An In vitro Study on the Postinfection Activities of Hydrated Lime and Lime Sulphur against Apple Scab (Venturia inaequalis)

The activities of hydrated lime and lime sulphur to stop infections of apple scab (Venturia inaequalis) were evaluated at 20 C using an in vitro test system based on isolated cuticles and scab conidia. Treatments were applied 24 or 48 h after inoculation that is 16 or 40 h after a predicted infection period, respectively. Efficacy of the postinfection treatments was assessed 72 h after inoculation by counting living primary stromata, which fluoresced brightly green when stained with fluorescein diacetate (FDA). A suspension of hydrated lime (5 g/l) as well as lime sulphur (1.5%) applied 16 h after infection killed early infection structures and stopped further development. Lime sulphur reduced the percentage of penetration to below 10% even with treatments 40 h after infection. The activity of hydrated lime is due to its alkaline pH of 12.45. Suspensions of calcium carbonate (6.75 g/l) showed no effect on percentage of penetration. Up to pH 12.0 effects of potassium hydroxide solutions on vitality of germinated conidia and percentage of penetration were weak. Values of pH 12.4 or higher were necessary to kill germinated conidia of V. inaequalis and their primary penetration structures. At pH 12.4 activity of KOH was comparable with the effect of hydrated lime. It is argued that the high pH disrupts the spore membranes. This is supported by the fact that dead spores and germ tubes did not fluoresce with FDA, which requires integrity of the membranes. For an individual spore the disruption of the spore membrane is an all-or-nothing event