Common scab caused by Streptomyces spp. is one of the major factors limiting successful potato (Solanum tuberosum) production in South Africa. Most potato cultivars are susceptible to the disease and huge losses are incurred due to a reduction in cosmetic value. Common scab symptoms that have been reported worldwide are variable, with circular, raised, tan-to-brown, corky lesions being the most common symptoms. However, a distinct atypical symptom was observed in isolated production regions (Western Free State and Mpumalanga) in South Africa since early 2010. Deep longitudinal fissures (3 to 12 mm) containing scab-like lesions were observed on the surface area of tubers from several potato cultivars (Mondial, BP1, and Buffelspoort). Lesions on Mondial were interesting since this cultivar, but not the susceptible BP1 and Buffelspoort cultivars, is tolerant to typical common scab in South Africa. Isolations were made from tuber lesions obtained from the two production regions and were plated onto yeast malt extract medium. Several pure culture strains were obtained that were positively identified as Streptomyces spp. based on morphology. Species identity of four strains was investigated using PCR primers targeting the 16S rRNA region of known species (3), which showed that the strains did not belong to any of the known pathogenic reference strains (S. scabiei, S. europascabiei, S. turgidiscabies, S. acidiscabies). Species identity of the strains was further investigated through sequencing of the 16S rRNA region (1.2 kb). The four strains had 100% sequence similarity (GenBank Accession. No. JQ241439) to each other and to 15 GenBank sequences that included several unknown Streptomyces spp., S. vinaceus, S. malachiticus, S. werraensis, S. cyaneus (the only published sequenced), and S. pseudogriseolus. The sequence of the isolates had only 95.5% identity to the most prevalent common scab pathogen, S. scabiei (GI154707840, ). The pathogenicity of the four strains, along with an S. scabiei reference isolate, was investigated using BP1 potato tubers and the double pot methodology (1). Fifteen-centimeter pots containing silica sand were each planted with one potato tuber and placed on top of a rectangular planting box (15 × 1.5 × 0.4 m) filled with native Hutton soil. The 15-cm pots were irrigated until the roots grew into the Hutton soil. Subsequently, irrigation was only applied to the Hutton soil, thus creating dry conditions in the tuber development area (15-cm pot), which is conducive for common scab development. The potato plants were inoculated with 10- to 14-day-old Streptomyces strains during the tuber initiation stage at a concentration of 1 × 106 spores/ml of sterile water, 10 ml per plant, and six replicates per strain. Ten weeks after inoculation, each of the four strains caused cracking and scabbing, similar to initial symptoms observed, on more than 90% of the tubers. The S. scabiei reference isolate caused typical circular, raised, brown, corky common scab lesions. Streptomyces was reisolated from the lesions, fulfilling Koch's postulates. To our knowledge, these results indicate that a previously unreported Streptomyces sp. is the causal agent of a new lesion type, fissure scab, on potato in South Africa that may lead to serious losses to the local potato industry.
References: (1) L. Marais and R. Vorster. Potato Res. 31:401, 1988. (2) D. J. Theron. Page 1 in: Guide to Potato Production in South Africa, 2003. (3) L. Wanner. Phytopathology 96:1363, 2006.