First report of Streptomyces acidiscabies causing potato scab in China

In spring 2004, potato scab disease was observed on potato tubers (Solanum tuberosum cv. Favorita) at approximately 5-10% incidence in fields in Shandong province, China. Scabby tubers had brown, superficial or pitted corky lesions that were somewhat different from typical symptoms (for example, superficial, raised or pitted lesions)of potato scab caused by Streptomyces scabies (Fig. 1). Both traditional and PCR-based methods were employed to identify the causal agent. 

A Streptomyces sp. strain CPS-3 was isolated from the lesion of a scabby potato tuber on oatmeal agar. It produced rectiflexuous spore chains, and the spores were white with smooth surface and 0.4-0.5 x 0.9-1.1 μm in size. The strain did not produce melanin on tyrosine-peptone-yeast extract agar medium, but produced diffusible yellow pigments, utilised all the International Streptomyces Project (ISP) sugars as single carbon sources (Shirling & Gottlieb, 1966), and L-hydroxyproline, L-histidine as sole nitrogen sources but not L-methionine. It grew at pH 4.0 and was susceptible to phenol (0.1%) and crystal violet (0.5 µg/ml), but not to penicillin (10 IU/ml) and streptomycin (20 μg/ml).It did not produce H2S. The 16S rRNA sequence of the strain was amplified using the primers S1 ( 5´-CATTCACGGAGAGTTTGATCC-3´) and S2 (5´-AGAAAGGAGGTGATCCAGCC-3´). The resulting 1460 bp sequence (GenBank Accession No. AY621376) had the highest similarity (99.8%) with S. acidiscabies type strain ATCC 49003 (AB026220). The strainis identical toS. acidiscabies based on the above characteristics (Lambert & Loria, 1989).

A pathogenicity test was carried out by inoculating young potato plants of cv. Favorita with 100 ml spore suspension (1 x 10⁵ cfu/ml) of the strain CPS-3 in a greenhouse where temperatures varied from 20 to 25°C. Brown, corky scab lesions identical to those observed in the original field developed on the immature potato tubers obtained from the inoculated plants 10 weeks after planting. No symptoms were observed on control plants inoculated with sterile water. The pathogen was re-isolated from the lesions of immature tubers and identified asS. acidiscabies according to the above methods. Potato scab caused by S. acidiscabies was previously reported in the USA (Bonde & McIntyre, 1968; Loria et al., 1997). To our knowledge, this is the first report of S. acidiscabies causing potato scab in China.