Cultural Practices to Modify Berry Physical Properties and Susceptibility to Cracking
Ethrel sprays, irrigation regimes, and girdling all had a significant influence on Flame Seedless cracking in the 2010 field trials at Arvin, CA. Among the 64 treatment combinations of these factors, ethrel sprays had the largest effects, particularly the first ethrel spray, which increased cracking rate by six-fold compared to plots with no spray. This suggests that the dose and timing of the ethrel spray may be a key issue in balancing the advantages for color development with disadvantages of cracking. The four irrigation treatments, 0.6X, 1X (grower standard), 1.7X and 2.3X, caused clear differences in plant water status as measured by stem water potential (SWP), with berries from the higher irrigation treatments having significantly increased berry size, but also increased cracking, and berries from the lowest irrigation having decreased size, firmness, and cracking. Cell turgor was also well correlated with berry firmness. The correlation between irrigation and cracking justifies the prospect of future usage of SWP and other linked berry properties to monitor and minimize cracking, and we will continue research to resolve these factors in order to provide growers with information about threshold values for cracking. Girdling also significantly increased cracking with the late girdle having higher effects than the early girdle. Late girdle may not be as necessary for this field considering its negative impact on cracking but minor effects on color development.
Tracking of individual berry growth showed that berries at around veraison expanded with an average rate of about 2.5% per day, but most of the expansion occurred at night. A peak short-term rate equivalent to 6.4-8.5% per day was exhibited at 7-8am, but the monitored berries that showed these rates did not crack. Skin mechanical properties in the standard and frequent irrigation treatments were tested with custom equipment (berry balloon system, BBS), and in both treatments, pressure, stress and skin strain at failure (cracking) progressively decreased over berry development, indicating, as expected, that cracking susceptibility increases over time. In the scanning electron microscope (SEM) the surfaces of field-cracked, soaking-cracked, and BBS-cracked berries all showed clear evidence of failure by cell wall breaking and not cell separation, contrary to current scientific thinking. The BBS also detected varietal differences (Flame seedless vs Syrah) in skin mechanical properties, and given the strong impacts of ethrel spray on cracking, it will be important to quantify skin properties with BBS for the different ethrel treatments in 2011. Flame seedless berries were soaked in water with only the stylar end or the longitudinal side immersed, and berries cracked at the stylar end in the former case but remained intact in the latter. Soaking of entire flame seedless berries in water confirmed that berries crack at a fairly low strain (<5%). These results and our SEM images showing broken cell walls, indicate that cracking is most likely a local event that is initiated by failure of a few cells rather than a consequence of an overall expansion of the flesh, as previously believed.