Effect of Molybdenum Foliar Sprays on Yield, Berry Size, Seed Formation, and Petiolar Nutrient Composition of “Merlot” Grapevines

Abstract

Abstract Field experiments with irrigated "Merlot" vines were carried out at 3 sites in the Mount Lofty Ranges of South Australia over 3 years to examine the effects of molybdenum (Mo) foliar sprays on bunch yield, berry size, and nutrient composition of petioles. Bunches were divided into different size grades for black and green berries. Basal petioles were sampled at flowering and veraison for nutrient analyses. In year 3, seed number per berry was assessed at sites 2 and 3. Two Mo foliar sprays (each spray contained 118 g Mo as sodium molybdate/ha in 410–800 L/ha of water) applied before flowering increased yield per vine and bunch weight in all experiments in year 2 and at site 3 in year 3. Yield responses ranged from 221% at site 1 to 750% at site 2 in year 2 and 70% for site 3, year 3. Average bunch weight increased from 243% at site 2 to 425% at site 1 for year 2 and by 69% at site 3 in year 3, and was the main yield component affected by Mo application. In year 1, the application of Mo did not affect yield or bunch weight at any site. In year 2, the application of Mo increased the yield of 5–15 mm colored berries by 301, 499, and 258% at sites 1, 2, and 3, respectively, and by 70% at site 3, year 3. Mo application increased the percent of berries, which had one or more functional seeds (when assessed at sites 2 and 3 in year 3). Molybdenum concentrations in petioles sampled at flowering and veraison increased in response to applied Mo in all years. Petiolar Mo concentrations in unsprayed vines were consistently higher in year 1 compared with other years. The effect of applied Mo on the concentration of other nutrients in basal petioles sampled at flowering and veraison were small and of little practical importance. Nitrate-N did not accumulate in the petioles of unsprayed plants in any year. Changes in petiolar Mo concentrations between flowering and veraison were dependent on supply. Nitrate-N, total-N, and phosphorus (P) concentrations declined with time, while calcium (Ca), manganese (Mn), and iron (Fe) tended to increase. At flowering, Mo concentrations in basal petioles of 0.05–0.09 mg/kg were associated with significant bunch yield response to applied Mo. Molybdenum deficiency can be a major factor in the occurrence of berry development disorders such as shot berry formation and hens and chickens (millerandage) in "Merlot" grapevines. The increased percent of colored berries with one or more functional seeds and the decrease in the proportion of green berries suggests that Mo application affected pollination and/or fertilization, and thereafter berry development. Acknowledgments The authors thank Dr. J. B. Robinson for comments made initially on the role of Mo in the "Merlot" problem and grapevine nutrition. We acknowledge the expertise of Mr. L. Palmer and Mrs. T. Fowles of Waite Analytical Services, of the Plant Science Department, the University of Adelaide, Waite Precinct for low limit detection of Mo in petioles and other plant nutrient analyses. We thank the staff of the Analytical Crop Management Laboratory at Loxton for nitrate and chloride analyses. Thanks to Mr. D. Maschmedt, Primary Industries and Resources, South Australia for the soil classifications. We are grateful to Ms. K. O"Brien for technical assistance in year 1. Thanks to Mr. Murray Leake, Nepenthe Viticulture for provision of soil temperature data. The Cooperative Research Centre for Viticulture (CRCV) is a joint venture between Australia's viticultural industry and leading research and educational organizations. It promotes cooperative scientific research to accelerate quality viticultural management from vine to plate. This work was conducted as part of the CRCV's Viticare Participatory On Farm Trials Program and we acknowledge crucial funding assistance from the Grape and Wine Research and Development Corporation. We thank the Central Committee of the Phylloxera and Grape Industry Board of South Australia for part funding costs of soil and plant analyses and bunch fractionation work. We are indebted to the participating growers; Messrs B. Miller, D. Handyside, and C. Eckert who provided vineyards and assisted in the planning and conduct of these field experiments as part of the Australian On Farm Trials project.