Cultural Practices to Modify Berry Physical Properties and Susceptibility to Cracking
Cultural treatments including ethrel sprays, irrigation regimes, and girdling were performed in 2011 on a Flame Seedless vineyard in Arvin, CA. Ethrel sprays, consistent with 2010 trials, had the largest effects, with the standard two ethrel sprays significantly increasing cracking compared to no spray. Cracking following ethrel spray significantly increased during the period of 63DAA to 68DAA, but remained at a similar level after 68DAA. This suggests that cracking may have happened soon after spraying ethrel at 60 and 65DAA, and it will be important to focus future efforts on this time period. Unlike 2010, irrigation treatments did not have significant effects on cracking, probably because the differences for vine stem water potential (SWP) among treatments in 2011 were not as substantial as in 2010 due to the relatively wet and cool weather in 2011. Half (0.5X) of the grower standard irrigation resulted in berries of less firmness and smaller size compared to grower standard (1X) and frequent (3.6X) irrigation. Irrigation trials that are designed to coincide the crack susceptible period may be more effective in reducing cracking by withholding water prior to spraying ethrel. Less than half irrigation during this time may be necessary, depending on the weather conditions. As in 2010, girdling did not significantly affect cracking. Skin mechanical properties were tested in the laboratory using custom equipment (the ?berry balloon system,? BBS), and in all treatments, skin stress and strain at failure (cracking) decreased markedly over berry development, indicating that cracking susceptibility increases as berries ripen. As a result, skin stress and strain at failure were highly and negatively correlated with berry brix. Skin mechanical properties did not differ for berries with or without ethrel spray in the field, or for berries from different irrigation treatments. The lack of an ethrel effect on skin properties was surprising, in view of the strong effect ethrel had on cracking in the field. However, exposing the berry skin to an ethrel solution (1.5 µM) in the lab while conducting BBS tests, caused significantly lower skin stress and strain at failure. Also, intact berries soaked in ethrel solution cracked much faster than berries soaked in water. Noticeably, 100%of flame seedless berries cracked within a few hours after soaking in ethrel solution and the soonest cracks were detected within 20 minutes of soaking. Hence, we are confident that ethrel is mechanistically important in cracking. Using ethrel in the soaking solution may have use as a relatively rapid screen for evaluating cracking susceptibility in different grape cultivars. In addition to changing skin properties, soaking tests also directly expose berries to water, but the combined results from soaking and BBS, strongly suggest that cracking susceptibility may increase very soon following an ethrel spray. Hence, the failure to detect differences in skin properties using field sampled berries may be due to the fact that only sound (non-cracked) berries had to be selected for the lab tests, and may not have been typical of the berries that did crack in the field. Microscopic studies showed that skin thickness decreased somewhat post veraison, perhaps contributing to the observed increase in cracking susceptibility over grape berry development. Preliminary soaking tests showed that Calcium could reverse the ethrel effect on accelerating cracking. Future studies will be necessary to determine if Calcium can also be used to reduce the cracking that is associated with ethrel sprays.