Pectobacterium carotovorum subsp. brasiliensis causing blackleg on potatoes in South Africa

In South Africa during the 2006/2007 potato growing season, outbreaks of blackleg occurred, causing severe economic losses in commercial potato production fields. Symptoms were initially observed on only one stem per plant, on which the top leaves rolled upwards, wilted and became necrotic. As symptoms progressed to the lower leaves with subsequent leaf desiccation, a light to dark brown discolouration of the vascular system at the stem base developed, followed by external darkening. Under prevailing wet and humid conditions stems became slimy and pale. In the stems, the pith became necrotic and hollow. These symptoms were similar to those described in Brazil, where the causal agent was identified as a new subspecies, Pectobacterium carotovorum subsp. brasiliensis (Pbcb). Isolations from plants showing typical blackleg symptoms were made on CVP medium. Sequences and phylogenetic analysis of the partial 16S–23S rDNA intergenic spacer region indicated that the isolates were Pbcb. Comparison of PCR-RFLP patterns of the 16S–23S rDNA of isolates to reference cultures confirmed the identity of the South African blackleg strains as Pbcb, identical to strain 8 isolated in Brazil. This is the first report of Pbcb in South Africa and it appears to be the most important causal agent of blackleg in South Africa. The disease poses a major potential threat to the South African potato industry especially in terms of seed exports, tuber quality and yield.