First report of Xanthomonas campestris pv. musacearum on banana in Uganda

Abstract

In October 2001, a wilting disorder of new aetiology was reported affecting banana (Musa sp.) within the Mukono district of Uganda. The disorder was characterized by a rapid yellowing and wilting of the younger leaves, a discoloration of the internal vascular vessels, occasionally a dieback initiating from the male floral parts with internal rotting of banana fruits. These symptoms were notably distinct from fusarium wilt (Fusarium oxysporum) and ‘Matooke wilt’ (a wilt-like disorder of unknown aetiology), but strongly resembled Moko disease caused by Ralstonia solanacearum, although this particular pathogen had not previously been recorded on banana in Africa. A bacterium was isolated on nutrient agar that was identified by fatty acid (Microbial ID Inc. [MIDI]) and metabolic (Biolog, Inc, Hayward, CA, USA) analyses as Xanthomonas axonopodis (ID probability score < 0·4) and Xanthomonas campestris (ID probability score ∼0·9), respectively. The presence of the Xanthomonas specific fatty acids 11:0 ISO, 11:0 ISO 3OH and 13:0 ISO 3OH was recorded. Pathogenicity tests on disease-free tissue culture-derived banana plantlets by stem inoculation with a bacterial suspension induced wilt symptoms consistent with field observations after 3 weeks. Reisolation and identification, as outlined above, confirmed Koch's postulates. Reference to the literature suggested the bacterium was Xanthomonas campestris pv. musacearum (Yirgou & Bradbury, 1968). However, this bacterium is relatively poorly described and not contained within either the MIDI or Biolog databases. To support the identification, rep-PCR (Louws et al., 1994) using ERIC and BOX primers was performed on the Ugandan banana isolate, cultures of X. campestris pv. musacearum from Ensete and Musa in Ethiopia (IMI 349461, IMI 349986, IMI350025) and other cultures of Xanthomonas spp. from Africa. These analyses revealed an identical DNA fingerprint for all isolates from Musa and Ensete, but distinct fingerprints for the isolates from other hosts. This is the first report of X. campestris pv. musacearum outside of Ethiopia, where it is recorded as a pathogen of ensete and, to a lesser extent, banana. Accordingly, this pathogen has been given the common name of ensete bacterial wilt, although the aptness of this now looks questionable. The risk posed by this new disease record to the contiguous banana plantation of Uganda is undetermined, but significant spread is already being observed. The causative organism for this new disease record has been deposited within the CABI Genetic Resource Collection as IMI 386970.

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