The biology and control of powdery scab (Spongospora subterranea) of potatoes
The aim of this work was to develop improved control strategies for powdery scab based on a better understanding of the biology of the causal organism, the fungus Spongospora subterranea. Spongospora subterranea readily infected the roots of bait plants, and this was exploited to develop a bioassay using potato microplants. The bioassay proved sensitive enough to detect changes in the concentration of inoculum and was used for the fast and efficient screening of chemicals for control. The most critical determinant in the severity of disease developing on progeny tubers appeared not to be the initial level of inoculum. Experiments demonstrated that severe infections only develop when the initial inoculum was free to multiply vegetatively within the host roots: conditions such as wet soils that favoured the free movement of, and invasion by, zoospores enabled even low levels of inoculum to result in severe infections. Tubers were shown to be most susceptible to infection early in their growth. When a range of crop irrigation regimes were evaluated in the field, irrigation applied over the tuber initiation period significantly increased infection by powdery scab. The fungicidal effect of zinc ions was clearly demonstrated using the bioassay, and significant control in the field was achieved using zinc products of different solubilities as seed treatments. The most effective soil treatments were those that were moderately soluble but persisted in the soil. Thus, although zinc was shown to be a potential means of control, the studies of the biology of the fungus demonstrated the great importance of cultural control.