 2013. First report of banana bunchy top disease caused by Banana bunchy top virus in banana and plantain (Musa spp.) in Nigeria. Plant Disease 97(2):290.
20. Jooste, A.E.C., Wessels, N. and van der Merwe, M. 2016. First Report of Banana bunchy top virus in Banana (Musa spp.) from South Africa. Plant Disease First online.
21. Vakili, N.G. 1969. Bunchy top disease of bananas in the Central Highlands of South Vietnam. Plant Disease Reporter 53(8):634-638.
22. Kawano, S., Su, H.J. 1993. Occurrence of banana bunchy top virus in Okinawa. Annals of the Phytopathological Society of Japan 59:53.
23. Bananej, K., Ghotbi, T. and Vahdat, A. 2007. First report of Banana bunchy top virus infecting banana in Iran. Plant Pathology 56(4):719.
24. Khalid, S., Soomro, M.H. and Stover, R.H. 1993. First report of banana bunchy top virus in Pakistan. Plant Disease 77(1):101.
25. Kagy, V., Thomas, J.E., Sharman, M., Mademba-Sy, F. 2001. First record of banana bunchy top disease in New Caledonia. Australasian Plant Pathology 30(1):71.
26. Darnell-Smith, G.P. 1919. "Bunchy Top" of Bananas in the Tweed River District. Agricultural Gazette of New South Wales:809-814.
27. Ocfemia, G.O. 1927. Second progress report on bunchy-top of abaca or Manila hemp. Phytopathology 17:255-257.
28. Su, H.J., Wu, R.Y. and Tsao, L.Y. 1993. Ecology of banana bunchy top virus disease. p.308-312. In: Proceedings of International Symposium on Recent Developments in Banana Cultivation Technology, Pingtung (TWN), 1992/12/14-18. Proceedings: international symposium on recent developments in banana cultivation technology.
29. Australia Hundred Years War on bunchy top in the April 2013 issue of InfoMus@
32. Lepoint, P., Sibomana, R., Niyongere, C. and Blomme, G. 2013. Cultural Practices for Banana Bunchy Top Disease Management: a Sustainable Option for Burundian Smallholders?. p.111-117. In: Van den Bergh, I., Amorim, E.P. and Johnson, V. (eds.). Proceedings of International ISHS-Promusa Symposium on Bananas and Plantains: Towards sustainable global production and improved use, Salvador (Bahia), Brazil, 2011/10/10-14. Acta Horticulturae 986. ISHS, Leuven, Belgium.
Banana bunchy top factsheet produced by the University of Hawaii at Manoa
Detailed Signs and Symptoms of Banana Bunchy Top by Scot Nelson, University of Hawaii at Manoa
Resources on banana bunchy top disease maintained by University of Hawaii's Scot Nelson
The BBTD Alliance website

See also on this website

Photos related to bunchy top in the Musarama image bank
News and blogs on bunchy top: