Cracking susceptibility of sweet cherries (Prunus avium L.) under different conditions

Rain and hail during ripening and harvesting season can cause yield losses up to 90 % in sweet cherry cultivations in Germany. Particularly, high yield losses after precipitation are due to the cracking and the following rotting process through bacteria and fungi's. In order to protect the crop and improve cherry quality they can be treated with chemicals dried or healthier covered with rain shelters. To date the cracking phenomenon of cherries is still not clearly understood. Therefore, in the present study the cracking resistance of three cherry varieties under different conditions was observed. We used 'Regina', 'Karina', and 'Summit' grafted on the rootstock GiSelA 5. The test-section in the field was covered with a plastic-foil (pc) as rain shelter and a bird net, whereby the control (c) was covered with a bird protection-net only. The cherry varieties have been harvested, weight and sorted into undamaged, rotten and cracked fruits. In order to compare the varieties under equal conditions the cracking sensitivity was also tested under laboratory conditions with the method from CHRISTENSEN (1996). The average yield per tree was significantly higher in the pc treatment in 'Summit' and 'Regina' than in the control. Furthermore, the average weight of rotten fruits per tree in these varieties was in c, without rain shelter, about two to three times higher as in the pc treatment. There was no significant difference in yield and rotten fruits among treatments in 'Karina'. But the percentage of cracked fruits between the cultivation methods was significant different. Hereby improvement of cherry quality was observed in the pc treatment with reduced numbers of useless fruits. Under field conditions 'Karina' was at least susceptible to cracking resistance followed by 'Regina' and 'Summit'. In the laboratory Cracking Test the result was different. Here, the variety 'Summit' showed the best cracking resistance compared to the other varieties. According to the method of CHRISTENSEN (1996) it was impossible to calculate a Cracking Index (CI) from 'Summit' because of a too little number of cracked fruits. The CI of 'Karina' was 22 under cover and 36.8 in the control without cover. 'Regina' performed better with a CI from 6.8 under plastic cover compared to 15.6 in the control. Therefore, the cracking susceptibility of varieties was different under field and laboratory conditions. In conclusion the distribution of the raindrops on the outer layers of the cherries, the ingredients (sugar) and morphology of cherries (fruit firmness) play an important role in the cracking process. The result with different cultivation methods and the different reaction of the fruits in the Cracking Index trial relate to morphological dissimilarities of tested varieties. Using rain shelters can clearly reduce the cracking of cherries and the following rotting process.