1Groupe de Recherche en Biologie des Actinomycetes, Département de biologie, Université de Sherbrooke, Sherbrooke, Québec, Canada, J1 K 2R1; 2Centre de Recherche en Horticulture, Université Laval, Québec, Québec, Canada, Gl K 7P4

Background and objectives
Common scab of potato is a disease caused by streptomycetes belonging to various species [1]. However, Streptomyces scabies is a pathogen frequently isolated from potato lesions in Canada. Leiner et al. [2] showed that S. scabies not only induced common scab lesions on potato tubers but also caused growth inhibition of seedlings. The level of contamination of a specific soil by S. scabies and other pathogenic streptomycetes is difficult to determine. Indeed, pathogenic strains cannot be easily differentiated from non-pathogenic ones using phenotypic criteria, and pathogenicity tests on potato are time- and space-consuming. The main objective of this work is to propose a method to isolate pathogenic strains directly from soil and to estimate the level of contamination of a soil by pathogenic streptomycetes.

Materials and methods
Actinomycetes were isolated from soil as follows. Sample of soil (1 g) was mixed to 1 ml of solution A (0.85% NaCi; 0.71% phenol). The soil suspension was kept at room temperature for 5 min. Dilutions of soil suspensions were then plated on actinomycete isolation agar (Difco) supplemented with 50 mg/l cycloheximide and 1 mg/l penicillin G. Plates were inoculated for 5-7 days at 30°C. A percentage of the isolated actinomycetes were purified and their pathogenicity was tested on radish seedlings as previously described [2]. Isolates causing significant growth reduction of radish seedlings were analysed for their ability to produce thaxtomin A in oatmeal broth.

Results and conclusions
Soil samples were obtained from four different geographical regions of Québec (Canada) and were collected before plantation. Results presented below are the mean of four replicates. Actinomycete populations in soil varied from 1000 to 200,000 c.f.u./g soil. About 200 actinomycetes of each site were tested for their pathogenicity on radish seedlings. For each site, approximately 10% of the actinomycetes caused a significant reduction of seedling growth. Half of the actinomycetes causing growth inhibition produced thaxtomin A, a phytotoxin associated with plant pathogenic actinomycetes. It is not known yet whether or not the isolates producing no detectable amount of thaxtomin have the ability to induce common scab of potato.

1. Goyer C, Otrysko B, Beaulieu C, 1996. Canadian Journal of Plant Pathology 18, 107-130.
2. Leiner RH, Fry BA, Carling DE, Loria R, 1996. Phytopathology 86, 709-713.