CHEMICAL CONTROL OF COMMON SCAB OF POTATO TUBERS

A number of workers have published results on the use of metallic compounds to control 'Common Scab' disease of potato tubers caused by Streptomyces scabies. Mader and Mader (1937) reported that Bordeaux mixture at 85 kg/hectare of copper sulphate markedly reduced scab, retarded flowering and increased yield, these results being confirmed by Dorozkein (1955). Houghland reported that aluminium chloride at 160 ppm inhibited growth of Streptomyces scabies in culture media but that up to 900 kg/hectare of aluminium sulphate did not control scab in field trials. In 1955, Spatz found that 6 kg/hectare of manganese sulphate reduced the percentage of badly scabbed tubers from 70 to 20 and in trials conducted by Guntz and Coppenet (1957), boron, copper, manganese, molybdenum, zinc and magnesium salts were tested and only manganese gave any marked control. Mygind (1961) reported that aluminium sulphate was not effective at economic rates and that manganese sulphate had an erratic effect unless large quantities were used.

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.