Pathogens Causing Soft Rot and Blackleg Symptoms in Potatoes in Peri-urban Harare Area

Abstract

Aims: To identify, using standard biochemical tests, a polymerase chain reaction and sequencing, the soft rot species presumed to be the cause of potato soft rot and blackleg affecting potatoes in commercial farms around the Harare area in Zimbabwe. Study Design: Biochemical tests, a polymerase chain reaction and sequencing were conducted on the collected isolates. Place and Duration of Study: Samples were collected at potato commercial farms within a 60 km radius of Harare in May – June 2012. Laboratory experiments were conducted at the University of Zimbabwe, Plant Pathology Laboratory. Methodology: 24 samples of infected potato tubers and stems were collected from eight farms. Eight representative isolates were selected for identification tests. A polymerase chain reaction using ADE 1/2 primers and sequencing using gyrB and recA genes were carried out. Pathogenicity tests were conducted to further confirm the identity of the bacteria. Plants were observed for seven days for symptom development. Results: Biochemical tests showed that seven of the isolates were very homogeneous in their physical, cultural and biochemical properties. They tested positive for catalase, oxidative/fermentative and oxidase activity, three reactions that confirm their identity as soft rot bacteria. One isolate slightly differed from the others although results for most of the tests were similar. Polymerase chain reaction results showed amplification of the 420 bp fragment from all the eight isolates indicative of Dickeya species. Phylogenetic analyses of the rec A and gyr B gene sequences showed high genetic homogeneity of 95% and above between the isolates and Dickeya dadantii reference strain. Pathogenicity tests showed that healthy plants inoculated with the isolated bacteria all produced typical blackleg symptoms. Conclusion: Physiological and molecular tests confirmed that the most common pathogen causing blackleg and soft rot in potatoes grown in areas around Harare is from the Dickeya genus.