Raisin-making has traditionally been a labor-intensive activity, but California growers are increasingly adopting mechanized production methods to reduce cost and thereby help ensure economic sustainability (Christensen, 2000; Sánchez et al., 2008). One mechanized production method is dry-on-vine (DOV). In the DOV method, canes bearing mature fruits are severed, causing them to dry into raisins on the trellised vines, from which they are mechanically harvested (Fidelibus et al., 2008). This method has seen limited adoption because traditional ‘Thompson Seedless’ raisin vineyards are poorly suited for DOV (Fidelibus et al., 2007b), and the establishment of new DOV vineyards requires a substantial capital investment (Sánchez et al., 2008).
Instead, most raisin grapes are currently shaken from the vines with a mechanical harvester and then deposited onto a long sheet of paper called a “continuous tray” to dry (Christensen, 2000; Sánchez et al., 2008). Later, another machine is used to pick up the dried raisins from the tray. The main problem with this method is that the harvest machines, which were designed for juice and wine grapes, can impart excessive mechanical damage to the grapes, rendering them unsuitable for raisin-making (Fidelibus et al., 2007a). Some of the mechanical damage is the result of tearing of the stem end, which occurs when the berries are detached from their pedicels (Studer and Olmo, 1974). This type of damage is minimized by severing the fruit-bearing canes ≈1 week before harvest, which reduces fruit detachment force (FDF) and causes the rachis to become dry and brittle so that the cluster tends to break apart during harvest, allowing some berries to remain attached to their pedicels (Fidelibus et al., 2007a; Studer and Olmo, 1971, 1974).
When executed well, cane severance reduces mechanical damage, but in practice, results are variable because environmental conditions affect the physical characteristics of the rachises (Studer, 2000), and pruning crews often miss some canes. If there is a high proportion of grapes on missed canes or on basal shoots, which are not severed, they must be removed before harvest to prevent them from reducing the overall quality of the crop (Studer and Olmo, 1974). Furthermore, the need for cane severance, which cannot be mechanized in most traditional vineyards, detracts from the industry's goal of complete harvest mechanization. Recently, we discovered that the application of methyl jasmonate (MeJA) to grapes can reduce FDF to a similar extent as cane severance and thus might offer an alternative to cane severance (Fidelibus et al., 2007a). The purpose of this study was to verify the concentration of MeJA needed for consistent loosening and to determine how the time between application and harvest may affect FDF and preharvest fruit abscission.
Ballinger, W.E. & Nesbitt, W.B. 1982 Postharvest decay of muscadine grapes (Carlos) in relation to storage temperature, time, and stem condition Amer. J. Enol. Viticult. 33 173 175
Christensen, L.P. 2000 Current developments in harvest mechanization and DOV 252 263 Christensen L.P. Raisin production manual University of California, Agricultural and Natural Resources Oakland, CA
Fidelibus, M.W., Cathline, K.A. & Burns, J.K. 2007a Potential abscission agents for raisin, table, and wine grapes HortScience 42 1626 1630
Fidelibus, M.W., Vasquez, S.J. & Cathline, K.A. 2007b Canopy separation and defoliation do not improve the dry-on-vine raisin-making method for ‘Thompson Seedless’ grapevines on traditional trellises J. Amer. Pomol. Soc. 61 61 70
Fidelibus, M.W., Christensen, L.P., Katayama, D.G. & Ramming, D.W. 2008 Early-ripening grapevine cultivars for dry-on-vine raisins on an open-gable trellis HortTechnology 18 740 745
Hartmond, U., Yuan, R., Burns, J.K., Grant, A. & Kender, W.J. 2000 Citrus fruit abscission induced by methyl-jasmonate J. Amer. Soc. Hort. Sci. 125 547 552
Kou, L., Luo, Y., Wu, D. & Liu, X. 2007 Effects of mild heat treatment on microbial growth and product quality of packaged fresh-cut table grapes J. Food Sci. 72 567 573
Studer, H.E. 2000 Raisin harvest mechanization: A bit of history 245 251 Christensen L.P. Raisin production manual University of California, Agricultural and Natural Resources Oakland, CA
Studer, H.E. & Olmo, H.P. 1971 The severed cane technique and its application to mechanical harvesting of raisin grapes Trans. ASAE 14 38 43