• Potato scab

Streptomyces species are the causal agents of several scab diseases on potato tubers. Streptomyces scabiei, S. stelliscabiei and S. caviscabiei have been associated with common scab in South Africa.  A new type of scab symptom, caused by Streptomyces species, was observed in South Africa from 2010 onwards. The disease was initially thought to be caused by a single Streptomyces species, however, subsequent isolations from similar symptoms on other potato tubers revealed diversity of the Streptomyces isolates. We characterised these isolates in order to determine what are the major species involved in the disease. The results of the phylogenetic and genome analyses revealed that there are three main species involved, namely, Streptomyces werraensis, Streptomyces pseu dogriseolus and a novel Streptomyces species that is described here as Streptomyces solaniscabiei sp. nov., with strain FS70T (= PPPPB BD 2226T = LMG 32103T) as the type strain. The glasshouse trial results showed that all three of the Streptomyces species are capable of producing fissure scab symptoms. None of the Streptomyces isolates from fissure scab contained the full PAI and the mechanism of disease initiation still needs to be determined.

In addition, the 16S rDNA sequences of the causal agents of fissure scab and common scab were aligned and a hypervariable region was identified. Primers were designed around this region to amplify a 125 bp fragment, and this was used to develop a high resolution real time PCR (qPCR-HRM) to distinguish between the different fissure scab species. The results correlated 100% with the result obtained by MLSA. In addition the qPCR-HRM test could also detect the three common scab species prevalent in South Africa.

Contact: Dr Rene Sutherland and Dr Michele Cloete

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Typical c​ommon scab symptoms
on a potato tuber.

Fissure scab sy​mptoms on a potato tuber.
  • Bacterial wilt of potatoes

Ralstonia solanacearum is regarded as one of the world's most important bacterial plant pathogens because of its aggressiveness, large host range, broad geographical distribution and long persistence in soil and water environments. The survival of R. ​solanacearum in South African soil under different temperature and moisture conditions are investigated and knowledge regarding the survival limits in soil will be helpful to define control strategies for this devastating pathogen.

ContactDr Rene Sutherland

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