Causes of Potato Scab
We have the privilege of working with Mark Fulford as our soil consultant.
When asked if he had any advice for gardeners with potato scab he suggests
- scab may be caused by a shortfall of available Phosphorus
- may be caused by a shortfall of soil fungi
- may be caused by low levels of  mycorrhizae in the soil

Agri phos triggers the plant's immune system.  one farmer puts 30# of SulPoMag/ac in furrow at planting.