Scab is caused by a fungus that develops in soil that has slightly less than the optimum amount of moisture. And scab can be most severe in neutral or alkaline soils, but is usually not found in acidic soils of a pH of 5.2 or less. It is usually worse in dry rather than moist soil.

One solution: green manuring (soybean, rye, or alfalfa can be grown and turned under making the soil more acidic). Avoid using lime, wood ashes, or fresh manure--these will cause the soil to become more alkaline. Scab will decrease if potatoes are rotated out of the garden for three to five years.

Try planting resistant varieties: Cayuga, Cherokee, Early Gem, Menominee, Ontario, Seneca--also Cinook, Huron, Avon, Sable, Cariboo, Norland, Netted Gem.

Here is a link that might be useful: HarvestToTable.com

Thanks for the advice - I had been using Caribe and Yukon Gold so will make a switch I guess. The area is quite sandy and aged horse manure is applied to the entire garden area. I will also see about planting in a different area - would more frequent watering help? since you mention soil moisture levels?

Thanks again

For what it is worth, my father and his cohort were absolutely phobic about putting horse manure on the potato patch. Claimed it caused scab. Never tested it myself. In the 30 and early 40's we farmed with horses so it was available. We used cow manure and hog manure but horse manure went on cornfields. Chicken manure was also forbidden on the potato patch. Grew vines but not potatoes.

Here is a link that might be useful: scab