Effect of foliar fertilization on yield and quality of table grapes

The aim of the study was to examine the response of table grapes, cv. "Italia", to foliar fertilizers. The experiment was carried out in 2002 and in 2003 in a table grape vineyard in the province of Bari/Italy. The foliar fertilizer formulations used were a highly concentrated seaweed suspension (Ascophyllum nodosum) jointly applied with a Ca fertilizer suspension (Wuxal Aminocal). Both products were used at 5 x 3 L ha(-1) between fruit set and bunch closure. As additional treatment, sprays with a slow release N fertilizer (methyleneurea; Azolon) were applied between fruit set and bunch closure at 5 x 5 L ha-1 with the aim of retarding maturation. Particularly in 2002, the combined application of a seaweed suspension and Ca significantly increased yield, average berry weight and sugar content. In 2003 a yield increase of 2.96 t ha(-1) was obtained. Berry weight increased from 7.0 mm (control) to 7.9 mm (seaweed suspension and Ca). Average bunch weight increased from 582 to 655 g. Soluble solids increased from 16.5 to 17.0 (degrees Brix). Application of methyleneurea, resulted in bunch weight of 627.5 g average (45.5 g greater than the control). Methyleneurea delayed maturity as evidenced by the lowest sugar content (16.2 degrees Brix). Results obtained in 2003 confirmed the trend seen in 2002, although crop response was much less pronounced. Results show that foliar sprays did not cause any visual toxicity on grapes. 2002 and 2003 were years with unusual weather leading to extreme stress conditions (summer-autumn in 2002 being extremely wet, while 2003 was hot and dry, both conditions negatively affecting grape production). Results support the use of seaweed suspensions rich in micronutrients and natural growth regulators in combination with Ca to improve yield and quality of "Italia" table grapes particularly under stress conditions. Use of liquid methyleneurea fertilizer is indicated to retard maturation until the end of the commercialization period (November-December).