CHEMICAL CONTROL OF COMMON SCAB OF POTATO TUBERS
The extensive work of Mortvedt et al (1960, 1961 and 1962) showed that in pot culture, scab decreased as manganese content increased over the range 0 – 20 ppm. Field trials produced variable results. In one trial there was no control at 56 and 112 kg/ha but in another, 168 and 506 kg/ha of manganese sulphate gave good control. Glasshouse experiments did not relate periderm manganese content to scab incidence and this also applied to copper. Copper sulphate at 22.5 and 56 kg/ha significantly reduced scab but caused some phytotoxicity.
McGregor and Wilson (1964 and 1966) confirmed the effect of manganese sulphate, stating that it gave fair control at 63 kg/ha when mixed with a compound fertiliser in the drill before planting. Mortvedt suggested that a high concentration of soluble manganese could explain the tendency for scab to be less prevalent in highly acid soils. Scab is most common on alkaline soils and if manganese is to be used for control it may have to be applied in a form which will not be oxidised to insoluble forms.