Xanthomonas campestris pv. musacearum: a major constraint to banana, plantain and enset production in central and east Africa over the past decade

Abstract

Taxonomy: Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; currently classified as X. campestris pv. musacearum (Xcm). However, fatty acid methyl ester analysis and genetic and genomic evidence suggest that this pathogen is X. vasicola and resides in a separate pathovar.

Isolation and detection: Xcm can be isolated on yeast extract peptone glucose agar (YPGA), cellobiose cephalexin agar and yeast extract tryptone sucrose agar (YTSA) complemented with 5-fluorouracil, cephalexin and cycloheximide to confer semi-selectivity. Xcm can also be identified using direct antigen coating enzyme-linked immunosorbent assay (DAC-ELISA), species-specific polymerase chain reaction (PCR) using GspDm primers and lateral flow devices that detect latent infections.

Host range: Causes Xanthomonas wilt on plants belonging to the Musaceae, primarily banana (Musa acuminata), plantain (M. acuminata × balbisiana) and enset (Ensete ventricosum).

Diversity: There is a high level of genetic homogeneity within Xcm, although genome sequencing has revealed two major sublineages.

Symptoms: Yellowing and wilting of leaves, premature fruit ripening and dry rot, bacterial exudate from cut stems.

Distribution: Xcm has only been found in African countries, namely Burundi, Ethiopia, Democratic Republic of the Congo, Kenya, Rwanda, Tanzania and Uganda.

Ecology and epidemiology: Xcm is transmitted by insects, bats, birds and farming implements. Long-distance dispersal of the pathogen is by the transportation of latently infected plants into new areas.

Management: The management of Xcm has relied on cultural practices that keep the pathogen population at tolerable levels. Biotechnology programmes have been successful in producing resistant banana plants. However, the deployment of such genetic material has not as yet been achieved in farmers' fields, and the sustainability of transgenic resistance remains to be addressed.

Keywords: Xanthomonas wilt; banana; enset; plantain.

Figure 1

Map showing the banana production zones in Africa and the dominant cultivars grown in the banana production zones: A, plantain‐dominated; B, East African Highland banana‐dominated; C, Cavendish‐dominated; D, Mshare‐dominated. Mshare is a banana accession of AA species originating from Tanzania. ABB and AAB bananas can be found at all locations in varying numbers (http://banana.mappr.info/blog/).

Figure 2

Distribution of Xanthomonas campestris pv. musacearum, the causal pathogen of banana Xanthomonas wilt (BXW), within the East and Central African region (Manyong et al. 2008).

Figure 3

Symptoms caused by Xanthomonas campestris pv. musacearum. (a) Yellowing and wilting of the youngest leaves. (b) Wilting of the malebud and premature ripening of the fruit. (c) Internal discoloration of the fruits. (d) Pockets of yellow exudate in cut pseudostem.

Figure 4

Banana inflorescences. (a) The bract lifts to expose the female flowers that develop into the fruit arranged in clusters. (b) Male flowers subtended by a bract are always the last to appear.