Bitter pit in apples is caused by a lack of calcium in the fruit's outer tissues that ought to be easily overcome by spraying with solutions containing calcium. Unfortunately, the apple's surface often forms an impenetrable barrier to these solutions.
"Golden Reinders" trees in Spain were sprayed to run-off with solutions of calcium chloride or calcium propionate (120 or 250 millimolar). Each spray also contained an additive intended to improve the uptake of calcium - one was a food additive, the sodium salt of the carboxymethyl ether of cellulose (CMC), while the other was a standard wetting agent (Tween 20).
The trees were sprayed monthly up until one month before harvest. Fruit sprayed with 250 millimolar calcium chloride plus 0.05 percent Tween 20 had the highest concentration of calcium in their outer tissues for most of the growing season.
However, at harvest, fruit treated with 250 millimolar calcium propionate plus 0.5 per cent CMC had the highest calcium concentration and the firmest flesh. Less bitter pit developed on fruit stored at 4 degsC if the trees had been sprayed with a 250 millimolar solution of either calcium compound plus 0.5 per cent CMC.
Improving the Performance of Calcium-Containing Spray Formulations to Limit the Incidence of Bitter Pit in Apple (Malus x domestica Borkh.) by Blanco, Fernandez and Val (2010). Scientia Horticulturae 127 (1): 23-28. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti
Dr Ken Cockshull is associate fellow, Warwick Crop Centre, University of Warwick.