الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Potato common scab disease has limited effect on tuber yield but can greatly affect tuber quality and therefore, severely reduce the marketability of table stock and tuber seeds. Streptomyces species belong to the actinomycetes, a group of gram positive bacteria. The taxonomic definition of Streptomyces has traditionally relied on morphological and physiological properties. Twenty two isolates had positive infection of radish seedlings, variation in aggressiveness were observed among isolates by using minitubers assay. The genetic variation among thirteen Streptomyces scabies isolates which differed in their sources and regions of isolation, and their pathogenic abilities were evaluated using Rapid- PCR technique with three primers. The investigated thirteen isolates were clustered in two main clusters with similarity ranged between 18.9-94.3%.Many strains of Streptomyces scabies can cause common scab symptoms on different fleshy roots or tubers. Eight potato cultivars were used for determining their relative susceptibility to common scab. No complete resistance was recorded by the tested cultivars. Furthermore Spunta cultivar was the most susceptible while, Astrix and Carsou cvs were the least susceptible ones where they contained the lowest amounts of reducing sugars, higher amounts of total phenols, and lower amounts of amino acids compared with the most susceptible cultivar (Spunta). The effects of eight fertilization programs based on (NPK) were evaluated using infested soil and potato cultivar (Spunta). It was found in all treatments higher number of non-infected tubers compared with control and also there were significant differences between treatments. Six isolates of non-pathogenic Streptomyces spp. and antagonistic bacteria were used to reduce severity of common scab disease where, the bacterial isolate Pf3 and Bs10 were the most effective antagonists tested against Streptomyces scabies. Other materials such as Streptrol, Starner, plant extract of Carwaya, Rotacin and Jojoba, significantly reduced severity of common scab disease. The tubers treated with these materials showed reduction in disease index (DI) than control where the most effective treatment recorded with using Rotacin. Application of tested bioagents in form of talk powder formula as soil treatment decreased the disease index compared with the control. Key words: Common scab, Streptomyces scabies, RAPD-PCR technique, Economic hosts, NPK fertilizers, Biological control, Biolog system, Pseudomonas asplanii.