Re your former horse paddock, Dad always used to say never manure the ground before potatoes as it will cause terrible scab.
Amend the soil with shrimp or crab meal, supposedly to stop or reduce scab.
I am trying out some varieties this year that you listed as 'resistant' (to some degree or another) - German Butterball, Nicola, Red Gold. Will see how they compare for scab. Also trying Agria, which is said to be scab susceptible in alkaline soil (but we are on the acid side here). And Yukon Gold we're hoping to get as my mom is fond of them whether they have scab or not.
I think it's true that the russets are resistant. Don't think I've ever seen one that was badly scabby, and a lot are untouched by it. Too bad I don't like em much, so dry and starchy when roasted, and basically turn into soup when boiled.
There is always a certain amount of scab expected but unless it's very bad it is just a minor cosmetic thing to us. Far worse is whatever blight causes the tubers to rot and stink in storage! I had a few of those in commercial bags of potatoes this winter... really gross.
