A thesis submitted in fulfilment for the degree of Doctor of Philosophy in Horticulture in Jomo kenyatta University of Agriculture and Technology

Résumé,

Banana buchy top disease (BBTD) is caused by Banana bunchy top virus (BBTV) which is transmitted by the aphid vector , Pentalonia nigronervosa Coquerel and through infected planting materials. BBTD is the most common and destructive viral disease that hinders banana production worldwide. It was reported for the first time in the Great Lakes Region of Africa (GLRA) in 1958 in the Democratic Republic of the Congo (DR Congo) and in 1987 in Burundi and Rwanda. Since then, no specific study has been undertaken on the disease in the GLRA. Therefore, in this study, the BBTD incidence, occurrence of the vector and farmers' awareness about the disease were assessed. Nucleotide sequences of GLRA isolates were characterized, whereas the varieties were secreened for their resistance / tolerance to the disease. Additionally, seasonal and spatial distribution of P. nigronervosa was assessed.

Asurvey was conducted from September to October 2008 across five provinces in Burundi, Eastern DR Congo and Western province, Rwanda. In total, 7830 banana mats were assessed in 261 farms where 30 mats were considered per farm. Farmers were also interviewed on BBTD management. The regional averages for BBTD incidence and aphid occurrence were 25 % and 46 %, respectively. Among the interviewed farmers, 90 % of them were able to recognize BBTD symtoms, while 95 % were not informed on the disease management options and that none of the farmers' varieties was resistant to the disease.

A study on molecular characterization was performed using 22 and 19 isolates - based on BBTV DNA-R and Coat protein, respectively. These GLRA isolates in phylogenetic analysis were classified in the South Pacific group and followed by the Indian isolate. Molecular analyses revealed that the disease occurred in the region from 1970s.
Compared to other South Pacific group isolates, the GLRA isolates showed a relatively higher genetic variability.

Screening for resistance to BBTD was conducted using 40 different Musa genotypes at the ISABU Mparambo research station, from March 2007 to September 2010.
There was no cultivar totally resistant to BBTD. However, five tolerant cultivars [ Come plantain' (AAB-plantain), 'FHIA-03 (AABB), 'Highgate' (AAA, Gros Michel subgroup), Igjindi' (AAB-plantain) and 'Saba' (ABB)] out of 40 genotypes tested were selected based on their relative advantages for farmers' adoption.

The last activity was carried out to determine how aphid vectors influence disease spread in different locations. At nine months after trial establishment, BBTD incidence ranging from 21.8 to 56.4 % was oserved in plots established within BBTD- affected banana fields, while 0 to 12.3 % incidence was reported in plots located outside BBTD-affected banana fields. Aphids were reported in the three study regions in Burundi with higher aphid numbers observed in the dry season (i.e, July and August). These aphid vectors were able to acquire and transmit the virus irrespective of altitude. A disease incubation period of 21 and 84 days was observed at low (780 masl ) and high (2090 masl) altitudes, respectively.

From these results, the use of tolerant cultivars could be hampered by the by the genetic variability of BBTV and widespread nature of aphids. Here, in context of small-scale agriculture, sustainable BBTD management consists of raising famers' awareness on early detection and eraducation of BBTV- infected mats and adopting cultural practices aimed at the control of aphid populations. In addition, quarantine measures should help to prevent disease spread between BBTD affected and unaffected areas within the African Great Lakes region.