Reactions of potato varieties to skin spot (Oospora pustulans) infection and transmission in different soils

Emergence delay and blanking caused by skin spot were shown to depend on the degree of seed infection and were intensified by heavier soil. From similarly affected seed they were significantly less with Kerr's Pink than with King Edward, Majestic or Redskin. Growth was very similar from normal and necrotic buds of Kerr's Pink but virtually no growth was recorded from necrotic King Edward buds, suggesting that sprout vigour influences field response to skin spot. Transmission of infection in King Edward to stem bases, stolons and progeny tubers depended on seed infection and even minimal inoculum caused stolon and tuber infection especially in heavier soils. With severely infected seed transmission was greatly reduced in light soil. This indicates one means of reduction of disease incidence. Colonisation by Oospora pustulans of stem bases and stolons probably provides inoculum multiplication centres. Maximum colonisation was detected generally in September and declined during senescence.