Rainfastness of lime sulphur and other inorganic fungicides used for scab control in apple and pear production


Abstract: The aim of this study was to determine, under controlled and field conditions, the rainfastness of new fungicide formulations available for organic apple and pear farming. The rainfastness was assessed by both (i) analysing the content of residual chemical elements present on the leaves subjected to various amounts of rain (from 0 to 30 mm), using an atomic emission spectrometer (ICP-AES) under controlled and field conditions, and (ii) measuring scab control effectiveness on inoculated seedlings following various amounts of artificial rain (from 0 to 30 mm) applied on the leaves under controlled environment. The field study in apple orchards showed that with protective applications of lime sulphur (2%), copper hydroxide (0.1%), wettable sulphur (1%), and potassium bicarbonate (1%) subjected to 33 mm of precipitation, leaves still contained 61, 48, 36, 43%, respectively, elemental sulphur, copper or potassium ions applied initially. This means that among sulphur formulations, lime sulphur showed significantly higher retention on leaves subjected to rain. A greenhouse study on seedlings showed that protective applications on dry leaves of lime sulphur (2%), wettable sulphur (1%), copper hydroxide (0.1%) and potassium bicarbonate (1%) significantly reduced pear scab severity, with more than 96% effectiveness on susceptible cultivars under high scab pressure. Rainfastness of lime sulphur appeared to be lower compared to wettable sulphur, copper and bicarbonate, providing scab-intensity control effectiveness of 85, 52, 28, 6%, and scab-severity control effectiveness of 92, 64, 75, 27%, respectively, after an application of 30 mm of artificial rain. Other formulations, such as Heliosoufre, Heliocuivre and Armicarb were also tested in this study.

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