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ThesisItem Restricted Characterization, population dynamics of Streptomyces spp. and management of potato scab(Punjab Agricultural University, Ludhiana, 2019) Sabhikhi, Hardeep Singh; Hunjan, MandeepScab of potato caused by various Streptomyces spp. has assumed serious proportions in certain potato growing regions of Punjab. A total of 2060 acres were surveyed during this study, out of which 490 acres were having scab infection. Maximum disease severity was observed in Kapurthala (7.9%), followed by Ludhiana (6.3%) and Jalandhar (6.2%) districts. During the surveys, it was observed that common scab type of symptoms were the most prevalent (89.7%), however disease induced by multiple type symptoms was viz., pitted or raised corky lesions was more severe. One hundred seven isolates of Streptomyces were collected, and characterized biochemically, pathotypically and genetically. All the Streptomyces spp. isolates could utilize hexose sugars, but not pentose sugars and most of the isolates produced melanoid pigment, an important virulence factor of this bacterium. Multiple antibiotic resistance was observed in 20 isolates. Pathotypic bioassay of Streptomyces spp. isolates revealed that Kufri Pukhraj had the highest virulence frequency (81.3%) while K. Himsona had the least (40.2%). Ninety six isolates were confirmed pathogenic. The 16srDNA amplification confirmed the identity of the genus, whereas sequence analysis of nine representative isolates revealed that Strep101 belonged to Streptomyces acidiscabies, Strep102 to reticuliscabiei, while Strep131, Strep137, Strep143, Strep145, Strep154, Strep170 and Strep173 to scabiei. All isolates were further characterized on the basis of pathogenicity genes, txtA and nec1.The txtA gene from Punjab isolates either matched with S. scabiei or S. turgidiscabies No txtA amplification was observed in S. acidiscabies, Strep101 and S. reticuliscabiei, Strep102. The nec1 gene was amplified in 84 out of 96 isolates and no nec1 was detected in S. reticuliscabiei, Strep102. Genetic fingerprinting carried out by using 17 RAPD primers clustered these into eight genetic groups. Many genetic groups had multiple virulence groups and vice-versa. To identify resistant donors, 41 potato germplasm lines were screened using representative isolates of different virulence groups. Most of the lines were susceptible to moderately susceptible. Three crop rotations viz., fallow-rice-potato, summer moong-rice-potato and sunhemp-rice-potato showed maximum per cent decrease in the population of Streptomyces spp. in soil and scab severity on potato tubers. Using disease free seed tuber or early harvesting of potato from scab sick field also observed that to reduce scab effectively. In vitro and in vivo evaluation of different chemicals against S. scabies showed that Streptocycline, streptomycin, boric acid and copper sulphate were effective. However, tuber seed treatment with chemicals viz., Kocide@0.2% and Streptocycline@100 ppm and antagonist Streptomyces spp. strain S25 and S27 had negative impact on the germination of potato seed tuber. For the management of scab, complete package involving use of tolerant cultivars, disease free seed or seed treatment, and cultural practices have to be followed.