Dear Mehi Lal

Conditions favouring development of the disease: Soil reaction (pH) and soil moisture the most important factors affecting seasonal incidence and regional distribution of scab, Common scab may be particularly severe when potatoes are grown in neutral or alkaline soils (pH 7.0 and above). Some races of the organism can damage tubers at a pH of 5.0 or below, but it is generally accepted that the disease is usually controlled satisfactorily if potato soils are kept at pH 5.0 to 5.2. Within the range of pH 5.2 to 8.0 the severity of the disease increases with increasing alkalinity. Also warm and dry soil conditions, particularly during  the critical period of susceptibility to scab (two and five weeks after tubers start to form), can markedly increase the incidence of the disease. 

Disease management: Use of resistant varieties, Rotation, Adequate moisture and timing of irrigation, Soil management

- Avoid excessive use of lime, fresh animal manures and wood ash. If lime is necessary, do not raise the soil pH above 5.2; ideally, use dolomitic lime after potatoes in the rotation.

- The lowering of soil pH to between 5.0 and 5.2 with applications of sulphur has proved useful in reducing the level of scab in some soils of high pH.  Use acid-producing fertilisers and use ammonium sulphate as a source of nitrogen.

Combined control measures: no one control measure is as effective as a combination of several. For example, controlled irrigation combined with sulphur treatment results in better control than either treatment used on its own.

Maintaining high soil moisture (80–90% of available water storage) during tuber initiation and the 6 to 8 weeks that follow reduces the severity of scab and usually controls the disease adequately. Other practices that help reduce the incidence of scab include green manure crops such as rye, millet, and oats Avoid soil application of animal wastes, which favors scab development. Use certified seed tubers free from common scab. And more

Pls. find the attached files

http://www.tandfonline.com/doi/full/10.1080/09583157.2015.1079809

http://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-01-13-0022-R

http://www.sciencedirect.com/science/article/pii/S1049964411000405

Compendium of Potato Diseases (2001) ed. by Stevenson, W.R. and et al, American Phytopathological Society, St. Paul, MN, USA.

Mulder, A. and Trukensteen, L. J. (2005) Potato diseases, 2508 AC Den Haag The Netherlands.

Potato Health management (2008) ed. by Johnson, D.A. American Phytopathological Society, St. Paul, MN, USA. 

http://ipm.illinois.edu/diseases/series900/rpd909/

http://www.potatodiseases.org/scab.html

http://www.nrcresearchpress.com/doi/abs/10.4141/P01-030#.V_YlCDZWGiM

https://www.planetnatural.com/pest-problem-solver/plant-disease/potato-scab/

https://www.researchgate.net/publication/232409239_Biological_management_of_common_scab_of_potato_through_Pseudomonas_species_and_vermicompost

http://link.springer.com/article/10.1007/s10327-015-0614-y

https://www.researchgate.net/profile/Birinchi_Sarma/publication/232409239_Biological_management_of_common_scab_of_potato_through_Pseudomonas_species_and_vermicompost/links/559ca75408ae898ed651fdde.pdf

http://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=3640&context=etd

Hoping this will be helpful           

Regards

Prof. Houda Kawas

Article Biological management of common scab of potato through Pseud...

Crop rotation is the best way to manage the disease. However, common scab of potato can also be managed by other ways. Please check the following links and PDF attachments.

http://www.tandfonline.com/doi/abs/10.1080/03235408.2013.870375

https://www.ncbi.nlm.nih.gov/pubmed/27088392

 Thanks Prof. Houda Kawas and Arvind Singh for nice information. Basically as per my knowledge  chemicals are not available except some for commercial cultivation of potato.