Since I have 20+ horses, I use well composted horse manure on my whole garden, Including potatoes. I've never had issues with scab. I also have chickens, so I use the chicken manure selectively on the garden after it has composted for a year or more. Last year I used it on my artichokes. I buried it in a trench and covered it and then planted them. Had the best crop ever. I haven't used chicken manure on the potatoes though. I remember a long time ago using chicken manure that was a bit too fresh on my tomato patch, and getting wonderful foliage.
