Biological control of common scab disease of potatoes caused by Streptomyces scabies

This thesis investigates biological control methods against the causal organism of potato common scab disease. This disease is found wherever potatoes are grown and can result in substantial financial losses to growers. The disease is caused by invasion of lenticels by a number of pathogenic streptomycetes which historically have been referred to as Streptomyces scabies. A multi-control strategy was adopted and targeted to two key areas of infestation: the transport of infested seed tubers to grower's properties and uninfected fields; and control after outplanting in infected fields. The pathogen used in this study was isolated from a commercial potato field in northern Tasmania. Using cultural and morphological characteristics, the organism was identified as Streptomyces scabies and coded T19. A novel approach was taken in an attempt to achieve biological control at the seed tuber stage. This required the isolation and propagation of a phage effective against Streptomyces scabies. A mini airlift bioreactor was designed and built to rapidly propagate the phage and to produce sufficient volume (6 litres) for experimental purposes. When mini tubers with a surface dried infestation of Streptomyces scabies were immersed in a phage solution and outplanted in a glasshouse trial, common scab lesions were restricted to less than one percent of the surface area of tubers compared to tubers without phage that had an average of 25 % lesion coverage. This phage were ineffective when applied as a soil drench. Since biological control in the field is likely to be achieved through the manipulation of populations of antagonistic soil borne bacteria, a number of streptomycetes and nonstreptomycete actinomycetes were isolated from a pathogen suppressive soil from northern Tasmania. In addition, other isolates were obtained from a private laboratory. Eleven of the 264 isolates tested that were antagonistic to Streptomyces scabies in an in vitro test were evaluated in the glasshouse trial. Lesions were reduced to 4% of tuber surface area when a mix of the 11 antagonistics and Streptomyces scabies were applied to the soil. When Streptomyces scabies was applied without antagonists, lesion surface area was 26%. This study demonstrates that potential exists for the biological control of potato common scab but recognises that more extensive trialing with a wider range of organisms should be undertaken