Common scab, Streptomyces Scabies, is a soil borne disease living in all types of soil all over the world. A common complaint in some varieties most years but it can be controlled.
There are over five hundred different species of Streptomyces, some of them causing common scab, many that don't. The Streptomyces multiply fast in soil that is higher than a pH of 5.5 and is dry around the newly forming tubers and cause scabs to develop on the tubers. Once the tuber has common scab, adding moisture to the soil will not remove the scab, it will stop more developing but the scabs already formed will grow with the tuber.
The critical point for common scab to occur happens two to three weeks after emergence of the crop. As the end of the stolon starts to swell after hooking, for the next 4 to 6 weeks is when the soil must remain moist. the crucial stage ends after the primary tubers have reached 25mm+. This is unless you are growing a salad crop and require any later tubers to be scab free. There is no need to irrigate before this point if the soil is damp as there is no danger to future crop from common scab. Save the water for Tuber Initiation ( TI ) and for bulking later. Too much water can cause problems too such as powdery scab, root rot and can cause other complications like soft rots and watery would rot.
Other Factors know to contribute to common scab forming is a neutral to alkaline soil. For best control, soil pH should be around 5.1 and moist. If you are growing a susceptible variety, such as Maris Piper or Desiree, monitoring of the Soil Moisture Deficit (SMD) is crucial in all parts of the field in all crops. Moisture does not leave the soil in the field as it would leave a sponge, it has varying rates all over a field. If there is a sandy loam, a patch of clay, a slope, a rock head near the surface, these will all affect the rate of soil dispersal.
All fields are different and keeping good field records, including any soil tests, any inputs like lime, FYM or nitrogen, weather conditions throughout, harvest conditions and a full QC report of the tubers harvested. This will enable you to build up a picture over the years to give a guide to problem areas and whether a more resistant variety should be grown in different areas. If you can't irrigate certain fields then there are less susceptible varieties such as Vales Sovereign and Orchestra that don't mind a dryer year when it comes to common scab that you can plant to get a marketable crop.