Towards precision spray applications to prevent rain-induced sweet cherry cracking: Understanding calcium washout due to rain and fruit cracking susceptibility

Cracking of sweet cherries (Prunus avium L.) due to rain leads to serious economic loss to fresh market sweet cherry growers. Previous studies have proven that calcium (Ca)-based spray applications prior to rain events reduce rain-induced cracks in sweet cherries. The Ca-based chemicals on the surface of fruit get diluted or washed out due to rain and understanding such washout rates is critical to decide upon re-application rates to prevent fruit cracking. Therefore, the main purpose of this study was to quantify the potential washout of sprayed Ca-based chemicals from cherry fruit surface and leaf samples at different rain levels (2.5, 5.0 and 10.0 mm) under field condition. Prior to the field Ca washout experiment, cracking susceptibility, represented as cracking index (CI) of three different sweet cherry varieties (Selah, Skeena and Rainier) with different concentrations of calcium chloride (CaCl2) and calcium nitrate (Ca(NO3)2) (T1 = 0.5% CaCl2, T2 = 1.0% CaCl2, T3 = 0.5% Ca(NO3)2, T4 = 1.0% Ca(NO3)2) was measured under laboratory condition. Similarly, the CI of fruits sampled from 'Selah' cherry trees sprayed with T1, T2, T3 and T4 after 10 mm of rain was also measured. The laboratory experiment data revealed that CI for non-treated fruits was in the range of 55-85 and susceptibility reduced significantly (4-37) when fruits were treated with Ca-based chemicals. The field experiments revealed that the Ca washout from both fruit and leaf linearly (R2 > 0.750) increased with rain levels. Maximum washout was up to 72% at 10.0 mm rain. Results also suggest that for 5.0 mm or less rain, the Ca washout from cherry fruit surface was generally below 50%. Findings of this study could serve as a basis for determining suitable Ca-based chemical re-application rates and concentration on sweet cherries to minimize the fruit cracking without affecting produce quality.