what causes potato scab?

does anyone know what causes this.  is it something in the soil or  the potato variety or something else.  my desirees are particularly bad this year (especially from one part of the allotment). yet my charlottes are relatively unscabbed.

plant a resistant variety, i also find that feeding with comfrey tea helps stop it, as does sprinkling some fbb at planting time.im plagued with it at home and i find these things help.

There are two sorts of scab, common scab and powdery scab. The one you are troubled with is probably common scab. Both are caused by bacteria in the soil.

Some varieties are more suseptable. Go to the Potato Council's website and it tells you the suseptability of each variety.

Light abrasive soils, dry conditions, alkaline soils encourage it.

Plenty of organic material to keep the moisture, avoid liming just before growing, plenty of moisture when the tubers are forming will reduce it.

Commercial growers make sure the crop has plenty of moisture for about a month after the tubers form.