Enhancing biological efficacy and rainfastness of foliar applied calcium chloride solutions by addition of rapeseedoil surfactants

The efficacy of a homologous series of biodegradable rapeseedoil derivatives (triglyceride ethoxylates; Agnique RSO® series, Cognis, Düsseldorf, Germany) with an average of 5, 10, 30, and 60 units of ethylene oxide (EO) as adjuvants for foliar‐applied calcium chloride (CaCl₂) was evaluated in Malus domestica L. Borkh. Previous experiments revealed that the surfactants used are not phytotoxic. All the surfactants applied decreased contact angles of spray solutions. The lipophilic surfactants RSO 5 and RSO 10 reduced contact angles more than the hydrophilic ones. Drying time of spray droplets was most prolonged when RSO 5 was added. Our data provide evidence that the most lipophilic surfactant RSO 5 can enhance Ca penetration, which was shown in isolated cuticles of tomato fruits (Lycopersicon esculentum cv. Panovy) as a model system. The surfactant effect was most pronounced during the first 4 h and reached a plateau after 24 h. Rainfastness of Ca on surfaces of Malus domestica leaves was improved when RSO 5 was added to CaCl₂ spray solutions. All surfactants applied improved rainfastness of Ca on ’︁Braeburn’ fruit surfaces compared to the treatment with CaCl₂ alone. Under field conditions, leaf/fruit application of CaCl₂/RSO 5 led to an increased Ca content in ’︁Braeburn’ apple fruits and a reduced bitter pit incidence.