Relationship of common scab incidence of potatoes grown in Tasmanian ferrosol soils with pH, exchangeable cations and other chemical properties of those soils

Scab conduciveness of 35 Tasmanian ferrosol soils, used for potato cropping, was compared in a glasshouse trial. Linear regressions showed no association between scab incidence and any of 12 individual soil chemical properties. However, using contingency tables, threshold levels were found for pH and exchangeable cations below which scab disease was less likely. The observed pH threshold in the range 5.0-5.2 was in accordance with similar findings from elsewhere and indicates the robustness of this threshold over a range of soil types and pathogen species. Scab was not observed on potatoes grown in soil with combined exchangeable Ca. Mg and K at 12 cmol(c)/kg or less. A strong correlation between soil pH and these exchangeable cations, particularly calcium. was found.