The last several years I have been having problems with my potato crop, how do I prevent Potato Scab?

Question
The last several years I have been having problems with my potato crop.  I have moved them into different locations within my garden space (about 100'x60'), without any better results.  This year I thought we would try fresh seeds and bought them from Zamzows, my preference is the red potato but reading about potato scab on line, they say the red potato is easier to get the scab than other varieties so we planted a few yellow yukon along with the others to check this out. 

The results were worse than they have been and the yellow yukon had the same problems.  I do not use any commercial fertilizers or chemicals trying to keep everything organic.  We use chicken manure mixed with pine shavings that we keep in the nests and under the roost the same as we have for years.  I have included a couple of pictures of the problem, wondering if you guys know for sure what I have and the solution to get rid of it. 

Thanks for your help

Answer
Potato scab is a soil borne bacteria and common in most potato growing areas.  there are three types of bacteria that will cause scab on most root crops including potatoes.  first rule in planting potatoes is to buy frees seed spud each years.  In Idaho seed potatoes must be certified free of any and all diseases.  Never plant any left over potatoes from last year or any that you have bought from a grocery store as they may carry the bacteria that cause scab. 

The soil pH will also help promote the growth of the bacteria that cause scab.  You will want to try to keep the soil pH down to close to 7.0 as possible.  This will slow the growth of the bacteria in the soil.  Many research universities recommend that the use of manure as a fertilizer will also promote the growth of scab bacteria.  Watering is important in controlling the scab.  Rotating the areas for planting potatoes is a controlling factor in reducing the scab.  Never plant potatoes or other root crops in that area for 3 to 4 years to help eliminate the scab on your crop.