Chemical Removal of Primary Inflorescence and Mature Leaves to Force Vine Regrowth and Fruiting in a Warmer Region for Winegrape Production

Previous research has demonstrated that grapevines can be forced to regrow while fruit ripening is shifted to the cooler portion of the growing season improving winegrape fruit quality in warm regions. In order to force vine regrowth, shoot tips, primary clusters, leaves, and laterals need to be removed, requiring a significant amount of labor if it is conducted manually. In order to reduce or eliminate the reliance for manual labor, nitrogen fertilizers including NH₄NO₃, (NH₄)₂SO₄, NH₄Cl, Ca(NO₃)₂, KNO₃, urea, and UAN-32 were screened in 2012 and 2013 at 4, 8, 12, 16, and 20 g/L N with a surfactant containing 0.005% polyalkyleneoxide-modified polydimethylsiloxane. Primary inflorescences, mature leaves, young leaves, and shoot tips were dipped into fertilizer solutions at either flower separation, full bloom, or after fruit set. NH₄NO₃, (NH₄)₂SO₄, NH₄Cl, Ca(NO₃)₂ resulted in adequate necrosis at flower separation and full bloom. Most fertilizers were ineffective after fruit set. KNO₃, urea, and UAN-32 were less effective at all stages. Greater degree of necrosis was observed on inflorescences, young leaves, and shoot tips than on mature leaves at both flower separation and full bloom. The most effective fertilizers, NH₄NO₃, (NH₄)₂SO₄, and NH₄Cl, were then further evaluated in 2013 for the optimal application rate by spraying into the forcing zone of the developing canopy at full bloom. Growing shoots were hedged to 6 nodes and all remaining primary clusters, leaves, and laterals were removed by hand at pea-sized berry stage to force vine regrowth and fruiting. (NH₄)₂SO₄ at 30 g/L N and NH₄Cl at 20 g/L N were the most effective, resulting in the greatest degree of necrosis of inflorescence and mature leaves, the greatest amount of hedge weight, the greatest number of forced shoots and vine regrowth, and the greatest number of forced clusters and the highest yield of forced crop. No severe injury was observed on the remaining portion of growing shoots. The experiments demonstrated the potential of nitrogen fertilizers in chemical removal of primary inflorescences and mature leaves and optimized the application rate and timing to reduce the labor requirement for crop forcing. It seems possible to force vines by chemically removing the primary inflorescences with nitrogen fertilizers before full bloom, and then mechanically hedging the growing shoots and removing the leaves and laterals at the time of forcing.