Powdery scab of potato, caused by the zoosporic pathogen Spongospora subterranea f.sp. subterranea (Sss), is an often underestimated disease, which has lead to lack of appropriate control strategies. A new wave of increasing importance of this disease has occurred in the last 30years in many countries, among them France, Pakistan, Japan, Australia, New Zealand, Costa Rica, USA, and, very recently, Colombia, Korea and China. A consequence is that occurrence of, and research on, powdery scab has shown a cyclic pattern. Additionally to its importance as a pathogen, Sss is a vector of potato mop top virus which itself can cause substantial damage. Factors such as intensification of potato production, increasing use of susceptible cultivars, more frequent irrigation and banning of mercury, previously used as an efficient seed tuber treatment, have all contributed to greater incidence of powdery scab. Since 1842, when Wallroth first described the disease in the scientific literature, many researchers, particularly before 1960, started to work with Sss, mostly because powdery scab became a problem in their respective countries. Today we know many aspects of the biology of the biotrophic protozoan organism that causes powdery scab, but we still lack basic knowledge on the etiology and epidemiology of the disease. We lack sufficient knowledge of the factors that affect survival of the resting structures, potato and alternative host infection and disease development. This knowledge is essential in predicting disease risk and developing effective powdery scab management strategie
