Rain-induced fruit cracking in sweet cherry (Prunus avium L.).

Production of sweet cherries (Prunus avium L.) around the world is centred on achieving high quality, blemish free fruit. One of the main concerns to Australian cherry growers and limitations to production is the unpredictable nature of raininduced fruit cracking, which occurs in the later stages of fruit development and renders fruit unsaleable in markets that attract premium fruit prices. Control of cracking remains unreliable and the underlying mechanism is not yet fully understood. This thesis examines cracking from a tree water relations perspective, starting with patterns of crack development, the relationship of rainfall and fruit properties with cracking incidence, and concludes with an exploration of the underlying mechanisms resulting in crack development. Cracking takes three distinct forms; stem end cuticular fractures, apical end cuticular fractures and large cracks, usually deep into the pulp, on the cheek of the fruit. This study has demonstrated that although all three types developed in the three-week period prior to commercial harvest, varieties displayed different levels of total cracking and distinctly different proportions of each crack type. Overall the extent of cracking was strongly controlled by season. While initial development of cracks coincided with rainfall, no relationship between amount of rain and incidence of cracking was found. There were also relationships between both crop load and tangential stress of the fruit skin with crack type and incidence. Influx of water to the fruit via the vascular system was recorded after rainfall, prompting an investigation of the influence of water uptake both via the vascular system and directly across the skin. Application of excess water to simulate rainfall, only to the root-zone induced large cracks in the side of the fruit. Application of water at a similar rate to the canopy induced cuticular cracks localised around the stem and apical end of the fruit with no increase in deep side cracks. This finding suggested different water uptake mechanisms driving development of side cracks and the shallower cuticular cracks at the ends of the fruit. An exploration of the driving forces responsible for vascular entry of excess water into the fruit proposed influx via the pedicel phloem and supported a role for adjacent leaves in control of water movement under conditions likely to initiate deep side cracks. The findings establish mode of water uptake into the fruit as the main determinant of crack type in a susceptible variety. Future management of cracking needs to consider both varietal and seasonal factors, and may need to be variety or crack type specific.