Tackling the banana menace

Lava Kumar, l.kumar@cgiar.org, and Rachid Hanna, r.hanna@cgiar.org

Banana bunchy top disease (BBTD) is emerging as a serious threat to banana and plantain production in several regions in West-Central and Southern Africa. IITA is undertaking research to understand the factors leading to the increase in BBTD incidence and spread with the ultimate goal of developing integrated strategies to protect bananas from this menace.

BBTV-infected plants in field. Photo by L. Kumar, IITA
BBTV-infected plants in field. Photo by L. Kumar, IITA
The livelihoods of over 70 million people are intimately linked to banana (banana and plantain), a major food staple and premier fruit. It occupies an important position in the agricultural economies in sub-Saharan Africa (SSA). Banana plantations produce fruits all year round, providing farmers with food and income, even during fallow seasons, thus contributing to food security.

Various types of banana are grown in SSA: plantain in the humid lowlands of West and Central Africa; highland cooking banana in East Africa; and introduced dessert banana in all the subregions.
The world’s greatest variability in the crops is held in West and Central Africa for plantain and Great Lakes Zone in Eastern Africa for highland banana. These regions are considered as secondary centers of banana diversity.

The foe
Bunchy top disease caused by banana bunchy top virus (BBTV) is emerging as a serious threat in several regions in Central and Southern Africa. This disease, first reported from Fiji in 1889, has been dubbed as one of the most destructive plant viral diseases. BBTV has been found in Asia, Africa, Australia, and the South Pacific islands. It belongs to the genus Babuvirus of the family Nanoviridae. The virus readily spreads in vegetative material such as corms and suckers. It also relies on the banana aphid, Pentalonia nigronervosa, its sole biological vector.

Banana with typical bunchy top disease symptoms. Photo by L. Kumar
Banana with typical bunchy top disease symptoms. Photo by L. Kumar
BBTV curtails plant growth, resulting in unproductive plants. Leaves of infected plants have pale chlorotic margins, dark green dots, and streaks along the veins which often extend down the midrib and petiole. Emerging leaves become progressively smaller and choked in the throat of the plant creating the “bunchy” appearance at the top.

Plants infected at an early age rarely produce fruit and eventually die. Those infected at a later growth stage may produce bunches, often with deformed fruits, but suckers have typical symptoms and remain unproductive. Due to the high destructive potential of BBTV on biological diversity and human activities, the Invasive Species Specialist Group (ISSG) of the World Conservation Union (IUCN) included BBTV in the list of 100 of the World’s Worst Invasive Alien Species.

Formidable challenges
Usually viral diseases are effectively controlled through the use of resistant sources. So far, banana varieties resistant to BBTV have not been found. Some types are tolerant or express symptoms relatively slowly. Eradication and exclusion are the only options found to be effective in controlling BBTV. These phytosanitary approaches rely heavily on early detection and destruction of the diseased mats, coupled with strict quarantine and indexing procedures to prevent further spread.

For instance, in Australia, strict legislation prevents the transfer of plant material from infected to uninfected areas. Skilled workers conduct regular inspection to identify and destroy any plant displaying disease symptoms. Thus, disease incidence has been kept at a very low level.

However, rigorous enforcement of phytosanitary procedures is expensive and extremely difficult to implement in SSA where socioeconomic conditions are poor and capacity is inadequate. In addition, banana cultivation is mainly carried out by subsistence farmers and large areas occur in the wild. Under these conditions, eradication is very difficult and this can affect livelihoods of the farmers. More research is necessary to devise control strategies that are congenial for application in SSA.

Distribution map of BBTV and the banana aphid in Africa. Map by IITA
Distribution map of BBTV and the banana aphid in Africa. Map by IITA
BBTV distribution in Africa
In Africa, the virus was first reported from Egypt over a century ago. In the 1950s, it was reported from Central Africa. The origin of this virus is not clear, but it may have been introduced into Africa through infected suckers brought by migrants from South Asia. Subsequent spread might have occurred from the movement of infected suckers by humans and natural spread by the vector aphids.

Despite its presence for at least five decades, the disease attracted very little attention until its recognition in the mid-1990s in Burundi, Malawi, and Rwanda where it was causing serious epidemics.

Our surveys from 180 sites in West, Central and Southern Africa point to 1994-1996 as the years when there was widespread appearance of the disease.

Presently, BBTV has been identified in Angola, Burundi, Cameroon, Congo, DR Congo, Equatorial Guinea, Gabon, Malawi, Mozambique, Rwanda, and Zambia.

Banana aphid and biocontrol
The banana aphid is widespread in SSA. Considerable variations in abundance were observed in our studies depending on locality, banana variety, and season. The aphid is likely to be responsible for much of the local spread of the disease and to some extent its long distance transport. Unlike annual crops, where the fallow period acts as a check for vectors, the perennial nature of banana allows aphids to survive all year round, aiding virus spread.

Aphid control will play a significant role in BBTD management.

Banana aphids are found between the whorls of the pseudostem and newly emerged leaves. Photo by L. Kumar
Banana aphids are found between the whorls of the pseudostem and newly emerged leaves. Photo by L. Kumar
The banana aphid is exotic to Africa and lacks indigenous natural enemies. Classical biological control is being explored that includes testing known natural enemies and exploration for new ones for introduction into Africa. IITA has successfully used this approach to control several exotic pests. Biological control could minimize the local spread of the disease, and perhaps reduce it to very low levels, when coupled with tolerant varieties.

Prospects
At present, BBTV spread is not controlled in SSA. Besides its effect on food security, the virus poses a serious threat to the diversity of plantain and highland banana.

Understanding its disease ecology, demographics of distribution, vector biology and ecology, awareness creation, and conventional and nonconventional approaches to tackle the virus and vector is expected to provide a reprieve in the medium term and sustainable solutions in the long term.