This disease has been a serious problem for potato growers in New Zealand and part of Europe for many years, and has recently been recognized as a problem in North America. Powdery scab is caused by the fungus Spongospora subterranea. It produces a resting spore stage (spore balls) that can survive in soil for at least six years. Initial infection occurs at early tuber set, and is favoured by high soil moisture and soil temperatures of 57-64 (14-18).