Abstract:
Soft rot and blackleg can cause severe economic
losses in potato production in South Africa and
Zimbabwe depending on climatic conditions. The aim
of the study was to identify the predominant bacteria
causing potato soft rot and blackleg in these countries.
Samples, comprising of stems and tubers from potato
plants with blackleg and soft rot symptoms were collected
from 2006–2009 from potato production areas
where disease outbreaks occurred. The isolates from
these plants and tubers yielded Gram negative,
pectinolytic bacteria on crystal violet pectate and inoculated
tubers. Identification was based on biochemical
and phenotypic characteristics, rep-PCR,
Amplified Fragment Length Polymorphisms and
sequences of gyrB and recA genes. Isolates from
Zimbabwe were identified as Pectobacterium carotovorum
subsp. brasiliensis (Pcb) (21 isolates), Dickeya
dadantii subsp. dadantii (Dd) (20 isolates), P. c. subsp.
carotovorum (Pcc) (16 isolates) and P. atrosepticum
(Pa) (4 isolates). Pcb, Pcc and Dd subsp. dadantii
were isolated from samples collected from all the
regions, while Pa was isolated from Nyanga the
coolest region in Zimbabwe. In South Africa, however,
Pcb was the most common causal agent of soft
rot and blackleg. P. atrosepticum was the only pathogen
isolated from samples collected in Nyanga,
Zimbabwe, and was not isolated from any South
African samples. AFLP analysis separated the Pcb
strains into 12 clusters, reflecting subdivision in terms
of geographic origin, and Pcc isolates were clearly
differentiated from Pcb isolates. A large degree of
DNA polymorphism was evident among these 12
clusters. The study identified all the pathogens associated
with the blackleg/soft rot disease complex.
